Coherence peak; a universal feature in high-Tc superconductors
The BCS (Bardeen-Cooper-Schriefer) theory tells us that two electrons are condensed to make a pair (Cooper pair) in the superconducting state, producing a superconducting coherence peak (SCP) just below the Fermi level. However, it has been a long-standing puzzle why single-layered high-Tc superconductors (HTSCs) such as La2-xSrxCuO4 (LSCO) do not exhibit the SCP in contract to multi-layered HTSCs such as Bi2Sr2CaCa2O8. This inconsistency has provoked a lot of debates in modeling the high-Tc mechanism, for example, an essential difference between single- and multi-layered HTSCs. This PRL paper reports the first direct observation of a SCP in LSCO, together with the precise momentum dependence by angle-resolved photoemission spectroscopy (ARPES). They found that the SCP is confined in a surprisingly narrow momentum region between the node and the antinode in sharp contrast to multi-layered HTSCs. The experimental results suggest that a pseudogap dominant around the antinode affects this anomalous momentum dependence of SCP in LSCO. Thus, the present high-resolution ARPES study has established the universality of a coherence peak in HTSCs and at the same time provided a key for understanding the difference in the superconducting transition temperature between single- and multi-layered HTSCs.
Friday, June 1, 2007
Thursday, May 31, 2007
5-31-07
Quantum phase transition in ferroelectric oxides
A new quantum phase transition (QPT) has been interestingly observed in
ferroelectric oxides. In this case, the coulomb interaction, which
creates the long-range order of dipole moments, competes with the
quantum mechanical fluctuation that destroys the order.
Usually, the phase transition, for example the freezing of water, is
governed by the thermal fluctuation; however, if the temperature
approaches the absolute zero temperature, the quantum mechanical effect
emerges in the macroscopic scale. In this study, we have clearly
observed the crossover of the critical behavior of the 腧soft mode腨 from
the classical to the quantum regime, where the soft mode is the
anharmonic lattice vibration that triggers the phase transition. This is
the first QPT brought about by the phonon (the quanta of the lattice
vibration), in contrast to those induced by electronic interactions. In
particular, the novel ferroelectric-paraelectric phase coexistence state
has been revealed in the vicinity of the so-called quantum critical
point for the first time. These findings have been achieved in the solid
solution of SrTi^16 O_3 and SrTi^18 O_3 by carefully controlling the
amplitude of the quantum fluctuation without modulating the chemical
natures of the material by a proper selection on the ^18 O-concentration.
***
Bose-Einstein condensation in the presence of an attractive interaction and its relation to the cosmological Q-balls
(1) It is well known that stable atomic Bose-Einstein condensates can
be formed when the interaction between the atoms is repulsive. An
attractive interaction, unless it is extremely small, leads to the
collapse of the Bose gas. The Bose-Einstein condensate of spin waves
(magnons) with a repulsive interaction was experimentally stabilized
in 1984 in superfluid 3He-B. In this paper it is shown that a
specific feature of this Bose condensate is that it survives the
attractive interaction between the magnons. The magnon condensate
with attractive interaction is formed in the potential well produced
by the texture of the order parameter of 3He, and is observed in
nuclear magnetic resonance experiments as a long-lived coherent
precession of magnetization, which can persist for an hour without
external pumping of energy.
(2) The structure and stability of the magnon condensate with an
attractive interaction are similar to those of certain objects
discussed in particle physics. These are the so-called Q-balls,
which are stabilized due to the conservation of the global charge Q,
typically the baryon or lepton number. In the magnon condensate the
role of the charge Q is played by the projection of the total spin of
the liquid in the direction of the magnetic field. At the quantum
level, Q-balls are formed due to a suitable attractive interaction
that binds the quanta of the scalar field into a large compact
object. Q-balls may contribute significantly to the dark matter and
baryon contents of the Universe. Observation of Q-ball objects in
3He-B confirms the possibility that stable cosmological Q-balls might
exist and should be searched experimentally.
***
Origin of Antiferroelectricity Explained from First Principles
The phenomenon of antiferroelectriciy wherin a compound develops two (or more) oppositely polarized sublattices was first discovered in 1938 in ammonium dihydrogen phosphate, commonly called ADP. ADP and its ferroelectric analogs have been used extensively in electro-optical applications. However, the atomistic details of why ADP becomes antiferroelectric has not been fully understood. A detailed understanding of such properties is needed in order too be able to hone their desirable characteristics. Now efficient algorithms for /ab-initio/ electronic structure calculations have enabled a group of solid state theorists at the Rosario National University in Argentina, working with chemists at Florida State University, to show that ADP's antiferroelectricity is a result of a delicate balance between the energetics of the two different types of hydrogen bonds in the lattice : O-H···O and N-H···O bonds. In a small range of atomic configurations, the optimal N-H···O bonding is enabled by the antiparallel alignment of molecular dipole moments in the unit cell and the crystal thus exhibits antiferroelectricity. The calculations show also that a small change in the atomic displacements could also render ferroelectricity to ADP, thus providing a theoretical basis of a long-standing observation that ADP appears to possess coexisting ferroeelectric and antiferroelectric regions whose relative population changes with temperature. (J. Lasave et al., Phys. Rev. Lett. (2007, in press)).
***
Potassium Bose-Einstein condensate: last but not least
A recent revolution in modern physics has been the achievement of Bose Einstein condensation, i.e. the formation of macroscopic quantum objects offering unprecedented possibilities for the study of the quantum world. This phenomenon has been demonstrated for a number of atomic species including all the stable elements in the first column of the periodic table, except one isotope of potassium. An experimental team at LENS, University of Florence, has now succeeded in creating a Bose-Einstein condensate of such last isotope, potassium-39, and found that, surprisingly, it is one of the most interesting among those studied so far. The team has overcome what appeared to be a fundamental obstacle to condensation of this species, i.e. the presence of an attractive interaction between pairs of atoms. The researchers have found that with the application of appropriate magnetic fields such interaction can be changed to repulsive and a condensate can be produced. Moreover, they have found that in a potassium-39 condensate the interaction can also be nulled with high accuracy, opening a route to the study of an ideal, i.e. non-interacting, quantum object. This novel Bose-Einstein condensate is expected to allow the study of some aspects of the quantum world that have not yet been explored.
***
Surveying with Electrons: Putting Lloyd's Mirror on the Map
A major puzzle for many researchers working in the field of materials
research is how to obtain 3D movies of surface dynamic events, often
during thin film growth or annealing. In a new approach, Monash
Physicists have shown how in situ electron microscopy can be used to
obtain surface height resolution in real-time. Just as the contour lines
on a topographic map provide mountaineers with the capacity to plan
their assault on a mountain, Lloyd's mirror combined with electron
microscopy can provide a relief map of surfaces in which the spacing
between fringes is directly related to height. In the classic 19th
century Lloyd's mirror experiment, light reflected off a mirror
interferes with light coming directly from the source. PRL XX reports a
novel 21st century version of the famous Lloyd's mirror, in which
ultraviolet light is used to illuminate gallium droplets on a gallium
arsenide substrate. The bright interference fringes result in the
emission of electrons, which are imaged in a surface electron
microscope. The fringes are sensitive to the 3D shape of the gallium
droplets, but are also distorted by the electric field due to the
topographic features. However, these distortions can be corrected using
image processing, to provide a real-time relief map showing surface
dynamics of the evolving metallic droplets. Lloyd's mirror electron
microscopy using a synchrotron light source will allow the technique to
image surface features with nanoscale resolution, opening up new
possibilities to make 3D movies of dynamic events during thin film
growth and processing.
***
Towards ultrafast x-ray physics: a novel method to manipulate x-ray
pulses
Theoretical calculations predict a novel physical effect for x rays that
may facilitate the generation of ultrashort x-ray pulses with tailored
shape. This prediction opens new vistas in science, in connection with
both existing x-ray facilities such as Argonne's Advanced Photon Source
and future x-ray free electron lasers. The effect is called
electromagnetically induced transparency (EIT) for x rays. A gas that
would otherwise strongly absorb, and thus be opaque to, x rays of a
given wavelength can be rendered transparent to the x rays by
simultaneously shining an intense laser into the gas. EIT for visible
light is well known and represents a unique tool for manipulating light.
Inner-shell vacancies produced by x-ray absorption have very short
lifetimes of the order of femtoseconds or shorter. Therefore, the laser
intensity required to induce EIT is so high that the standard
description of EIT becomes invalid. Before this study, it was
questionable whether EIT may exist at all in the x-ray regime. The
theoretical analysis carried out in this paper demonstrates that
existing intense-laser technology may be harnessed to imprint the
temporal shape of optical laser pulses onto x-ray pulses.
***
Unique due to relativity
Heavy element polonium enters harmfully our lives via air, soil and
plants such as tobacco or tea, but very little is known about its
physical and chemical properties. It has not been clear why polonium, as
the only element from the Periodic Table, crystallizes in the simple
cubic structure, one of the simplest structures ever. Our paper shows,
on the basis of the fundamental quantum mechanics, that this
unique structure of polonium is, similarly as the yellow luster of
gold, due to relativistic effects. We also draw the attention to
polonium strong elastic anisotropy, which has no equal in other solids,
and demonstrate that this is an inherent peculiarity of its crystal
structure. Finally, as a challenge for future experimental studies, we
predict a mechanical instability of simple cubic polonium at relatively
low pressures resulting in a mixture of two lower-symmetry structures.
***
The quantum nature of an atom enables its simultaneous propagation in two different regions of space: this property has been used by the research group of J. Vigué (Université P. Sabatier and CNRS, Toulouse) to make an absolute measurement of the refraction index of gases for lithium atomic waves. This experiment is based on an atom interferometer, in which each atom follows two paths. Their maximum distance, equal to 100 micrometers, is sufficient to introduce a material object between them so as to apply a weak gas pressure on one of them.
The only comparable experiments have been done on sodium by the research group of D. Pritchard at MIT but they have not achieved an absolute measurement.
This index describes the attenuation and the delay or advance of the atomic wave induced by collisions with the atoms of the gas. The comparison of the present attenuation measurements with previous values is excellent. The delay or advance of the atomic wave can be accessed only by the present atom interferometry method and the measured values are in agreement with theoretical estimates.
Finally, this type of experiments changes our representation of atoms, from point-like particles to strongly delocalised quantum waves!
***
Plasma Density Fluctuations Are Universal, But This Was Hidden Beyond A
Picturesque Smile
A unique parabolic relation links two statistical quantities related to
plasma density fluctuations. As in many other physical systems, a
significant effort is dedicated to finding universal aspects in a
statistical description of plasma turbulence. The probability of
occurence of any event can be given by its probability density function
(PDF hereafter). For a PDF, the degree of asymmetry and the weight of
its wings are contained in two parameters called skewness and kurtosis,
respectively. In this paper, we compute these two parameters for around
ten thousands signals, measured over the whole cross-section of a
toroidal magnetised plasma for a broad range of experimental conditions.
For the first time in plasma physics, universality has been sought by
plotting the kurtosis against the skewness, revealing two proofs for
universality. Firstly, all the experimental points are not randomly
distributed but form a picturesque smile. Secondly, all the PDF of the
measured signals are universally described by one and only one
analytical distribution. Fluctuations in the frequency range of the
paradigmatic drift-interchange instability are necessary and sufficient
to assure that PDF can be described by this specific distribution.
A new quantum phase transition (QPT) has been interestingly observed in
ferroelectric oxides. In this case, the coulomb interaction, which
creates the long-range order of dipole moments, competes with the
quantum mechanical fluctuation that destroys the order.
Usually, the phase transition, for example the freezing of water, is
governed by the thermal fluctuation; however, if the temperature
approaches the absolute zero temperature, the quantum mechanical effect
emerges in the macroscopic scale. In this study, we have clearly
observed the crossover of the critical behavior of the 腧soft mode腨 from
the classical to the quantum regime, where the soft mode is the
anharmonic lattice vibration that triggers the phase transition. This is
the first QPT brought about by the phonon (the quanta of the lattice
vibration), in contrast to those induced by electronic interactions. In
particular, the novel ferroelectric-paraelectric phase coexistence state
has been revealed in the vicinity of the so-called quantum critical
point for the first time. These findings have been achieved in the solid
solution of SrTi^16 O_3 and SrTi^18 O_3 by carefully controlling the
amplitude of the quantum fluctuation without modulating the chemical
natures of the material by a proper selection on the ^18 O-concentration.
***
Bose-Einstein condensation in the presence of an attractive interaction and its relation to the cosmological Q-balls
(1) It is well known that stable atomic Bose-Einstein condensates can
be formed when the interaction between the atoms is repulsive. An
attractive interaction, unless it is extremely small, leads to the
collapse of the Bose gas. The Bose-Einstein condensate of spin waves
(magnons) with a repulsive interaction was experimentally stabilized
in 1984 in superfluid 3He-B. In this paper it is shown that a
specific feature of this Bose condensate is that it survives the
attractive interaction between the magnons. The magnon condensate
with attractive interaction is formed in the potential well produced
by the texture of the order parameter of 3He, and is observed in
nuclear magnetic resonance experiments as a long-lived coherent
precession of magnetization, which can persist for an hour without
external pumping of energy.
(2) The structure and stability of the magnon condensate with an
attractive interaction are similar to those of certain objects
discussed in particle physics. These are the so-called Q-balls,
which are stabilized due to the conservation of the global charge Q,
typically the baryon or lepton number. In the magnon condensate the
role of the charge Q is played by the projection of the total spin of
the liquid in the direction of the magnetic field. At the quantum
level, Q-balls are formed due to a suitable attractive interaction
that binds the quanta of the scalar field into a large compact
object. Q-balls may contribute significantly to the dark matter and
baryon contents of the Universe. Observation of Q-ball objects in
3He-B confirms the possibility that stable cosmological Q-balls might
exist and should be searched experimentally.
***
Origin of Antiferroelectricity Explained from First Principles
The phenomenon of antiferroelectriciy wherin a compound develops two (or more) oppositely polarized sublattices was first discovered in 1938 in ammonium dihydrogen phosphate, commonly called ADP. ADP and its ferroelectric analogs have been used extensively in electro-optical applications. However, the atomistic details of why ADP becomes antiferroelectric has not been fully understood. A detailed understanding of such properties is needed in order too be able to hone their desirable characteristics. Now efficient algorithms for /ab-initio/ electronic structure calculations have enabled a group of solid state theorists at the Rosario National University in Argentina, working with chemists at Florida State University, to show that ADP's antiferroelectricity is a result of a delicate balance between the energetics of the two different types of hydrogen bonds in the lattice : O-H···O and N-H···O bonds. In a small range of atomic configurations, the optimal N-H···O bonding is enabled by the antiparallel alignment of molecular dipole moments in the unit cell and the crystal thus exhibits antiferroelectricity. The calculations show also that a small change in the atomic displacements could also render ferroelectricity to ADP, thus providing a theoretical basis of a long-standing observation that ADP appears to possess coexisting ferroeelectric and antiferroelectric regions whose relative population changes with temperature. (J. Lasave et al., Phys. Rev. Lett. (2007, in press)).
***
Potassium Bose-Einstein condensate: last but not least
A recent revolution in modern physics has been the achievement of Bose Einstein condensation, i.e. the formation of macroscopic quantum objects offering unprecedented possibilities for the study of the quantum world. This phenomenon has been demonstrated for a number of atomic species including all the stable elements in the first column of the periodic table, except one isotope of potassium. An experimental team at LENS, University of Florence, has now succeeded in creating a Bose-Einstein condensate of such last isotope, potassium-39, and found that, surprisingly, it is one of the most interesting among those studied so far. The team has overcome what appeared to be a fundamental obstacle to condensation of this species, i.e. the presence of an attractive interaction between pairs of atoms. The researchers have found that with the application of appropriate magnetic fields such interaction can be changed to repulsive and a condensate can be produced. Moreover, they have found that in a potassium-39 condensate the interaction can also be nulled with high accuracy, opening a route to the study of an ideal, i.e. non-interacting, quantum object. This novel Bose-Einstein condensate is expected to allow the study of some aspects of the quantum world that have not yet been explored.
***
Surveying with Electrons: Putting Lloyd's Mirror on the Map
A major puzzle for many researchers working in the field of materials
research is how to obtain 3D movies of surface dynamic events, often
during thin film growth or annealing. In a new approach, Monash
Physicists have shown how in situ electron microscopy can be used to
obtain surface height resolution in real-time. Just as the contour lines
on a topographic map provide mountaineers with the capacity to plan
their assault on a mountain, Lloyd's mirror combined with electron
microscopy can provide a relief map of surfaces in which the spacing
between fringes is directly related to height. In the classic 19th
century Lloyd's mirror experiment, light reflected off a mirror
interferes with light coming directly from the source. PRL XX reports a
novel 21st century version of the famous Lloyd's mirror, in which
ultraviolet light is used to illuminate gallium droplets on a gallium
arsenide substrate. The bright interference fringes result in the
emission of electrons, which are imaged in a surface electron
microscope. The fringes are sensitive to the 3D shape of the gallium
droplets, but are also distorted by the electric field due to the
topographic features. However, these distortions can be corrected using
image processing, to provide a real-time relief map showing surface
dynamics of the evolving metallic droplets. Lloyd's mirror electron
microscopy using a synchrotron light source will allow the technique to
image surface features with nanoscale resolution, opening up new
possibilities to make 3D movies of dynamic events during thin film
growth and processing.
***
Towards ultrafast x-ray physics: a novel method to manipulate x-ray
pulses
Theoretical calculations predict a novel physical effect for x rays that
may facilitate the generation of ultrashort x-ray pulses with tailored
shape. This prediction opens new vistas in science, in connection with
both existing x-ray facilities such as Argonne's Advanced Photon Source
and future x-ray free electron lasers. The effect is called
electromagnetically induced transparency (EIT) for x rays. A gas that
would otherwise strongly absorb, and thus be opaque to, x rays of a
given wavelength can be rendered transparent to the x rays by
simultaneously shining an intense laser into the gas. EIT for visible
light is well known and represents a unique tool for manipulating light.
Inner-shell vacancies produced by x-ray absorption have very short
lifetimes of the order of femtoseconds or shorter. Therefore, the laser
intensity required to induce EIT is so high that the standard
description of EIT becomes invalid. Before this study, it was
questionable whether EIT may exist at all in the x-ray regime. The
theoretical analysis carried out in this paper demonstrates that
existing intense-laser technology may be harnessed to imprint the
temporal shape of optical laser pulses onto x-ray pulses.
***
Unique due to relativity
Heavy element polonium enters harmfully our lives via air, soil and
plants such as tobacco or tea, but very little is known about its
physical and chemical properties. It has not been clear why polonium, as
the only element from the Periodic Table, crystallizes in the simple
cubic structure, one of the simplest structures ever. Our paper shows,
on the basis of the fundamental quantum mechanics, that this
unique structure of polonium is, similarly as the yellow luster of
gold, due to relativistic effects. We also draw the attention to
polonium strong elastic anisotropy, which has no equal in other solids,
and demonstrate that this is an inherent peculiarity of its crystal
structure. Finally, as a challenge for future experimental studies, we
predict a mechanical instability of simple cubic polonium at relatively
low pressures resulting in a mixture of two lower-symmetry structures.
***
The quantum nature of an atom enables its simultaneous propagation in two different regions of space: this property has been used by the research group of J. Vigué (Université P. Sabatier and CNRS, Toulouse) to make an absolute measurement of the refraction index of gases for lithium atomic waves. This experiment is based on an atom interferometer, in which each atom follows two paths. Their maximum distance, equal to 100 micrometers, is sufficient to introduce a material object between them so as to apply a weak gas pressure on one of them.
The only comparable experiments have been done on sodium by the research group of D. Pritchard at MIT but they have not achieved an absolute measurement.
This index describes the attenuation and the delay or advance of the atomic wave induced by collisions with the atoms of the gas. The comparison of the present attenuation measurements with previous values is excellent. The delay or advance of the atomic wave can be accessed only by the present atom interferometry method and the measured values are in agreement with theoretical estimates.
Finally, this type of experiments changes our representation of atoms, from point-like particles to strongly delocalised quantum waves!
***
Plasma Density Fluctuations Are Universal, But This Was Hidden Beyond A
Picturesque Smile
A unique parabolic relation links two statistical quantities related to
plasma density fluctuations. As in many other physical systems, a
significant effort is dedicated to finding universal aspects in a
statistical description of plasma turbulence. The probability of
occurence of any event can be given by its probability density function
(PDF hereafter). For a PDF, the degree of asymmetry and the weight of
its wings are contained in two parameters called skewness and kurtosis,
respectively. In this paper, we compute these two parameters for around
ten thousands signals, measured over the whole cross-section of a
toroidal magnetised plasma for a broad range of experimental conditions.
For the first time in plasma physics, universality has been sought by
plotting the kurtosis against the skewness, revealing two proofs for
universality. Firstly, all the experimental points are not randomly
distributed but form a picturesque smile. Secondly, all the PDF of the
measured signals are universally described by one and only one
analytical distribution. Fluctuations in the frequency range of the
paradigmatic drift-interchange instability are necessary and sufficient
to assure that PDF can be described by this specific distribution.
Monday, May 21, 2007
Phys Rev Hot Paper: 5-21-07
WHY IS WATER SO GOOD AT NEUTRALIZING ELECTRIC FIELDS
Compared to other liquids made of polar molecules water has a far
greater power
to dissolve ionic substances. This is a consequence of the network of
hydrogen bond links
between pairs of neighboring molecules, which greatly enhances
the ability of water to neutralize electric fields. This is because
(a) the hydrogen bonds
effectively increase the average dipole moment of each molecule in
the liquid with respect
to the vapor phase, and (b) they enable the molecules to react in
unison to an applied field.
The actual magnitude of these two effects has been an open and
frequently debated question
for more than 60 years. This paper provides a precise and
indisputable answer from a computer
simulation based on fundamental quantum theory, i.e. a simulation in
which the molecular
dynamics derives from the dynamics of the nuclei and the electrons
with no need for empirical input.
The results are in good agreement with experiment not only for liquid
water but also for solid ice.
In the simulation the effects that are by far the most important
occur at short range,
between a molecule and its first shell of neighbors. Thus, even the
minuscule quantities of water
that can be found in protein pockets should be capable to
substantially shield electrostatic
interactions which play an essential role in protein structure and
function.
***
New Superstring Predictions for Gravitational Scattering
It has been known for over 20 years that superstring theory predicts
deviations from Einstein's general relativity for the scattering of
gravitational waves at high-energies.
In this letter, these high-energy deviations are determined with much more
accuracy using a new more powerful formalism for computing superstring
amplitudes.
***
Drunk cars
If you drive a car in a huge empty park at constant speed while randomly
turning the wheel, you perform a well-known type of random motion called
persistent Brownian motion. If now instead of keeping the speed constant
you randomly press the gas and break pedals while still randomly turning
the direction wheel, the resulting movement is no longer well described
as plain persistent Brownian motion, and the statistics of the movement
change. In this work the new features introduced by the fluctuations in
the speed are exposed. In particular, it is shown that there is a complex
transient, together with changes in the asymptotic properties of the
movement due to the speed fluctuations. Our results could be relevant to
understand the motion of small self-propelled particles subject to large
fluctuations in their motion. It is suggested that some crawling cells,
which exhibited fluctuations in the speed and the direction of motion,
could be described as such "drunk cars".
Compared to other liquids made of polar molecules water has a far
greater power
to dissolve ionic substances. This is a consequence of the network of
hydrogen bond links
between pairs of neighboring molecules, which greatly enhances
the ability of water to neutralize electric fields. This is because
(a) the hydrogen bonds
effectively increase the average dipole moment of each molecule in
the liquid with respect
to the vapor phase, and (b) they enable the molecules to react in
unison to an applied field.
The actual magnitude of these two effects has been an open and
frequently debated question
for more than 60 years. This paper provides a precise and
indisputable answer from a computer
simulation based on fundamental quantum theory, i.e. a simulation in
which the molecular
dynamics derives from the dynamics of the nuclei and the electrons
with no need for empirical input.
The results are in good agreement with experiment not only for liquid
water but also for solid ice.
In the simulation the effects that are by far the most important
occur at short range,
between a molecule and its first shell of neighbors. Thus, even the
minuscule quantities of water
that can be found in protein pockets should be capable to
substantially shield electrostatic
interactions which play an essential role in protein structure and
function.
***
New Superstring Predictions for Gravitational Scattering
It has been known for over 20 years that superstring theory predicts
deviations from Einstein's general relativity for the scattering of
gravitational waves at high-energies.
In this letter, these high-energy deviations are determined with much more
accuracy using a new more powerful formalism for computing superstring
amplitudes.
***
Drunk cars
If you drive a car in a huge empty park at constant speed while randomly
turning the wheel, you perform a well-known type of random motion called
persistent Brownian motion. If now instead of keeping the speed constant
you randomly press the gas and break pedals while still randomly turning
the direction wheel, the resulting movement is no longer well described
as plain persistent Brownian motion, and the statistics of the movement
change. In this work the new features introduced by the fluctuations in
the speed are exposed. In particular, it is shown that there is a complex
transient, together with changes in the asymptotic properties of the
movement due to the speed fluctuations. Our results could be relevant to
understand the motion of small self-propelled particles subject to large
fluctuations in their motion. It is suggested that some crawling cells,
which exhibited fluctuations in the speed and the direction of motion,
could be described as such "drunk cars".
Thursday, May 17, 2007
Phys Rev Hot Papers: 5-17-07
Manipulating the mechanisms of crystallization in simple systems
Controlling the structure (or polymorph) in which a molecule
crystallizes is a long-standing issue. Since polymorphs have
different physical properties, it is crucial for many applications
(e.g. in the making of pharmaceuticals) to understand and control
this phenomenon. In this paper, we use molecular simulations to shed
light on the molecular mechanisms underlying the choice of a specific
polymorph during the crystallization of spherical particles. We show
how, by modifying the temperature and pressure of crystallization, we
succeed in manipulating the mechanisms of crystal growth. We not only
determine the conditions enabling us to control the overall structure
of the crystallite but we also control its purity. Furthermore, we
provide new insight on how one structure can form on the surface of
another and on how we can prevent this phenomenon known
experimentally as cross-nucleation. In (a), we show a cross-
nucleation event as large domains of a structure (yellow) grow on top
of another (grey) while cross-nucleation is greatly reduced in (b).
***
The nuclear trigger for X-ray bursts
X-ray bursts belong to the most fascinating of astrophysical phenomena.
They are explained as thermonuclear explosions in the outer atmosphere
of accreting neutron stars. The thermonuclear explosion is triggered by
a single reaction, 15O(alpha,gamma)19Ne which has been experimentally
determined for the first time after two decades of failed attempts. The
paper not only reports on the successful experiment but demonstrates the
impact of the results on X-ray burst ignition and X-ray burst
periodicity in the framework of a thermonuclear explosion model. The new
results define stringent boundaries for the actual accretion rate in
observed X-ray bursters.
***
Lightspeed? Not so fast
What would happen if light were slowed to the speed of sound?
Tasgal, Band, and Malomed looked at pulses of light (solitons)
in an optical fiber with a Bragg grating, i.e., a periodic modulation
of the index of refraction of the fiber. Bragg reflection off the
periodic modulation can act as a uniformly distributed mirror that
couples forward- and backward-moving photons. This kind of fiber
allows light pulses as slow as zero velocity, though the current
experimental velocities are a sixth the speed of light (but dropping).
Interactions with sound had not been included in studies of this system.
Near the speed of sound, the electrostrictive coupling of light and sound
waves can result in braking, bringing the light pulse abruptly from
supersonic to subsonic. For reasons not yet completely understood,
this sonic braking can decelerate the light pulses sharply down to almost
zero velocity. (See the figure.) This form of slowed or stopped light
can exist at room temperature, in relatively unexotic materials
(as opposed to in Bose-Einstein condensates, which exist near absolute
zero temperature). The coupling of the light to sound provides extra
complications for theorists to grapple with, but it may simplify the work
of experimentalists in bringing light to a stop.
Figure caption: A slightly supersonic soliton spins off sound waves
and thereby slows to a velocity well below the speed of sound. The
top panel shows the light intensity versus time and position in the
fiber, and the bottom panel shows the sound wave amplitude, with the
newly generated sound pulses propagating away from the surviving
subsonic soliton.
***
Low energy interaction of the ozone cation with electrons
Ozone is one of the most important triatomic molecules, since it absorbs harmful
solar ultraviolet radiation and therefore plays a large role in protecting life
on Earth. We have studied the low energy interaction between an electron and
the ozone cation. Such an interaction leads to the almost exclusive rupture of
all the covalent bonds in the ozone molecule. Due to the relatively high
exothermicity of the reaction, oxygen fragments are predominantly formed in
{^3}P and {^1}D states, though formation in the {^1}S state is highly
unfavourable. Oxygen in the {^1}D state is responsible for the red aurora and
red airglow.
***
Equilibrium Flux Patterns in Type-I Superconductors
Patterns play an important role in strongly correlated systems with long
range interactions where different phases can co-exist. In type-I
superconductors, topologically rich physics is observed in so-called
intermediate state that appears due to finite size of the system. For about
80 years a model first suggested by Lev Landau was used in textbooks on
superconductivity. In this model stripy (lamellar or labyrinth) pattern was
suggested. Any hysteresis in macroscopic response (such as magnetization)
was considered due to various imperfections or particular edge structure.
In this work we show that 1) topology of the intermediate state depends on
macroscopic sample shape. We show experiments in spheres, hemispheres and
cones where so-called geometric barrier is absent; 2) equilibrium pattern is
represented by flux tubes, not stripes; 3) the difference in pattern
geometry gives raise to topological hysteresis even in perfect samples.
We conclude that it is impossible to deduce equilibrium topology from energy
minimization arguments. We hope it will prompt development of new methods
in the physics of complex systems.
Real time visualization of magnetic fields is available at:
http://www.cmpgroup.ameslab.gov/supermaglab/video/Pb.html
***
A handle on chaos in electron billiards
Regular or chaotic motion of balls on billiard tables is “win” or “lose”. This paper investigates how electrons, restricted in their motion to nanometer-sized billiards, can win or lose, in technical words can increase or decrease the billiard’s electrical resistance. This “game” is controlled by the application of a variable magnetic field, switching the electron motion between predominantly regular and predominantly chaotic. The detailed interpretation of the experimentally observed quantity – the electrical resistance – involves classical and quantum-mechanical theoretical concepts and relates prominent structure in the resistance to regular or chaotic behaviour. Also, an intriguing question since the advent of quantum theory - the correspondence of these two concepts – is addressed and shown to be applicable to the electron billiards. Regarding technological aspects, the results are of great relevance for the development of nano-electronics within the next decade, when devices will become electron-billiard-like due to further miniaturization.
***
Giving Faraday's law a new spin
Faraday's law of induction is a basic principle of physics dating from 1831 and
which explains the electromotive force that drives electrical currents in
generators and transformers. In Physical Review Letters [date], Barnes and
Maekawa show that, for magnetic materials, there is a correction to this law.
Electrical currents usually correspond to a flow of electrons, elementary
particles that carry a tiny electrical charge. In 1930, Dirac showed that, in
order to be consistent with Einstein's special relativity and quantum mechanics,
an electron must also have "spin", in effect, be a tiny magnet. The above
modifications to Faraday¿s law occur when this spin is accounted for.
"Spintronics" is an emerging electronics technology in which the electron spin
is used in an essential manner. While the Barnes and Maekawa corrections are
small for transformers or generators, they embody the requirements of energy
conservation for the spintronic devices being developed by the likes of IBM,
Freescale and Hitachi. Barnes and Maekawa show that the corrected Faraday's law
can be elegantly restated in terms of the time rate of change of the so called
¿Berry phase¿.
***
Quantum Repeater
For long-distance quantum communication one must realize quantum
network via quantum repeater protocol, a combination of entanglement
swapping, entanglement purification and quantum memory. In a seminar paper,
Duan et al. proposed a promising quantum repeater protocol (DLCZ) with
atomic ensembles and linear optics. However, in DLCZ protocol entanglement
generation and entanglement swapping rely on single-photon Mach-Zehnder-type
interference, which is sensitive to phase instabilities. This drawback is
severe enough to make long-distance quantum communication impossible. In
this paper, we present a robust quantum repeater architecture building on
DLCZ protocol. The architecture is based on two-photon Hong-Ou-Mandel-type
interference which relaxes the long distance stability requirements by about
7 orders of magnitude, from sub wavelength for the single photon
interference required by DLCZ to the coherence length of the photons. The
prize which has to be paid for this benefit are spurious contributions of
states with higher number of excitations. We subtly design the protocol so
that the spurious contributions can be automatically eliminated during the
entanglement connection process. Our protocol provides an exciting
possibility for robust and realistic long-distance quantum communication.
***
Gating electrons at a free silicon surface
Gating electrons at a free silicon surface terminated with a monolayer
of hydrogen enclosed in a vacuum cavity provides a revolutionary
technique in which to create a high quality two-dimensional electron
system (2DES) in silicon. It has been known for some time that a
silicon surface terminated with hydrogen is an ideal electronic surface
with its atomic flatness and low number of defects. In this paper we
present the first systematic low temperature electron transport
measurements on a hydrogen terminated silicon surface whereby the
quality of our 2DES can be attested by the first observation of the
integer quantum Hall effect on a (111) orientated silicon surface.
Unlike 2DES created by conventional methods at an interface between
silicon and an amorphous glass (SiO2), the hydrogen terminated
silicon-vacuum interface results in an order of magnitude less disorder.
As a result we observe the ground state in silicon (111) surfaces to be
sensitive to the wafer orientation and in-plane magnetic fields. We
believe these results are seminal for two reasons: new ground states may
occur in the quantized Hall regime in a multi-component system such as
Si (111). Secondly because the 2DES is directly below the hydrogen
terminated silicon surface, a new breed of atomic-scale quantum devices
may arise whereby electrons can couple to molecules or atoms
specifically grafted on to the surface.
***
Polytetrahedral nature of the dense disordered packings of hard spheres
by A.V. Anikeenko, N.N. Medvedev
The paper is a new examination of the structure of non-crystalline
and partly-crystalline dense packings of hard spheres. It throws some light
on the unanswered question concerning the disordered dense packing is why
its maximum density is ~0.64. This problem is a "non-crystalline
equivalent" of the long-standing Kepler conjecture about maximal density
(~0.74) for crystalline packing of identical spheres which was solved only
recently. The performed analysis of computer models justifies the
polytetrahedral nature of the dense disordered packings of spheres, i.e.
the packings contain an appreciable fraction of tetrahedral configurations
of spheres that prefer to coalesce via their faces to form a variety of
locally dense aggregates which are inconsistent with the formation of a
lattice. Such tetrahedra are not perfect, the gaps between the neighboring
spheres may be as large as 25% of the diameter. These tetrahedra coincide
with the class of quasi-regular tetrahedra introduced by Hales in his proof
of the Kepler conjecture. The paper is of intense interest to both
mathematicians and condensed matter physicists in general. The model of
hard spheres is very successfully in trying to understand the structures of
liquids, glasses, colloids and granular matter.
***
Novel schemes of measurement-based quantum computation
Imagine one has a quantum system with many constituents prepared in a
certain entangled state in a laboratory. This could be a state of a
many-body system like of cold atoms in optical lattices or of atoms in an
array of cavities, entangled via a light bus. Could this state be used
for quantum computing, based merely on local measurements, but abandoning
the need for any unitary control to realize quantum gates?
In our work [1], we show that with a great deal of flexibility, many-body
states allow for quantum computation in such a fashion that fully
abandons the need for unitary control during the computation. The
celebrated result by Briegel and Raussendorf showed that local
measurements on a single very specific state, the so-called cluster state
[2], gives rise to universal quantum computation. This cluster state has
a number of interesting, but also rare properties. In our work, we hence
address the question: what if the state is simply not a cluster state,
but just some other state?
Our work opens up an avenue to follow such a line of thought: We find
many new models for quantum computing, and also many resource states that
have radically different correlation and entanglement properties. For
example, do we really need to have no long-range correlations in such a
resource state? This would be bad, ruling out many ground states from
scratch. But, fortunately, one can overcome this limitation. This
suggests that to some extent, the theorist does not necessarily have to
approach the experimentalist, asking for the preparation of a particular
state that may possibly be fragile with respect to decoherence effects.
But that the theorist may construct a computational model based on the
very state that can relatively feasibly be prepared.
***
How to make negative charge positive
In this paper we show that some negatively charged particles
may produce a positive charge density. An everyday experience tells us
that it is impossible. Indeed, ordinary particles like negatively charged
electrons produce negative charge density everywhere, any time.
However, the situation is different for W-bosons, fundamental particles
which mediate the weak interaction (that is responsible for
beta-decay of unstable nuclear isotopes).
The reason why W-bosons demonstrate this misterious behavior stems from
the fact that they are described by the relativistic quantum mechanics.
The spin S of W-bosons is large, S=1
(electrons have spin 1/2). In naive terms the W-boson rotates vigorously
around its axes. Precisely this strong rotation, combined with effects
of special relativily, leads to the puzzling phenomenon of the
"wrong sign" of the charge distribution. This effect does not contradict
the charge conservation, i.e. no charge disappered or created.
For example, W-bosons at rest have a given negative charge.
For fast, relativistic W-bosons there are some areas of space with
positive charge and some areas with negative charge, but the total charge
does not change.
***
Are protons getting lighter with time?
It is common knowledge that protons are 1837 times heavier than electrons.
Recent astronomical observations [Reinhold and others, PRL, 96, 151101 (2006)]
suggest that this ratio was even bigger in the past. If proven, that
would require reconsideration of the most fundamental physical theories,
including Einstein's general relativity.
In this paper we have shown that the proton-to-electron mass ratio has not
changed by more than 2.5 parts per million during the last 6 billion years.
We used microwave spectra from the galaxy B0218+357, which is
approximately 6 billion light years away from the Milky Way. These spectra
include famous ammonia transitions, which were used by Townes to build the
first maser [Nobel Prize 1964]. The frequency of these transitions appears to
be extremely sensitive to the mass ratio in question. Comparison of
ammonia lines with microwave lines of other molecules revealed no
relative frequency shift. This allowed us to place the most
stringent limit on the space-time variation of the proton-to-electron
mass ratio.
Controlling the structure (or polymorph) in which a molecule
crystallizes is a long-standing issue. Since polymorphs have
different physical properties, it is crucial for many applications
(e.g. in the making of pharmaceuticals) to understand and control
this phenomenon. In this paper, we use molecular simulations to shed
light on the molecular mechanisms underlying the choice of a specific
polymorph during the crystallization of spherical particles. We show
how, by modifying the temperature and pressure of crystallization, we
succeed in manipulating the mechanisms of crystal growth. We not only
determine the conditions enabling us to control the overall structure
of the crystallite but we also control its purity. Furthermore, we
provide new insight on how one structure can form on the surface of
another and on how we can prevent this phenomenon known
experimentally as cross-nucleation. In (a), we show a cross-
nucleation event as large domains of a structure (yellow) grow on top
of another (grey) while cross-nucleation is greatly reduced in (b).
***
The nuclear trigger for X-ray bursts
X-ray bursts belong to the most fascinating of astrophysical phenomena.
They are explained as thermonuclear explosions in the outer atmosphere
of accreting neutron stars. The thermonuclear explosion is triggered by
a single reaction, 15O(alpha,gamma)19Ne which has been experimentally
determined for the first time after two decades of failed attempts. The
paper not only reports on the successful experiment but demonstrates the
impact of the results on X-ray burst ignition and X-ray burst
periodicity in the framework of a thermonuclear explosion model. The new
results define stringent boundaries for the actual accretion rate in
observed X-ray bursters.
***
Lightspeed? Not so fast
What would happen if light were slowed to the speed of sound?
Tasgal, Band, and Malomed looked at pulses of light (solitons)
in an optical fiber with a Bragg grating, i.e., a periodic modulation
of the index of refraction of the fiber. Bragg reflection off the
periodic modulation can act as a uniformly distributed mirror that
couples forward- and backward-moving photons. This kind of fiber
allows light pulses as slow as zero velocity, though the current
experimental velocities are a sixth the speed of light (but dropping).
Interactions with sound had not been included in studies of this system.
Near the speed of sound, the electrostrictive coupling of light and sound
waves can result in braking, bringing the light pulse abruptly from
supersonic to subsonic. For reasons not yet completely understood,
this sonic braking can decelerate the light pulses sharply down to almost
zero velocity. (See the figure.) This form of slowed or stopped light
can exist at room temperature, in relatively unexotic materials
(as opposed to in Bose-Einstein condensates, which exist near absolute
zero temperature). The coupling of the light to sound provides extra
complications for theorists to grapple with, but it may simplify the work
of experimentalists in bringing light to a stop.
Figure caption: A slightly supersonic soliton spins off sound waves
and thereby slows to a velocity well below the speed of sound. The
top panel shows the light intensity versus time and position in the
fiber, and the bottom panel shows the sound wave amplitude, with the
newly generated sound pulses propagating away from the surviving
subsonic soliton.
***
Low energy interaction of the ozone cation with electrons
Ozone is one of the most important triatomic molecules, since it absorbs harmful
solar ultraviolet radiation and therefore plays a large role in protecting life
on Earth. We have studied the low energy interaction between an electron and
the ozone cation. Such an interaction leads to the almost exclusive rupture of
all the covalent bonds in the ozone molecule. Due to the relatively high
exothermicity of the reaction, oxygen fragments are predominantly formed in
{^3}P and {^1}D states, though formation in the {^1}S state is highly
unfavourable. Oxygen in the {^1}D state is responsible for the red aurora and
red airglow.
***
Equilibrium Flux Patterns in Type-I Superconductors
Patterns play an important role in strongly correlated systems with long
range interactions where different phases can co-exist. In type-I
superconductors, topologically rich physics is observed in so-called
intermediate state that appears due to finite size of the system. For about
80 years a model first suggested by Lev Landau was used in textbooks on
superconductivity. In this model stripy (lamellar or labyrinth) pattern was
suggested. Any hysteresis in macroscopic response (such as magnetization)
was considered due to various imperfections or particular edge structure.
In this work we show that 1) topology of the intermediate state depends on
macroscopic sample shape. We show experiments in spheres, hemispheres and
cones where so-called geometric barrier is absent; 2) equilibrium pattern is
represented by flux tubes, not stripes; 3) the difference in pattern
geometry gives raise to topological hysteresis even in perfect samples.
We conclude that it is impossible to deduce equilibrium topology from energy
minimization arguments. We hope it will prompt development of new methods
in the physics of complex systems.
Real time visualization of magnetic fields is available at:
http://www.cmpgroup.ameslab.gov/supermaglab/video/Pb.html
***
A handle on chaos in electron billiards
Regular or chaotic motion of balls on billiard tables is “win” or “lose”. This paper investigates how electrons, restricted in their motion to nanometer-sized billiards, can win or lose, in technical words can increase or decrease the billiard’s electrical resistance. This “game” is controlled by the application of a variable magnetic field, switching the electron motion between predominantly regular and predominantly chaotic. The detailed interpretation of the experimentally observed quantity – the electrical resistance – involves classical and quantum-mechanical theoretical concepts and relates prominent structure in the resistance to regular or chaotic behaviour. Also, an intriguing question since the advent of quantum theory - the correspondence of these two concepts – is addressed and shown to be applicable to the electron billiards. Regarding technological aspects, the results are of great relevance for the development of nano-electronics within the next decade, when devices will become electron-billiard-like due to further miniaturization.
***
Giving Faraday's law a new spin
Faraday's law of induction is a basic principle of physics dating from 1831 and
which explains the electromotive force that drives electrical currents in
generators and transformers. In Physical Review Letters [date], Barnes and
Maekawa show that, for magnetic materials, there is a correction to this law.
Electrical currents usually correspond to a flow of electrons, elementary
particles that carry a tiny electrical charge. In 1930, Dirac showed that, in
order to be consistent with Einstein's special relativity and quantum mechanics,
an electron must also have "spin", in effect, be a tiny magnet. The above
modifications to Faraday¿s law occur when this spin is accounted for.
"Spintronics" is an emerging electronics technology in which the electron spin
is used in an essential manner. While the Barnes and Maekawa corrections are
small for transformers or generators, they embody the requirements of energy
conservation for the spintronic devices being developed by the likes of IBM,
Freescale and Hitachi. Barnes and Maekawa show that the corrected Faraday's law
can be elegantly restated in terms of the time rate of change of the so called
¿Berry phase¿.
***
Quantum Repeater
For long-distance quantum communication one must realize quantum
network via quantum repeater protocol, a combination of entanglement
swapping, entanglement purification and quantum memory. In a seminar paper,
Duan et al. proposed a promising quantum repeater protocol (DLCZ) with
atomic ensembles and linear optics. However, in DLCZ protocol entanglement
generation and entanglement swapping rely on single-photon Mach-Zehnder-type
interference, which is sensitive to phase instabilities. This drawback is
severe enough to make long-distance quantum communication impossible. In
this paper, we present a robust quantum repeater architecture building on
DLCZ protocol. The architecture is based on two-photon Hong-Ou-Mandel-type
interference which relaxes the long distance stability requirements by about
7 orders of magnitude, from sub wavelength for the single photon
interference required by DLCZ to the coherence length of the photons. The
prize which has to be paid for this benefit are spurious contributions of
states with higher number of excitations. We subtly design the protocol so
that the spurious contributions can be automatically eliminated during the
entanglement connection process. Our protocol provides an exciting
possibility for robust and realistic long-distance quantum communication.
***
Gating electrons at a free silicon surface
Gating electrons at a free silicon surface terminated with a monolayer
of hydrogen enclosed in a vacuum cavity provides a revolutionary
technique in which to create a high quality two-dimensional electron
system (2DES) in silicon. It has been known for some time that a
silicon surface terminated with hydrogen is an ideal electronic surface
with its atomic flatness and low number of defects. In this paper we
present the first systematic low temperature electron transport
measurements on a hydrogen terminated silicon surface whereby the
quality of our 2DES can be attested by the first observation of the
integer quantum Hall effect on a (111) orientated silicon surface.
Unlike 2DES created by conventional methods at an interface between
silicon and an amorphous glass (SiO2), the hydrogen terminated
silicon-vacuum interface results in an order of magnitude less disorder.
As a result we observe the ground state in silicon (111) surfaces to be
sensitive to the wafer orientation and in-plane magnetic fields. We
believe these results are seminal for two reasons: new ground states may
occur in the quantized Hall regime in a multi-component system such as
Si (111). Secondly because the 2DES is directly below the hydrogen
terminated silicon surface, a new breed of atomic-scale quantum devices
may arise whereby electrons can couple to molecules or atoms
specifically grafted on to the surface.
***
Polytetrahedral nature of the dense disordered packings of hard spheres
by A.V. Anikeenko, N.N. Medvedev
The paper is a new examination of the structure of non-crystalline
and partly-crystalline dense packings of hard spheres. It throws some light
on the unanswered question concerning the disordered dense packing is why
its maximum density is ~0.64. This problem is a "non-crystalline
equivalent" of the long-standing Kepler conjecture about maximal density
(~0.74) for crystalline packing of identical spheres which was solved only
recently. The performed analysis of computer models justifies the
polytetrahedral nature of the dense disordered packings of spheres, i.e.
the packings contain an appreciable fraction of tetrahedral configurations
of spheres that prefer to coalesce via their faces to form a variety of
locally dense aggregates which are inconsistent with the formation of a
lattice. Such tetrahedra are not perfect, the gaps between the neighboring
spheres may be as large as 25% of the diameter. These tetrahedra coincide
with the class of quasi-regular tetrahedra introduced by Hales in his proof
of the Kepler conjecture. The paper is of intense interest to both
mathematicians and condensed matter physicists in general. The model of
hard spheres is very successfully in trying to understand the structures of
liquids, glasses, colloids and granular matter.
***
Novel schemes of measurement-based quantum computation
Imagine one has a quantum system with many constituents prepared in a
certain entangled state in a laboratory. This could be a state of a
many-body system like of cold atoms in optical lattices or of atoms in an
array of cavities, entangled via a light bus. Could this state be used
for quantum computing, based merely on local measurements, but abandoning
the need for any unitary control to realize quantum gates?
In our work [1], we show that with a great deal of flexibility, many-body
states allow for quantum computation in such a fashion that fully
abandons the need for unitary control during the computation. The
celebrated result by Briegel and Raussendorf showed that local
measurements on a single very specific state, the so-called cluster state
[2], gives rise to universal quantum computation. This cluster state has
a number of interesting, but also rare properties. In our work, we hence
address the question: what if the state is simply not a cluster state,
but just some other state?
Our work opens up an avenue to follow such a line of thought: We find
many new models for quantum computing, and also many resource states that
have radically different correlation and entanglement properties. For
example, do we really need to have no long-range correlations in such a
resource state? This would be bad, ruling out many ground states from
scratch. But, fortunately, one can overcome this limitation. This
suggests that to some extent, the theorist does not necessarily have to
approach the experimentalist, asking for the preparation of a particular
state that may possibly be fragile with respect to decoherence effects.
But that the theorist may construct a computational model based on the
very state that can relatively feasibly be prepared.
***
How to make negative charge positive
In this paper we show that some negatively charged particles
may produce a positive charge density. An everyday experience tells us
that it is impossible. Indeed, ordinary particles like negatively charged
electrons produce negative charge density everywhere, any time.
However, the situation is different for W-bosons, fundamental particles
which mediate the weak interaction (that is responsible for
beta-decay of unstable nuclear isotopes).
The reason why W-bosons demonstrate this misterious behavior stems from
the fact that they are described by the relativistic quantum mechanics.
The spin S of W-bosons is large, S=1
(electrons have spin 1/2). In naive terms the W-boson rotates vigorously
around its axes. Precisely this strong rotation, combined with effects
of special relativily, leads to the puzzling phenomenon of the
"wrong sign" of the charge distribution. This effect does not contradict
the charge conservation, i.e. no charge disappered or created.
For example, W-bosons at rest have a given negative charge.
For fast, relativistic W-bosons there are some areas of space with
positive charge and some areas with negative charge, but the total charge
does not change.
***
Are protons getting lighter with time?
It is common knowledge that protons are 1837 times heavier than electrons.
Recent astronomical observations [Reinhold and others, PRL, 96, 151101 (2006)]
suggest that this ratio was even bigger in the past. If proven, that
would require reconsideration of the most fundamental physical theories,
including Einstein's general relativity.
In this paper we have shown that the proton-to-electron mass ratio has not
changed by more than 2.5 parts per million during the last 6 billion years.
We used microwave spectra from the galaxy B0218+357, which is
approximately 6 billion light years away from the Milky Way. These spectra
include famous ammonia transitions, which were used by Townes to build the
first maser [Nobel Prize 1964]. The frequency of these transitions appears to
be extremely sensitive to the mass ratio in question. Comparison of
ammonia lines with microwave lines of other molecules revealed no
relative frequency shift. This allowed us to place the most
stringent limit on the space-time variation of the proton-to-electron
mass ratio.
Thursday, May 10, 2007
Phys Rev Hot Papers: 5-10-07
Photons help to confine electrons in graphene.
One of the most unusual quantum-mechanical phenomena that can be
realized in a graphene is the reflectionless transmission of electrons
through a potential barrier of arbitrary strength. This effect was
predicted in Ref. [1] and recently coined as the "Klein paradox" . The perfect
transmission of quasi-particles might lead to difficulties in confining electrons in
future graphene based electronic devices. In our paper [2] we have demonstrated that the resonant interaction of propagating quasi-particles with the radiation of a moderate intensity applied in the longitudinal (with respect to the interface) direction, leads to a pronounced suppression of the quasi-particle transmission through the n-p junction formed in a graphene. This effect occurs due to formation of a non-equilibrium dynamic gap between electron and hole bands in the quasi-particle spectrum. The value of the gap can be controlled by variation of the intensity of an external radiation. Propagation of quasi-particles is possible due to the non-equilibrium interband tunnelling. This specific type of the tunnelling results also in an N-type of current-voltage characteristics (CVC). The suppression of the quasi-particle transmission allows one to control confinement of electrons in diverse structures fabricated in graphene like, e.g., n-p-n transistors, single electron transistors, quantum dots, by variation of the intensity and
frequency of the external radiation.
***
What do advanced lightweight materials and the cytoskeleton have in
common?
Imagine a game of Jacks Straws in which the first player, having dropped the
straws, decides to glue them to each other where they touch, whilst
the other players have lunch. Later, the first of the young players
to try to remove a straw quickly realizes it is impossible without
moving any of the others. Being of a curious nature, the young
scientist decides to investigate the behavior of the clump of straws.
Are they all connected to each other, or can some straws be
separated? How much can the clumps be bent or stretched? In
this paper, we use simulations to answer these questions, and find a
transition from networks which are bendy to networks which are quite
stiff based entirely on the internal structure. Deciphering the role
of geometry in the mechanics of such networks can help us learn how
nature designs lightweight load bearing structures, such as spongy
bone or the cytoskeleton in cells. This, in turn, will ultimately
help us to design lightweight, yet sturdy, materials for a wide range
of applications.
***
Did the Universe undergo the stage of catalyzed nuclear fusion?
Three minutes after the Big Bang most of the light elements such as
hydrogen, helium, lithium (and their isotopes) were formed as the Universe
continued to expand and cool. This process, known as Big Bang
Nucleosynthesis, is well understood and has been used as a sensitive probe
of particle physics in the early Universe. In my paper, I show that in
many extensions of the Standard Model of particles and fields, a new
spectacular phenomenon will take place: the catalysis of nuclear reactions
by metastable heavy particles (e.g. charged supersymmetric particles) that
can interact via the electromagnetic or strong force. The Catalyzed Big
Bang Nucleosynthesis is triggered by the binding of nuclei with exotic
particles, and in particular leads to the explosive production of Li6
isotope at about 5 hours after the Big Bang. The reaction rate for the
synthesis of Li6 receives the catalytic enhancement of up to eight orders
of magnitude. Confronting the predictions of lithium abundance with
observations leads to the conclusion that even one such exotic particle
per 10^16 photons may significantly alter the nucleo-chemical history of
the Universe.
***
Reduction of quantum information loss due to strong driving
Are quantum computers a reachable vision? An essential requirement is that
one can identify suitable computational units, so called "Q-bits", which
keep their quantum state for a long period of time and that can be
externally manipulated as desired for computation. Unfortunately quantum
states change chaotically with time due to noise in the external
environment, which acts analogously to a friction force in the classical
world. This friction quickly destroys the information stored in a Q-bit. In
this paper it is demonstrated that for an important class of quantum
dissipative systems this loss can be significantly reduced. A prototype for
such a system is a semiconductor nano-sized dot where the friction results
from the interaction of the Q-bit with the surrounding crystal lattice. The
trick is to control the strength of the driving force. In contrast to common
expectations it is shown that in the class of quantum systems considered
here the friction is strongly reduced when the field is larger than a
threshold set by the properties of the environment. This result reveals
fascinating dynamical properties of an important prototype quantum system
and opens new perspectives to combat information loss in these systems.
***
Dynamics in Terms of Geometrically Defined Motions
Many mechanical systems occurring in nature follow equations of motion
that are nonlinear, often resulting in very complex patterns of motion
that are called chaotic. For this type of motion, nearby orbits,
corresponding to the setting of slightly different initial conditions for
the evolution, separate exponentially in time. The complex orbits of such
systems often appear to be a flow on geometrically defined surfaces.
Mathematical models of such systems are often given in the literature for
which dynamical flow follows geometrically determined orbits (geodesics)
in a very similar way to the motions of particles in a gravitational
field. The curved surfaces correspond to a geometry determined by a metric
function and a connection form, specified a priori in the model, fixing
the angles between lines and distances between points. Actual dynamical
systems are, however, not usually described in such terms. It is shown in
this paper that the standard form of the equations governing mechanical
motion can, quite generally, be cast into this type of model by
introducing a metric which induces appropriate scaling (conformal) on the
space of free particle motions. The resulting geodesics can be put into
correspondence with the actual motions of the particles by transforming to
a new connection form, a procedure which preserves the geometrical
interpretation of the theory. The computation of deviation of nearby
orbits, based entirely on the geometry of the space, is shown to provide
new useful criteria for instability which are very sensitive and widely
applicable.
***
Conduction Electrons Sense Devil's Staircase
A coupling between spins and electrons allows us to control resistivity by
magnetic fields. Indeed, applied magnetic fields can change perovskite
manganites from an antiferromagnetic insulator to a ferromagnetic metal. On
the other hand, frustrated Ising spin systems have been studied for decades
because of their interesting magnetism typified by the devil's staircase;
Magnetization, M, changes stepwise with having a value of MS/n as a function
of a magnetic field (MS and n denote a saturated magnetization and an
integer, respectively). Then, what happens if frustrated Ising spins coexist
and couple with conduction electrons? In this paper, we show one of the
clear-cut answers for this query, that is, giant two-staged
magnetoresistance with uniaxial anisotropy in SrCo6O11. In this material,
spin-polarized conduction electrons do sense the devil腦s staircase thanks
to a unique layered structure consisting of alternatively stacking
Ising-spin layers and conductive layers, which was confirmed by NMR studies
and band structure calculations. Our work demonstrates a prospect for novel
spintronic functions arising from a coupling between frustrated Ising spins
and spin-polarized conduction electrons.
One of the most unusual quantum-mechanical phenomena that can be
realized in a graphene is the reflectionless transmission of electrons
through a potential barrier of arbitrary strength. This effect was
predicted in Ref. [1] and recently coined as the "Klein paradox" . The perfect
transmission of quasi-particles might lead to difficulties in confining electrons in
future graphene based electronic devices. In our paper [2] we have demonstrated that the resonant interaction of propagating quasi-particles with the radiation of a moderate intensity applied in the longitudinal (with respect to the interface) direction, leads to a pronounced suppression of the quasi-particle transmission through the n-p junction formed in a graphene. This effect occurs due to formation of a non-equilibrium dynamic gap between electron and hole bands in the quasi-particle spectrum. The value of the gap can be controlled by variation of the intensity of an external radiation. Propagation of quasi-particles is possible due to the non-equilibrium interband tunnelling. This specific type of the tunnelling results also in an N-type of current-voltage characteristics (CVC). The suppression of the quasi-particle transmission allows one to control confinement of electrons in diverse structures fabricated in graphene like, e.g., n-p-n transistors, single electron transistors, quantum dots, by variation of the intensity and
frequency of the external radiation.
***
What do advanced lightweight materials and the cytoskeleton have in
common?
Imagine a game of Jacks Straws in which the first player, having dropped the
straws, decides to glue them to each other where they touch, whilst
the other players have lunch. Later, the first of the young players
to try to remove a straw quickly realizes it is impossible without
moving any of the others. Being of a curious nature, the young
scientist decides to investigate the behavior of the clump of straws.
Are they all connected to each other, or can some straws be
separated? How much can the clumps be bent or stretched? In
this paper, we use simulations to answer these questions, and find a
transition from networks which are bendy to networks which are quite
stiff based entirely on the internal structure. Deciphering the role
of geometry in the mechanics of such networks can help us learn how
nature designs lightweight load bearing structures, such as spongy
bone or the cytoskeleton in cells. This, in turn, will ultimately
help us to design lightweight, yet sturdy, materials for a wide range
of applications.
***
Did the Universe undergo the stage of catalyzed nuclear fusion?
Three minutes after the Big Bang most of the light elements such as
hydrogen, helium, lithium (and their isotopes) were formed as the Universe
continued to expand and cool. This process, known as Big Bang
Nucleosynthesis, is well understood and has been used as a sensitive probe
of particle physics in the early Universe. In my paper, I show that in
many extensions of the Standard Model of particles and fields, a new
spectacular phenomenon will take place: the catalysis of nuclear reactions
by metastable heavy particles (e.g. charged supersymmetric particles) that
can interact via the electromagnetic or strong force. The Catalyzed Big
Bang Nucleosynthesis is triggered by the binding of nuclei with exotic
particles, and in particular leads to the explosive production of Li6
isotope at about 5 hours after the Big Bang. The reaction rate for the
synthesis of Li6 receives the catalytic enhancement of up to eight orders
of magnitude. Confronting the predictions of lithium abundance with
observations leads to the conclusion that even one such exotic particle
per 10^16 photons may significantly alter the nucleo-chemical history of
the Universe.
***
Reduction of quantum information loss due to strong driving
Are quantum computers a reachable vision? An essential requirement is that
one can identify suitable computational units, so called "Q-bits", which
keep their quantum state for a long period of time and that can be
externally manipulated as desired for computation. Unfortunately quantum
states change chaotically with time due to noise in the external
environment, which acts analogously to a friction force in the classical
world. This friction quickly destroys the information stored in a Q-bit. In
this paper it is demonstrated that for an important class of quantum
dissipative systems this loss can be significantly reduced. A prototype for
such a system is a semiconductor nano-sized dot where the friction results
from the interaction of the Q-bit with the surrounding crystal lattice. The
trick is to control the strength of the driving force. In contrast to common
expectations it is shown that in the class of quantum systems considered
here the friction is strongly reduced when the field is larger than a
threshold set by the properties of the environment. This result reveals
fascinating dynamical properties of an important prototype quantum system
and opens new perspectives to combat information loss in these systems.
***
Dynamics in Terms of Geometrically Defined Motions
Many mechanical systems occurring in nature follow equations of motion
that are nonlinear, often resulting in very complex patterns of motion
that are called chaotic. For this type of motion, nearby orbits,
corresponding to the setting of slightly different initial conditions for
the evolution, separate exponentially in time. The complex orbits of such
systems often appear to be a flow on geometrically defined surfaces.
Mathematical models of such systems are often given in the literature for
which dynamical flow follows geometrically determined orbits (geodesics)
in a very similar way to the motions of particles in a gravitational
field. The curved surfaces correspond to a geometry determined by a metric
function and a connection form, specified a priori in the model, fixing
the angles between lines and distances between points. Actual dynamical
systems are, however, not usually described in such terms. It is shown in
this paper that the standard form of the equations governing mechanical
motion can, quite generally, be cast into this type of model by
introducing a metric which induces appropriate scaling (conformal) on the
space of free particle motions. The resulting geodesics can be put into
correspondence with the actual motions of the particles by transforming to
a new connection form, a procedure which preserves the geometrical
interpretation of the theory. The computation of deviation of nearby
orbits, based entirely on the geometry of the space, is shown to provide
new useful criteria for instability which are very sensitive and widely
applicable.
***
Conduction Electrons Sense Devil's Staircase
A coupling between spins and electrons allows us to control resistivity by
magnetic fields. Indeed, applied magnetic fields can change perovskite
manganites from an antiferromagnetic insulator to a ferromagnetic metal. On
the other hand, frustrated Ising spin systems have been studied for decades
because of their interesting magnetism typified by the devil's staircase;
Magnetization, M, changes stepwise with having a value of MS/n as a function
of a magnetic field (MS and n denote a saturated magnetization and an
integer, respectively). Then, what happens if frustrated Ising spins coexist
and couple with conduction electrons? In this paper, we show one of the
clear-cut answers for this query, that is, giant two-staged
magnetoresistance with uniaxial anisotropy in SrCo6O11. In this material,
spin-polarized conduction electrons do sense the devil腦s staircase thanks
to a unique layered structure consisting of alternatively stacking
Ising-spin layers and conductive layers, which was confirmed by NMR studies
and band structure calculations. Our work demonstrates a prospect for novel
spintronic functions arising from a coupling between frustrated Ising spins
and spin-polarized conduction electrons.
Monday, May 7, 2007
Phys Rev Hot Papers: 5-07-07
How "relativistic" electrons in graphene avoid localization
Recent breakthrough in fabrication of graphene (monoatomic graphite
layer) and subsequent transport measurements revealed remarkable
electronic properties of this material. One of the most striking
observations is the minimal conductivity of undoped samples, which is
of the order of conductance quantum and stays constant from room
temperature down to 1K, showing no trace of quantum localization by
disorder. In this paper, we develop a theory of electron transport in
graphene with impurities that explain the experimental findings. The
following properties of graphene are crucial for our theory. First,
its low-energy states are split into two "valleys" in the
energy-momentum landscape. Second, the electrons in each valley
behave as massless relativistic particles. Third, experimentally
relevant scatterers in graphene -- charged impurities or so-called
ripples -- are of long-range character. As a result, the dominant
scattering cannot transfer the particle from one valley into the
other. We derive a quantum field theory describing electrons in this
situation and show that, due to the valley decoupling, the electrons
are characterized by very peculiar topological properties. As a result
the theory is at a novel quantum critical point, implying, in
partiuclar, that the conductivity is temperature-independent. This
criticality bears similarity with the famous quantum Hall transition
but occurs without any magnetic field! The non-trivial topology of
our theory explains also another striking experimental observation --
the anomalous, half-integer, quantum Hall effect in graphene in strong
magnetic field.
***
Enabling a Mother Qubit to deliver three lovely qubits
In quantum computers, interacting qubits perform computations.
In topological quantum computers, on the other hand, their offspirings
called topological qubits perform computations merrily. Their topological
character make them ghost like and oblivious to the material environment.
Quantum computation for them becomes a `merry go round', getting entangled
and picking up non-Abelian Berry phases, without ever losing their
coherence, even in an unfriendly and decohering environment.
Kitaev model, a remarkable, non trivial and exactly solvable 2 dimensional
quantum spin model exemplifies key ides of this topological quantum
computation. In our theoretical work on Kitaev model, where we present
certain exact results for dynamical properties, birth of a
Topological Qubit, from a `mother qubit' is made visible. We show that
changing the quantum state of one qubit results in the birth of a
triplet of topological qubits: an immobile `Siamese twin' of
`pi-fluxes' and a very dynamic `Majorana fermion' which frees itself
away from the mother qubit and is ever ready for quantum computation,
from the time of its birth.
Our finding will have a role in creating and organizing quantum computations
in the much awaited future quantum computers.
***
A Paul trap for neutral atoms
This paper presents the first trapping of ground-state neutral atoms in a macroscopic AC electric trap. Similarly to ions in a Paul trap (Nobel Prize, 1989), trapping is achieved here by alternating between two electric fields that result in a swinging motion of the atoms. For the first time, this motion in the trap is directly visualized using 2D images of the atom cloud.
The special feature of an AC electric trap is the ability to trap atoms or molecules in their ground state. The ground state is always attracted towards high electric fields, but it cannot be trapped in a static field, as static fields cannot possess a maximum in free space. Trapping in the ground state is therefore only possible when AC fields are used.
In any AC trap, an electric field is created with a saddle point at the trap center, resulting in attractive forces in one direction and repulsive forces along the other two directions. The electric field configuration is then switched to a second configuration in which the roles of the forces are reversed. Cycling between these two configurations leads to stable dynamic confinement of the particles.
***
Speeding up microfluidics
The term lab-on-a-chip summarizes the effort to miniaturize chemical
production lines into millimeter-sized devices. Due to the down scaling
of the dimensions, the fluid dynamics is altered and viscous forces now
take over inertia. This circumstance hampers rapid mixing in the
microchannels of lab-on-a-chip devices because now diffusion is the
dominant mechanism. A second challenge in microfluidic application is
the actuation of liquids. Currently the lab-on-a-chip devices rely on
additional fabrication steps and an elaborate integration of connections
and wiring, e.g. for pneumatic or electrical controls. An
interdisciplinary research team from the University of Twente (The
Netherlands) and Shimadzu Corporation (Germany) came up with a
revolutionary approach to overcome both limits. The team lead by
Claus-Dieter Ohl sped up microfluidics with single vapor bubbles thus
they surpassed the viscous constrains inherently found in lab-on-a-chip
devices. These short lived vapor bubbles are created with a focused
laser pulse of a few nanosecond duration. The miniature vapor explosions
accelerate the flow to 20m/s and more. Additionally, when the bubble
grows close to a channel wall liquid flow becomes focused and
accelerated to even higher speeds. This allows liquid pumping at fast
pace. And third advantage of the new technique implementing lasers
pulses is the easy applicability: on transparent lab-on-a-chip systems
any spot can be addressed just by positioning the laser spot thus
external connection are obsolete.
***
Emergence of collectivity in a few-electron system
The existence and nature of end and central plasmon resonances in a linear
atomic chain, the 1-dimensional analog to ¿surface¿ and ¿bulk¿ plasmons in
2-dimensional metals, has first been predicted by time-dependent density
functional theory. It is well known that electronic energies and
wavefunctions of such atomic chains are quantized, and can be understood
by the ¿particle-in-a-box¿ model. Whether these quantized states can
support collective electron oscillations has, however, not been
investigated. The calculation reported here shows the emergence and
development of collective excitations, as the chain length increases atom
by atom. In the long-chain limit, these collective modes converge to a
single longitudinal resonance and two transverse ones, which are localized
at the ends and center of the chains. These collective modes bridge the
gaps, in concept and scale, between the collective excitation in atomic
physics and nanoplasmonics. It also outlines a route to atomic-scale
engineering of collective excitations via atomic manipulation.
***
Dancing With the Spins
Using a novel device called the spin dynamo, researchers demonstrated that the
electron spins dance in a small stage with more than 20 different steps, rather
than the few steps we know before.
Watching the magnificent ball dancing in the fabulous Imperial Hofburg Palace in
Vienna, Austrian, is a very different experience as watching the ABC¿s TV show
of ¿Dancing with the Stars¿, where pairs dance in a compact studio stage. So is
for scientists studying the dance of electron spins in samples with different
dimensions. The dance of spins plays the most important role in operating your
computer hard disks and memory cells. Whenever you click on the mouse to save
your data, millions of electron spins in your computer disk dance collectively
to change their patterns, and thereby save the information.
Because everyone wants to have a computer that stores more data and faster,
scientists are given the task to study how electron spins dance in
nanostructures that are invisible to the naked eye. The fundamental questions
are how do electron spins influence each other (the collective phenomenon), and
how are they influenced by the finite stage where they are dancing (the boundary
condition problem). Find the answer is the major challenge of two emerging
disciplines known as the ¿spintronics¿ and ¿nanomagnetism¿.
Now Can-Ming Hu and his colleagues at the University of Manitoba in Canada has
created a novel device called ¿spin dynamo¿ [1], which generates current from
the dance of electron spins. Using this device, they have made a break through
in studying how electron spins dance in a small piece of magnet, where they
have observed more than 20 different ¿steps¿ of dancing for the electron spins
[2]! Previously, only a few steps of them have been seen. The observed new
phenomena allow the team to precisely explain how the size of a finite stage
influences the dance of spins, which is a long standing problem in the field of
magnetism, and it¿s solution is pivotal for the architecture of the next
generation computer memories.
Recent breakthrough in fabrication of graphene (monoatomic graphite
layer) and subsequent transport measurements revealed remarkable
electronic properties of this material. One of the most striking
observations is the minimal conductivity of undoped samples, which is
of the order of conductance quantum and stays constant from room
temperature down to 1K, showing no trace of quantum localization by
disorder. In this paper, we develop a theory of electron transport in
graphene with impurities that explain the experimental findings. The
following properties of graphene are crucial for our theory. First,
its low-energy states are split into two "valleys" in the
energy-momentum landscape. Second, the electrons in each valley
behave as massless relativistic particles. Third, experimentally
relevant scatterers in graphene -- charged impurities or so-called
ripples -- are of long-range character. As a result, the dominant
scattering cannot transfer the particle from one valley into the
other. We derive a quantum field theory describing electrons in this
situation and show that, due to the valley decoupling, the electrons
are characterized by very peculiar topological properties. As a result
the theory is at a novel quantum critical point, implying, in
partiuclar, that the conductivity is temperature-independent. This
criticality bears similarity with the famous quantum Hall transition
but occurs without any magnetic field! The non-trivial topology of
our theory explains also another striking experimental observation --
the anomalous, half-integer, quantum Hall effect in graphene in strong
magnetic field.
***
Enabling a Mother Qubit to deliver three lovely qubits
In quantum computers, interacting qubits perform computations.
In topological quantum computers, on the other hand, their offspirings
called topological qubits perform computations merrily. Their topological
character make them ghost like and oblivious to the material environment.
Quantum computation for them becomes a `merry go round', getting entangled
and picking up non-Abelian Berry phases, without ever losing their
coherence, even in an unfriendly and decohering environment.
Kitaev model, a remarkable, non trivial and exactly solvable 2 dimensional
quantum spin model exemplifies key ides of this topological quantum
computation. In our theoretical work on Kitaev model, where we present
certain exact results for dynamical properties, birth of a
Topological Qubit, from a `mother qubit' is made visible. We show that
changing the quantum state of one qubit results in the birth of a
triplet of topological qubits: an immobile `Siamese twin' of
`pi-fluxes' and a very dynamic `Majorana fermion' which frees itself
away from the mother qubit and is ever ready for quantum computation,
from the time of its birth.
Our finding will have a role in creating and organizing quantum computations
in the much awaited future quantum computers.
***
A Paul trap for neutral atoms
This paper presents the first trapping of ground-state neutral atoms in a macroscopic AC electric trap. Similarly to ions in a Paul trap (Nobel Prize, 1989), trapping is achieved here by alternating between two electric fields that result in a swinging motion of the atoms. For the first time, this motion in the trap is directly visualized using 2D images of the atom cloud.
The special feature of an AC electric trap is the ability to trap atoms or molecules in their ground state. The ground state is always attracted towards high electric fields, but it cannot be trapped in a static field, as static fields cannot possess a maximum in free space. Trapping in the ground state is therefore only possible when AC fields are used.
In any AC trap, an electric field is created with a saddle point at the trap center, resulting in attractive forces in one direction and repulsive forces along the other two directions. The electric field configuration is then switched to a second configuration in which the roles of the forces are reversed. Cycling between these two configurations leads to stable dynamic confinement of the particles.
***
Speeding up microfluidicsThe term lab-on-a-chip summarizes the effort to miniaturize chemical
production lines into millimeter-sized devices. Due to the down scaling
of the dimensions, the fluid dynamics is altered and viscous forces now
take over inertia. This circumstance hampers rapid mixing in the
microchannels of lab-on-a-chip devices because now diffusion is the
dominant mechanism. A second challenge in microfluidic application is
the actuation of liquids. Currently the lab-on-a-chip devices rely on
additional fabrication steps and an elaborate integration of connections
and wiring, e.g. for pneumatic or electrical controls. An
interdisciplinary research team from the University of Twente (The
Netherlands) and Shimadzu Corporation (Germany) came up with a
revolutionary approach to overcome both limits. The team lead by
Claus-Dieter Ohl sped up microfluidics with single vapor bubbles thus
they surpassed the viscous constrains inherently found in lab-on-a-chip
devices. These short lived vapor bubbles are created with a focused
laser pulse of a few nanosecond duration. The miniature vapor explosions
accelerate the flow to 20m/s and more. Additionally, when the bubble
grows close to a channel wall liquid flow becomes focused and
accelerated to even higher speeds. This allows liquid pumping at fast
pace. And third advantage of the new technique implementing lasers
pulses is the easy applicability: on transparent lab-on-a-chip systems
any spot can be addressed just by positioning the laser spot thus
external connection are obsolete.
***
Emergence of collectivity in a few-electron system
The existence and nature of end and central plasmon resonances in a linear
atomic chain, the 1-dimensional analog to ¿surface¿ and ¿bulk¿ plasmons in
2-dimensional metals, has first been predicted by time-dependent density
functional theory. It is well known that electronic energies and
wavefunctions of such atomic chains are quantized, and can be understood
by the ¿particle-in-a-box¿ model. Whether these quantized states can
support collective electron oscillations has, however, not been
investigated. The calculation reported here shows the emergence and
development of collective excitations, as the chain length increases atom
by atom. In the long-chain limit, these collective modes converge to a
single longitudinal resonance and two transverse ones, which are localized
at the ends and center of the chains. These collective modes bridge the
gaps, in concept and scale, between the collective excitation in atomic
physics and nanoplasmonics. It also outlines a route to atomic-scale
engineering of collective excitations via atomic manipulation.
***
Dancing With the Spins
Using a novel device called the spin dynamo, researchers demonstrated that the
electron spins dance in a small stage with more than 20 different steps, rather
than the few steps we know before.
Watching the magnificent ball dancing in the fabulous Imperial Hofburg Palace in
Vienna, Austrian, is a very different experience as watching the ABC¿s TV show
of ¿Dancing with the Stars¿, where pairs dance in a compact studio stage. So is
for scientists studying the dance of electron spins in samples with different
dimensions. The dance of spins plays the most important role in operating your
computer hard disks and memory cells. Whenever you click on the mouse to save
your data, millions of electron spins in your computer disk dance collectively
to change their patterns, and thereby save the information.
Because everyone wants to have a computer that stores more data and faster,
scientists are given the task to study how electron spins dance in
nanostructures that are invisible to the naked eye. The fundamental questions
are how do electron spins influence each other (the collective phenomenon), and
how are they influenced by the finite stage where they are dancing (the boundary
condition problem). Find the answer is the major challenge of two emerging
disciplines known as the ¿spintronics¿ and ¿nanomagnetism¿.
Now Can-Ming Hu and his colleagues at the University of Manitoba in Canada has
created a novel device called ¿spin dynamo¿ [1], which generates current from
the dance of electron spins. Using this device, they have made a break through
in studying how electron spins dance in a small piece of magnet, where they
have observed more than 20 different ¿steps¿ of dancing for the electron spins
[2]! Previously, only a few steps of them have been seen. The observed new
phenomena allow the team to precisely explain how the size of a finite stage
influences the dance of spins, which is a long standing problem in the field of
magnetism, and it¿s solution is pivotal for the architecture of the next
generation computer memories.
Friday, May 4, 2007
Phys Rev Hot Papers: 5-4-07
Metal Contacts at the Atomic Scale
The atomic process in which two metallic surface contact each other has
been characterized. The formation of a contact between two bodies is a
process which always involves the formation of at least one atomic
contact. Many tribological problems such as adhesion or friction will
then depend on the formation of such a contact. We show that for many
situations this process occurs smoothly without a mechanical instability
that would lead to a jump to contact. We observe that the configuration
and material composition of the electrodes before contact largely
determine the presence or absence of a jump. Through a combination of
experiments, atomistic simulations, and first-principles transport
calculations the first contact in between two metallic surfaces is shown
to be formed, in most of the times, by either a single atom , two atoms
aligned or two parallel atoms.
***
Sending a camel through the eye of a needle, the mechanics of DNA
translocation through nanopores
The Bibilical phrase, according to some historians, actually refers
to a certain gate in Jerusalem called
the "Needle's Eye" that was so narrow that a camel could barely squeeze
through and only if unencumbered by baggage. About ten years ago
scientists were quite amazed to
discover that single molecules of DNA will squeeze through holes that
are only just bigger than their diameter,
if a small electrical voltage is applied across it. Since the entry
of a DNA blocks the current that would normally flow
through the hole, the characteristics of the DNA can be inferred by
monitoring the amount of this current.
The intriguing possibility that information about the actual base
sequence can be coaxed out of this current signal
has become the holy grail of a sizable group of researchers pursuing
this as a possible ultrafast DNA sequencing
technology. A major obstacle is that unlike the proverbial camel
trying to go through the Needle's Eye, the DNA
zips through at the galloping clip of a thousand to a million bases
per second -- too fast for scientists to "read" the
sequence. In this paper, we take the first step towards slowing down
the DNA, which is to provide an understanding
of what determines its speed in the first place! It is shown that the
same hydrodynamic resistance that determines
the speed with which a wire can be drawn through a die in the
classical engineering science of metallurgy
also operate at these ultra small molecular scales and is responsible
for determining how fast the DNA
crosses the pore.
The atomic process in which two metallic surface contact each other has
been characterized. The formation of a contact between two bodies is a
process which always involves the formation of at least one atomic
contact. Many tribological problems such as adhesion or friction will
then depend on the formation of such a contact. We show that for many
situations this process occurs smoothly without a mechanical instability
that would lead to a jump to contact. We observe that the configuration
and material composition of the electrodes before contact largely
determine the presence or absence of a jump. Through a combination of
experiments, atomistic simulations, and first-principles transport
calculations the first contact in between two metallic surfaces is shown
to be formed, in most of the times, by either a single atom , two atoms
aligned or two parallel atoms.
***
Sending a camel through the eye of a needle, the mechanics of DNA
translocation through nanopores
The Bibilical phrase, according to some historians, actually refers
to a certain gate in Jerusalem called
the "Needle's Eye" that was so narrow that a camel could barely squeeze
through and only if unencumbered by baggage. About ten years ago
scientists were quite amazed to
discover that single molecules of DNA will squeeze through holes that
are only just bigger than their diameter,
if a small electrical voltage is applied across it. Since the entry
of a DNA blocks the current that would normally flow
through the hole, the characteristics of the DNA can be inferred by
monitoring the amount of this current.
The intriguing possibility that information about the actual base
sequence can be coaxed out of this current signal
has become the holy grail of a sizable group of researchers pursuing
this as a possible ultrafast DNA sequencing
technology. A major obstacle is that unlike the proverbial camel
trying to go through the Needle's Eye, the DNA
zips through at the galloping clip of a thousand to a million bases
per second -- too fast for scientists to "read" the
sequence. In this paper, we take the first step towards slowing down
the DNA, which is to provide an understanding
of what determines its speed in the first place! It is shown that the
same hydrodynamic resistance that determines
the speed with which a wire can be drawn through a die in the
classical engineering science of metallurgy
also operate at these ultra small molecular scales and is responsible
for determining how fast the DNA
crosses the pore.
Thursday, May 3, 2007
Phys Rev Hot Papers: 5-3-07
Photos of the quantum-classical transition
The transition between the quantum and the classical ¿world¿ has been
visualized experimentally for the first time, providing vivid evidence
of the correctness of the basic ideas of the theory of decoherence. This
theory resolves the longstanding problem of the incompatibility of the
quantum mechanical superposition principle with our everyday experience
of a ¿classical¿ world, a problem most drastically illustrated by the
famous Schrödinger¿s cat paradox. Understanding of decoherence is
essential from a fundamental point of view, and experimental control of
decoherence is crucial for applications, e.g. quantum computers. In the
present experiment, a novel mechanism of decoherence has been studied,
Coulomb interaction of elementary particles without inner degrees of
freedom, namely free electrons in a biprism interferometer, with a truly
macroscopic and dissipative environment, namely the electron gas inside
a semiconducting plate.
The closer the electrons pass to the surface of the plate, the stronger is the disturbance (e.g. heating) of the
electron gas beneath the flight paths of the beam electrons. In turn,
which-path information, entanglement and decoherence increase. This
manifests itself in decreasing contrast of the interference fringes
(which are perpendicular to the surface of the plate) with decreasing
altitude of the electrons above the plate. The decrease in contrast
demonstrates the continuous transition from quantum to classical.
***
How to Rip a Fluid
In a simple experiment on a mixture of water, soap, and salt, we show thata rigid object (like a knife) passes through a gel-like material as if it
were a liquid at slow speeds, but rips it up like a soft solid if it is
pulled rapidly. Most materials in real life do not follow the textbook
cases of solid, liquid, or gas; examples like blood, saliva, toothpaste,
and cell cytoplasm are called viscoelastic (viscous like a fluid, elastic
like a solid). This article focuses on the response of such a material to
increasingly extreme conditions of flow.
As a child will swish its finger
through an unknown liquid to discover what it is, in this experiment we
pull a cylinder through a viscoelastic gel of surfactant and organic salt
in water, to learn its responses. What happens is: flow at slow speeds,
cutting at intermediate speeds, and tearing at the highest speeds. Because
the material is not a solid however, it heals in the wake of the tear, and
recovers completely after several hours. We find that the material
strength of the solid is essentially the surface tension of the liquid -
this fact unifies the response across the time scales from flow to
fracture.
Wednesday, May 2, 2007
Phys Rev Hot Papers: 5-2-07
Self-assembly of a model quasicrystal
Although this year marks the 25th anniversary of the discovery of
quasicrystals, the mechanism of their formation is only poorly understood
yet. One reason is the lack of suitable theoretical models of atomic
interactions that lead to stable quasicrystals. A realistic model would be a
formidable task and simplifications have to be made. Michael Engel and
Hans-Rainer Trebin from Stuttgart University in Germany have now developed
interactions that allow the first direct observation of quasicrystal
growth. Their simple model consists of identical particles moving in two
dimensions favoring two distinct interparticle distances. The competition
between the distances can favor a local particle arrangement with ten-fold
symmetry, which is not compatible with periodicity. A quasicrystal is then
self-assembled in computer simulations at elevated temperatures. Upon
cooling
the quasicrystal undergoes a reversible phase transition into a complex
periodic crystal. A remarkably large variety of other crystals and a
quasicrystal with twelve-fold symmetry have also been found. In the
future the
model system might be experimentally realized with colloidal particles.
***
Essential building-block for large-scale quantum communication
In the present paper we have experimentally demonstrated an essential
building-block for large-scale quantum communication. We demonstrate
efficient creation of photonic entanglement using spatially distributed,
narrow-band, and memory built-in single-photon sources. This progress may
pave the way for future global quantum networks.
Currently, the channel length of quantum communication is limited to about
150 km due to photon losses and decoherence. Intuitively, one would expect a
"repeater", widely used in the traditional communication networks, will help
to extend the distance of quantum communication to an arbitrary desired
length. However, the quantum nature requires very special properties for
such a quantum repeater. Efficient schemas require a quantum memory to be
scalable.
In our experiment, atomic ensembles are used as quantum memory (stationary
qubit) while single photons are used as information carrier (flying qubit).
The inherent memory built-in properties of single photon sources make the
generation of entanglement--fundamental resource for quantum
communication--of independent single photons coming from remote sites very
efficient, based on which the quantum network is scalable.
***
Anomalous thermodynamics of Coulomb interacting massless Dirac fermions in two spatial dimensions
Low temperature thermodynamics of metals is a part of standard
condensed matter curriculum. It is well know, yet still remarkable,
that on the basis of the low temperature dependence of specific
heat, an experimentalist would be hard pressed to distinguish a
non-interacting electron gas from Coulomb interacting Fermi liquid.
In both cases the specific heat vanishes linearly with temperature.
This paper considers the effect of Coulomb interactions on the
specific heat of a two dimensional semimetal, which is in some sense
a critical point between a metal and a semiconductor. If the
electrons were free, the specific heat of a semimetal would vanish
quadratically with temperature, the extra power of temperature comes
from the linear depletion of single particle states at low energies.
Remarkably, the Coulomb interactions suppress this non-interacting
result by logarithmic factors, whose strength is given by the
effective fine structure constant, differing from the 1/137 by the
ratio of the speed of light to the Fermi velocity. Experimental
observation of such effect, would firmly place two dimensional
semimetals, such as graphene, into the category of non-Fermi liquid.
Although this year marks the 25th anniversary of the discovery of
quasicrystals, the mechanism of their formation is only poorly understood
yet. One reason is the lack of suitable theoretical models of atomic
interactions that lead to stable quasicrystals. A realistic model would be a
formidable task and simplifications have to be made. Michael Engel and
Hans-Rainer Trebin from Stuttgart University in Germany have now developed
interactions that allow the first direct observation of quasicrystal
growth. Their simple model consists of identical particles moving in two
dimensions favoring two distinct interparticle distances. The competition
between the distances can favor a local particle arrangement with ten-fold
symmetry, which is not compatible with periodicity. A quasicrystal is then
self-assembled in computer simulations at elevated temperatures. Upon
cooling
the quasicrystal undergoes a reversible phase transition into a complex
periodic crystal. A remarkably large variety of other crystals and a
quasicrystal with twelve-fold symmetry have also been found. In the
future the
model system might be experimentally realized with colloidal particles.
***
Essential building-block for large-scale quantum communication
In the present paper we have experimentally demonstrated an essential
building-block for large-scale quantum communication. We demonstrate
efficient creation of photonic entanglement using spatially distributed,
narrow-band, and memory built-in single-photon sources. This progress may
pave the way for future global quantum networks.
Currently, the channel length of quantum communication is limited to about
150 km due to photon losses and decoherence. Intuitively, one would expect a
"repeater", widely used in the traditional communication networks, will help
to extend the distance of quantum communication to an arbitrary desired
length. However, the quantum nature requires very special properties for
such a quantum repeater. Efficient schemas require a quantum memory to be
scalable.
In our experiment, atomic ensembles are used as quantum memory (stationary
qubit) while single photons are used as information carrier (flying qubit).
The inherent memory built-in properties of single photon sources make the
generation of entanglement--fundamental resource for quantum
communication--of independent single photons coming from remote sites very
efficient, based on which the quantum network is scalable.
***
Anomalous thermodynamics of Coulomb interacting massless Dirac fermions in two spatial dimensions
Low temperature thermodynamics of metals is a part of standard
condensed matter curriculum. It is well know, yet still remarkable,
that on the basis of the low temperature dependence of specific
heat, an experimentalist would be hard pressed to distinguish a
non-interacting electron gas from Coulomb interacting Fermi liquid.
In both cases the specific heat vanishes linearly with temperature.
This paper considers the effect of Coulomb interactions on the
specific heat of a two dimensional semimetal, which is in some sense
a critical point between a metal and a semiconductor. If the
electrons were free, the specific heat of a semimetal would vanish
quadratically with temperature, the extra power of temperature comes
from the linear depletion of single particle states at low energies.
Remarkably, the Coulomb interactions suppress this non-interacting
result by logarithmic factors, whose strength is given by the
effective fine structure constant, differing from the 1/137 by the
ratio of the speed of light to the Fermi velocity. Experimental
observation of such effect, would firmly place two dimensional
semimetals, such as graphene, into the category of non-Fermi liquid.
Wednesday, April 25, 2007
Phys Rev Hot papers: 4-25
Minimum detection efficiency for a loophole-free atom-photon Bell
experiment
Atom-photon systems can refute Einstein's view
43 years after Bell's discovery, sometimes referred to as ``the most
profound discovery of science'', that Einstein's local realistic
vision of the universe was in conflict with quantum physics,
scientists have strong evidences that Einstein was wrong. However,
there is still no conclusive experiment. These experiments require
the detection of pairs of particles, usually photons, in distant
locations. The problem is that the imperfect efficiency of
photo-detectors makes the results of all performed experiments still
compatible with Einstein's view. In this paper we show that if we
use asymmetric systems like atom-photon systems, instead of
photon-photon systems, then it would be possible to perform
loophole-free experiments which refute Einstein using currently
available technology.
***
Presence of pseudogap correlations in the superconducting state of underdoped
cuprates.
On spite of the large effort done since their discovery more than twenty
years ago, there is not an accepted theory to describe the high
temperature superconductors.
Their anomalous properties and the existence of a pseudogap in
the spectrum above the critical temperature
are well established, but not understood.
On the contrary, an important part of the community
considers that the superconducting state is more standard and
can be described (1) by the BCS model with a gap with nodes
along the diagonal (d-wave type). Two
very recent experiments (2,3), which we analyze in this
letter, have changed this view. We show that the superconducting state is
non-conventional because the pseudogap physics remains below the
superconducting transition. These results put strong
constraints to a valid theory of high-temperature superconductors.
The clue for this discovery comes from Raman experiments, which probe
charge excitations. In a superconductor, the Raman spectrum show peaks
due to the breaking of Cooper pairs. The intensity of this peak measures
the strength of the superconductivity while the peak frequency
gives the gap in the spectrum. In a BCS superconductor
both strength and gap are given
by the superconducting order parameter and show similar dependencies on
external parameters. Surprisingly, it was seen (2) that below a given doping,
in the antinodal region decreasing the doping produces a strong
suppression of the intensity together with an
increasing energy scale. The nodal behavior is more BCS-like.
We have now been able to show that this
strange behavior is consequence of the presence of pseudogap correlations in
the superconducting state.
The
pseudogap is strongest in the antinodal region.
and competes with
superconductivity, which weakens the strength of
the superconductivity in this region.
On spite of this
weakening, superconductivity is not completely suppressed
in the antinodal region.
This is ensured by the
existence of an antinodal pair-breaking peak and the smooth evolution of its
characterictic energy with doping.
This picture also explains similar two-scale behavior
found in photoemission(3), which is well reproduced.
***
Black Holes on the Move
New computer simulations show that colliding, supermassive black
holes can receive enormous kicks, potentially large enough to knock
them clear out of the galaxies where they normally reside. The kicks
are caused by a burst of gravitational waves that accompanies the
violent coalescence. Pairs of black holes, or "binaries", are
believed to form when two galaxies merge; the two black holes
eventually coalesce into one. The new simulations show that if the
black holes are rapidly spinning prior to their coalescence, the
gravitational radiation emitted during the final plunge can impart a
velocity as large as 4000 km/s to the coalesced black hole. This
velocity substantially exceeds the escape velocities even from the
centers of giant galaxies, implying that supermassive black holes are
sometimes completely removed from the galaxies in which they form.
The result would be "naked" black holes moving, undetected, through
intergalactic space.
experiment
Atom-photon systems can refute Einstein's view
43 years after Bell's discovery, sometimes referred to as ``the most
profound discovery of science'', that Einstein's local realistic
vision of the universe was in conflict with quantum physics,
scientists have strong evidences that Einstein was wrong. However,
there is still no conclusive experiment. These experiments require
the detection of pairs of particles, usually photons, in distant
locations. The problem is that the imperfect efficiency of
photo-detectors makes the results of all performed experiments still
compatible with Einstein's view. In this paper we show that if we
use asymmetric systems like atom-photon systems, instead of
photon-photon systems, then it would be possible to perform
loophole-free experiments which refute Einstein using currently
available technology.
***
Presence of pseudogap correlations in the superconducting state of underdoped
cuprates.
On spite of the large effort done since their discovery more than twenty
years ago, there is not an accepted theory to describe the high
temperature superconductors.
Their anomalous properties and the existence of a pseudogap in
the spectrum above the critical temperature
are well established, but not understood.
On the contrary, an important part of the community
considers that the superconducting state is more standard and
can be described (1) by the BCS model with a gap with nodes
along the diagonal (d-wave type). Two
very recent experiments (2,3), which we analyze in this
letter, have changed this view. We show that the superconducting state is
non-conventional because the pseudogap physics remains below the
superconducting transition. These results put strong
constraints to a valid theory of high-temperature superconductors.
The clue for this discovery comes from Raman experiments, which probe
charge excitations. In a superconductor, the Raman spectrum show peaks
due to the breaking of Cooper pairs. The intensity of this peak measures
the strength of the superconductivity while the peak frequency
gives the gap in the spectrum. In a BCS superconductor
both strength and gap are given
by the superconducting order parameter and show similar dependencies on
external parameters. Surprisingly, it was seen (2) that below a given doping,
in the antinodal region decreasing the doping produces a strong
suppression of the intensity together with an
increasing energy scale. The nodal behavior is more BCS-like.
We have now been able to show that this
strange behavior is consequence of the presence of pseudogap correlations in
the superconducting state.
The
pseudogap is strongest in the antinodal region.
and competes with
superconductivity, which weakens the strength of
the superconductivity in this region.
On spite of this
weakening, superconductivity is not completely suppressed
in the antinodal region.
This is ensured by the
existence of an antinodal pair-breaking peak and the smooth evolution of its
characterictic energy with doping.
This picture also explains similar two-scale behavior
found in photoemission(3), which is well reproduced.
***
Black Holes on the Move
New computer simulations show that colliding, supermassive black
holes can receive enormous kicks, potentially large enough to knock
them clear out of the galaxies where they normally reside. The kicks
are caused by a burst of gravitational waves that accompanies the
violent coalescence. Pairs of black holes, or "binaries", are
believed to form when two galaxies merge; the two black holes
eventually coalesce into one. The new simulations show that if the
black holes are rapidly spinning prior to their coalescence, the
gravitational radiation emitted during the final plunge can impart a
velocity as large as 4000 km/s to the coalesced black hole. This
velocity substantially exceeds the escape velocities even from the
centers of giant galaxies, implying that supermassive black holes are
sometimes completely removed from the galaxies in which they form.
The result would be "naked" black holes moving, undetected, through
intergalactic space.
Tuesday, April 24, 2007
Phys Rev Hot Papers: 4-24
Gold Nanostructures Fabricated by Ion Bombardment
Low energy ion beams were used to both produce gold nanoclusters, and
to measure their electronic properties. Nanostructured materials have
opened possibilities for new technology and basic science research in
areas ranging from microelectronics to catalysis. Changes in
electronic structure that scale with the cluster dimensions, i.e.,
quantum size effects, can strongly contribute to the chemical,
electrical and optical properties of the materials. A formidable
effort is thus underway to develop methods for producing
nanostructures in a controlled manner. In this work, a thin gold film
was grown on a titanium dioxide substrate, and then bombarded with
low energy argon ions. The structures, which formed as a result of
the interplay between sputtering and surface diffusion during ion
bombardment, were imaged with scanning tunneling microscopy, and
their chemical composition was quantified with x-ray photoelectron
spectroscopy. The neutralization of scattered alkali-metal ions was
then used to probe the electronic properties of the material. It was
demonstrated that low energy ion beams enable simple, but efficient
methods for the controlled production and characterization of
nanostructures that exhibit quantum size effects.
Results to appear in Physical Review Letters as P. Karmakar, G. F.
Liu, Z. Sroubek and J. A. Yarmoff, “Ion beam-induced formation and
interrogation of Au nanoclusters”.
***
ATTRACTION BETWEEN QUADRUPOLES CAN GIVE RISE TO NEW STATES OF MATTER
We show that attraction between strange objects called "quadrupoles" may
cause
a system to look different along different orientations in space. This
apparently
weird phenomenon can be more common than could be guessed a priori and is
present
in such different systems as magnets, polymers and tightly bound electrons.
Attraction between masses (or gravitational force) may glue matter producing
compact objects like planets. The driving unit of gravitational attraction,
the mass, is only defined by a number, the mass value. A more complex kind
of
attraction happens between a magnet and a needle: the effect of the magnet
is to
align the needle in a particular direction in space; in this way we can
define
the South and North poles and navigate in a definite direction. The
equivalent to the
mass in the magnet-needle attraction mechanism is called a "dipole". Unlike
the mass,
we need two numbers or coordinates to define a dipole in a plane, they
define a particular
direction, from south to north, say.
We have found that a rather wide class of complex systems, ranging from
systems with magnetic
properties like computer memories, to polymers which give rise to everyday
plastics or even more subtle
systems composed of many electrons in interaction at low temperatures, can
show attraction between
a third kind of object, which is neither a mass, nor a dipole, but a
"quadrupole". This object only
defines an orientation in space, but without specifying a direction. In
other words, a quadrupole
is like the needle but one cannot distinguish north from south. Attraction
between quadrupoles can give
rise to labyrinthine-like patterns like the one shown in the figure. The
understanding of their behavior
can be relevant to control magnetic memories and to design new materials
with sharply defined
characteristics.
***
Twisting and Failure Induced by Bending of Multiwalled Carbon Nanotubes
Xiaoyan Li1, Wei Yang1,2,* and Bin Liu1
1Department of Engineering Mechanics, Tsinghua University, Beijing 10084, People’s Republic of China
2Zhejing University, Hangzhou 310027, People’s Republic of China
*Electronic address: yw-dem@tsinghua.edu.cn
The carbon nanotubes (CNTs) are extraordinarily flexible under larger strains, and resist failure under repeated bending. Recently, experimental observations show that the multiwalled carbon nanotubes (MWCNTs) exhibit the characteristic wavelike distortion under bending load. These rippling structures dramatically reduce the stiffness of MWCNTs. In this paper, we report that a twisting mode concurs with the rippling (see Fig. 1) via atomistic simulations. The twisting mode originates from the lattice mismatch caused by external bending load, and enhances the local stain relaxation of CNTs with larger radii. Owing to the nucleation and propagation of defects under thermal fluctuations, further development of twisting causes necking and even facture of inner nanotubes prior to the failure of outer nanotubes (see Fig. 2). Occasionally, a monatomic chain comes into being under the combination of bending and twisting (see Fig. 3). This distinct deformation mode emerges as an important topic for future studies and applications to monatomic wires as probes, emitters and connectors at the nanoscale.
***
Realization of negative refraction in a natural material
Negative refraction reverses all known electromagnetic phenomena of
classical optics and promises exciting applications, e.g. as imaging beyond
the wavelength limit. Negative refraction can only be realized in systems
with negative dielectric constant and negative magnetic permeability and
it is generally believed that nature does not provide such materials and
artificial systems like metamaterials or photonic crystals have to be used
instead. In this work we demonstrate experimentally that such materials
indeed do exist at least at THz frequencies: Ferromagnetic metals reveal
negative refraction close to the frequency of ferromagnetic resonance.
The experimental realization utilizes a colossal magnetoresistance
manganite
as an example. In this material the negative refractive index can be
achieved
even at room temperature using external magnetic fields. Within the same
idea such common ferromagnetic metals like iron or nickel may be tuned to
a regime with negative refraction.
Figure: Experimental demonstration of a negative refraction in a bulk
manganite.
***
Black Holes to obey new quantum rules.
Black holes, those mythical objects capable of engulfing whole
stars at the center of galaxies, are also enigmatic for they are
believed to posses properties only assigned to the small, quantum
realm. Now, a new quantum behavior of black holes, speculated to
exist by Jacob Bekenstein some time back, has been
corroborated by a delicate calculation.
In a forthcoming paper to appear in the Physical
Review Letters, A. Corichi of the National University in Mexico
(UNAM) and J. Diaz-Polo and E. Fernandez-Borja of the University
of Valencia, Spain have shown that the Bekenstein-Hawking entropy
of black holes, as explained by the formalism know as loop quantum
gravity (LQG), exhibits this particular discrete property. Just as
J. Bekenstein conjectured more than 30 years ago, the entropy of a
black hole can only take some special values, in integer multiples
of a basic unit. So far, no formalism available was known to
reproduce this conjectured behavior, even LQG. In the forthcoming
paper, the authors show that, by some unexpected feature of the
theory, this `quantization' property arises in a natural manner.
Even after 30 years of unraveling the mysteries of black holes, they
continue to surprise us every day.
Monday, April 23, 2007
Phys Rev Hot Papers: 4-23
Ultracold Collision of Ytterbium Atom
Cold collision properties of a bosonic isotope of ytterbium (Yb) atoms are now completely clarified. Yb is the only atom for which a spinless
Bose-Einstein condensation (BEC) has been realized . In addition,
Yb recently attracts considerable attention as a future optical frequency standard. The ultracold collision between atoms plays a very important role such as the determination of the stability and dynamics of the BEC as well as the collisional shift of the atomic clock.
Two previous measurements about the ultracold collision of Yb were,
however, inconsistent. This Letter has solved this contradiction
by clarifying an unexpectedly large contribution from the d-wave
scattering even at a few tens of micro-Kelvin, so called, d-wave shape
resonance, and the scattering length has been determined with two
orders of magnitudes higher precision than previous works. The newly determined Van der Waals coefficient clarified the important contribution of the inner core electrons.
Cold collision properties of a bosonic isotope of ytterbium (Yb) atoms are now completely clarified. Yb is the only atom for which a spinless
Bose-Einstein condensation (BEC) has been realized . In addition,
Yb recently attracts considerable attention as a future optical frequency standard. The ultracold collision between atoms plays a very important role such as the determination of the stability and dynamics of the BEC as well as the collisional shift of the atomic clock.
Two previous measurements about the ultracold collision of Yb were,
however, inconsistent. This Letter has solved this contradiction
by clarifying an unexpectedly large contribution from the d-wave
scattering even at a few tens of micro-Kelvin, so called, d-wave shape
resonance, and the scattering length has been determined with two
orders of magnitudes higher precision than previous works. The newly determined Van der Waals coefficient clarified the important contribution of the inner core electrons.
Sunday, April 22, 2007
Phys Rev Hot Papers: 4-22
Stochastic resonant signaling in cells allows for quick transmission
of noisy signals
Cells live in a fluctuating environment in which signals and noise
keep bombarding the cell receptors. Noisy signals propagate inside
the cell via microscopic chemical reaction events. The problem of how
signals can be precisely detected, smoothly transduced, and reliably
processed under noisy conditions is a research topic of great current
interest. Papoian's Lab at the University of North Carolina at Chapel
Hill has discovered the phenomenon of stochastic resonant signaling
in cellular signaling cascades, where noisy signals are fastest
transmitted when biochemical reaction rates are tuned in a special
way. Their calculations, reported in the upcoming issue of Physical
Review Letters, indicate that this phenomenon is ubiquitous. Future
experimental and computational studies will reveal whether biological
networks have evolved to take advantage of this possibility. For
example, extremely fast signal detection might be evolutionarily
advantageous in certain processes such as visual signal transduction.
In addition, the physical principles discovered by Papoian's lab may
be used to design biochemical networks which can reliably detect
signals under very noisy conditions. Papoian's lab work may also help
experimentalists to rationalize the commonly observed response
variability, from cell to cell, to the same external stimulus.
of noisy signals
Cells live in a fluctuating environment in which signals and noise
keep bombarding the cell receptors. Noisy signals propagate inside
the cell via microscopic chemical reaction events. The problem of how
signals can be precisely detected, smoothly transduced, and reliably
processed under noisy conditions is a research topic of great current
interest. Papoian's Lab at the University of North Carolina at Chapel
Hill has discovered the phenomenon of stochastic resonant signaling
in cellular signaling cascades, where noisy signals are fastest
transmitted when biochemical reaction rates are tuned in a special
way. Their calculations, reported in the upcoming issue of Physical
Review Letters, indicate that this phenomenon is ubiquitous. Future
experimental and computational studies will reveal whether biological
networks have evolved to take advantage of this possibility. For
example, extremely fast signal detection might be evolutionarily
advantageous in certain processes such as visual signal transduction.
In addition, the physical principles discovered by Papoian's lab may
be used to design biochemical networks which can reliably detect
signals under very noisy conditions. Papoian's lab work may also help
experimentalists to rationalize the commonly observed response
variability, from cell to cell, to the same external stimulus.
Friday, April 20, 2007
Phys Rev Hot Papers: 4-20
Light induced terahertz surface plasmons
Summary
Manipulation of electromagnetic waves below wavelength scale has been a challenging task for decades. Recent discovery of higher-than-unity transmission of light in periodic metal nanoholes, primarily due to resonant excitation of surface plasmons, has opened up a new avenue to subwavelength photonics. In this paper, we report light induced terahertz surface plasmons. This unique approach leads to direct observation of instantaneous transition between two interesting optical phenomena, out-of-plane photonic crystal effect and surface-plasmon resonance, in a thin semiconductor film perforated with an array of subwavelength holes. By use of optical pump-terahertz probe technique, the dielectric function of semiconductors is essentially altered by intense ultrafast laser pulses due to photo-generated free carriers. As a result, the semiconductor array becomes metallic and favors the coupling and propagation of surface plasmons in the terahertz frequency region. This finding demonstrates a new path to tunable surface plasmons, particularly, the ultrafast tuning of surface plasmons will be feasible in arrays made from semiconductors that possess fast carrier lifetime. Thus, it would be promising in device applications such as tunable terahertz sources, switches, filters, and modulators.
****************
Dark Stability -Storage of an Optical Vortex
A joint group of researchers from the Weizmann Institute of Science and the Technion Institute of Technology in Israel stored and later retrieved an optical vortex beam in rubidium vapor, employing Electro-magnetic Induced Transparency (EIT) in the medium.
Imagine yourself circling the earth along the equator while constantly updating your analog clock to the local time. After returning to the origin, the hand of your clock have completed a full circle. The number of complete turns of the clock's hand after this round-trip, is a topological invariant known as the winding number. Small changes in the local time across the globe can not change the winding number of the clock's hand upon completion of the trip.
Facing a light vortex, one observes a ring of light surrounding a dark center. The phase of the light at the rim is winding n times around the dark center, n being an integer. This phase is much like the hands of the clock. When two light waves with opposite phase meet, they destructively interference and cancel each other. The dark center of a light vortex is a result of this destructive interference. Light coming from a point on the rim to the center will be exactly canceled by light coming from the opposite rim since they have opposing phases.
Storing light in an EIT medium is done by continuously transferring a "signal" light pulse onto the atomic level's coherence, by switching off a second "control" beam. By switching back this control beam, the atoms are forced to emit the stored "signal" pulse.
In this work the spatial two dimensional phase and amplitude of an optical vortex was stored. Since this beam's winding number is topologically stable against local deformations, the structure of the restored vortex remained invariant, and the dark center remained dark even though the atoms storing the beam diffused significantly during the storage time. In a control experiment, the researchers used a similar ring shaped beam with a dark center but with a flat phase, and showed that it is not stable against atomic diffusion and was in fact filled with light upon retrieval.
The importance of this experiment lies in the prospect of using optical vortices in quantum communication and computation schemes, as well as in more elaborate two-dimensional information storage schemes.
*****
Playing with bricks on the atomic scale: making icosahedral and decahedral
particles in a bath of liquid helium.
Small particles of matter, consisting of just a few thousand atoms or
less, are very different from their large cousins. Like bricks in a
child's toy set, they come in a variety of symmetric shapes, including
icosahedra and decahedra of the Platonic shape fame. For a few compounds,
it is possible to make such particles using methods of chemistry. Minute
amounts of others can be produced in the so-called supersonic beams. In
our work, we report a technique of producing atomic-scale icosahedral and
decahedral particles in large amounts. Thus far, noble-gas elements (neon,
argon) were used, and the particles were stored in liquid helium at low
temperatures. However, particles made of other compounds that are stable
at higher temperatures can also be produced with this technique. Small
particles possess unique properties which find applications in such
diverse areas as electronics, energy storage, and catalysis. Our work
opens new opportunities for research of the properties of the atomic-scale
particles, and may lead to future technological applications.
**********
Unveiling Quasi-Periodicity within Periodic Media
Quasi-periodicity is the intriguing concept of order without periodicity.
Such materials, known as quasicrystals, are obtained by a definite, but not
periodic ordering in space of a certain material parameter - e.g. atom
position in natural quasicrystals (as discovered by Schectman et al in
1984), or the value of the linear or nonlinear electric susceptibility, in
man-made linear and nonlinear photonic crystals. Unlike periodic crystals,
quasicrystals lack translational symmetry, thus they cannot be constructed
using a translated repetition of a single building block. Until recently,
quasi-periodic behavior was studied only in materials which are
quasi-periodically ordered. However, we have found a scattering condition,
which unveils quasi-periodic behavior within a periodic media. This is a
physical manifestation of one of the methods to create models for
quasicrystals by referring to an abstract high-dimensional periodic
structure and then projecting part of it into an irrationally oriented
subspace. To verify our finding experimentally, we used a basic nonlinear
optical process - three-wave-mixing - within an artificially constructed
periodic nonlinear photonic crystal. Under the new scattering condition, the
photons participating in two different processes exhibited an incommensurate
momentum conservation relation, thus revealing quasi-periodicity.
Phys Rev Hot Papers: 4-19
How electron stripes yield ferroelectricity
A novel type of ferroelectricity, called “electronic ferroelectricity” – with an essential connection to charge ordering – is strongly suggested by new micrographic evidence. Conventional theory of solids holds that ferroelectricity in general originates from atomic structural polarizations – a familiar example is the notable off-center shift in the perovskite BaTiO3. Yet our new work appearing in Physical Review Letters (LY10555) reveals that ferroelectric LuFe2O4 has a curious ground state distinguished by electron stripes. We discovered that these electron stripes manifest a frustrated charge density wave with a remarkable ferroelectric polarization. This 3-dimensional charge ordering state, occurring at a low temperature of about 20K, was directly revealed for the first time by our in-situ transmission electron microscopy (TEM). A remarkable series of richly varied structural phenomena were also recorded as we lowered the temperature from 300K to 20K. The clear micrographic results have enabled us to detect new details about spontaneous polarization.
P.S. fig.1 (a) Electron diffraction image showing the weak satellite spots from charge stripe order. (b) Model for charge stripes and ferroelectric polarization.
**********************
Orange reflection from a three-dimensional photonic crystal in the
scales of the weevil Pachyrrhynchus congestus pavonius (Curculionidae)
Welch,Victoria/Lousse,Virginie/Deparis,Olivier/Parker,Andrew/Vigneron, Phys Rev E.
The three-dimensional structure which causes the colouration of the
tropical weevil \textit{Pachyrrhynchus congestus pavonius} was studied,
using a combination of electron microscopy, optical spectroscopy and
numerical modelling. The orange scales which cover the coloured rings
on the animal's body were opened, to display the structure responsible
for the colouration. This structure is a three-dimensional photonic
polycrystal, each grain of which showing a face-centred cubic symmetry.
The measured lattice parameter and the observed filling fraction of
this structure explains the dominant reflected wavelength in the reddish
orange. The long-range disorder introduced by the grain boundaries
explain
the paradoxical observation that the reflectance, although generated by
a photonic-crystal, is insensitive to changes in the viewing angle.
*********
Turning electrons around : Echo as a measure for reversibility
An echo, a phenomenon known for sound (acoustic) waves, can be
observed with electronic waves in atoms. While a sound echo
results from the reflection of a sound wave at a hard object,
the echo of an electronic wave is a reflection induced by a rapid change
of an external field. With such tricks we can reverse the motion of the
electron leading to the recurrence of the initial wave. Using a technique
similar to the spin echo (the echo of the nuclear spins of molecules) we
demonstrate in this paper the echoes of electronic waves in atoms by
reversing the arrow of time. Like sound waves, electron waves may be
damped (decohere) during the propagation due to interactions with the
environment and the intensity of echoes is reduced. Measurements of echoes
can be thus used to quantify how much information initially stored in
atoms survives during the time propagation and can be retrieved.
*********
X-ray standing wave detects atoms taking tiny steps.
When you pull a string of a guiter to generate a musical note, a standing wave pattern is generated on the string. This is a mechanical wave. We generate standing waves of X-rays, which are electromagnetic waves, in an artificially made layered structure (multilayer) - like those used in the read head in your computer. Standing waves of X-rays can detect an impurity atom taking tiny steps (~ 0.2 nanometer) in such a multilayer. We cause such minute atomic movements (of Fe) in a Pt/C multilayer containing Fe impurity by shooting an energetic ion beam. We track the movements of Fe using an X-ray standing wave as we shoot more and more ions to the multilayer, where Fe atoms are driven out of C layers and captured in the Pt layers. This converts a nonmagnetic material into a ferromagnetic material by forming FePt magnetic nanoparticles. The method holds promise for future Terabit magnetic storage devices as nanometer sized ferromagnetic dots can be created in a nonmagnetic medium using a focused ion beam.
**************
Subscribe to:
Posts (Atom)