LX12358
Pendulum in Fermi liquid
When a pendulum is immersed in a liquid, its oscillation frequency is
generally reduced because of the flow of the liquid around the bob.
This is not necessarily the case in a Fermi liquid. In ordinary
liquids atoms or molecules continuously collide with each other. This
leads to standard hydrodynamic description of the fluid, which is
essentially similar as in water. A Fermi liquid is different because
at low temperatures the bob of the pendulum excites quasiparticles
which are ballistic, i.e. they fly off like a ball hit by a bat. In
spite of being ballistic, the quasiparticles interact with each
other, as argued by Lev Landau in 1957. We show that for attractive
interactions, the frequency of the pendulum is increased by immersing
it in a the Fermi liquid. Such a case is realized in isotopic
mixtures of helium (3He & 4He). Experiments made in this system show
evidence of the frequency increase. This "Landau force" is a new
application of Landau's theory, which is one of the central paradigms
of condensed matter physics.
***
LY12754
A phonodiode for detection of ultrashort acoustic pulses
We describe an all-electrical method for the detection of ultrashort acoustic pulses using a semiconductor device. The device could be named a ‘phonodiode’ in analogy with a photodiode used for detecting light. Such devices could find applications in high speed acoustically-driven switching of electronic circuits, direct conversion of terahertz sound to terahertz electrical signals, and heterodyne mixing of terahertz acoustical and electrical signals.
There has in recent years been a rapid expansion in the area of phononics, which is concerned with the physics and applications of sub-terahertz coherent phonons (the quanta of vibration, or acoustic, energy). This has been driven by development of femtosecond laser-based techniques for generating picosecond-duration acoustic phonon pulses. The traditional method of detecting the acoustic signals is to use an optical probe. We demonstrate high-speed electrical detection of the acoustic pulses using a metal-semiconductor contact (Schottky diode). We show that the device detects, with good sensitivity and temporal resolution, the acoustic pulses generated by femtosecond laser excitation of a metal film. A phonodiode, used in conjunction with an electrical method of generation of coherent phonon pulses, e.g. a saser, could make an integrated all electrical system for manipulations with coherent sub-terahertz phonon pulses.
Tuesday, January 18, 2011
EX10520
NANO-SPIDERS HURRY ALONG TRACKS
We study the movement of nano-scale walkers, called "molecular
spiders" that travel along one-dimensional tracks. Our analysis
reveals a useful transient behavior: the spiders move faster than
ordinary random walkers over significant distances and times. In other
words they travel faster than diffusion. The spiders in our model are
inspired by physical experiments (Pei et al., J. Am. Chem. Soc. 128
12693-12699, 2006; Lund et al., Nature 465 206-210 (2010)), in which
spider molecules are synthesized with a rigid chemically inert body
attached to legs made of single-stranded segments of DNA; they walk on
a surfaces coated with single-stranded DNA complementary to the legs.
The legs attach and detach to the surface strands, causing the spider
to move over the surface. When a leg detaches from a strand for the
first time, it cuts it in half. This change is irreversible and the
legs subsequently bind to cut strands more weakly.
Previous analyses dealt with so-called asymptotic behavior---if time
goes to infinity, the spiders diffuse. Our analysis is based on
stochastic computer simulations, which enable us to predict how the
strand modification affects the motion of spiders in the context of
real experiments that last for a finite time and where spiders cover a
finite distance. This has important applications if spiders walking
along one-dimensional tracks are used as a transport mechanism for molecular
payloads. We hope to use our simulations to help chemists design
faster spiders. In the future spiders might be used for various
medical purposes or to assemble nano-devices built from molecules
which spiders will be able to pick up and carry along tracks.
***
BU11301
Magnetization switched off by crystal vibration
We have connected magnetic phase transition observed in magnetocaloric
MnAs near room temperature (315 K) with particular vibration of crystal
structure. We have shown that excitation of this vibration (with
increasing temperature or stretching the sample) leads to disordering of
magnetic moments and disappearance of total magnetization. Discovery of
such a strong coupling between crystal vibrations and magnetization
enabled us to form a simple explanation of the mechanism of both:
discontinuous magneto-structural phase transition at 315 K as well as
continuous return at 393 K (without magnetic ordering) to the
low-temperature structure. This particular interplay between magnetic
and lattice properties leads to large entropy change around this
discontinuous transition and, consequently, to giant magnetocaloric
effect promising applications in cheap and green refrigeration at room
temperature. Our theoretical predictions agree well with all
experimental findings published earlier.
***
LV12016
SPONTANEOUS RIPPLING OF GRAPHENE MIMICS CONDENSATION OF RELATIVISTIC
HIGGS FIELD
Great part of the current excitement about graphene (the one-atom thick
carbon layer) comes from the fact that it is the only known membrane
(genuine two-dimensional material) with conducting properties. A
remarkable and puzzling property of this material is its strong tendency
to develop ripples (modulations of the vertical displacements) by which
the membrane freezes spontaneously into a corrugated configuration. In
this paper we have shown that the interaction between the membrane
distortions and its conduction electrons places the flat phase of
graphene in unstable equilibrium, closely mimicking the state of the
relativistic Higgs field prior to its decay into a ground-state
condensate filling the space -the mechanism that gives mass to
elementary particles in high-energy physics. We have seen that the
effective potential for both quantum fields (the vertical displacement
in graphene, the Higgs field in relativistic physics) has the same
typical "mexican-hat" shape, which makes the unstable state at the top
to spontaneously decay by rolling down to a lowest-energy state with
broken symmetry (the aggregate of ripples in one case, the Higgs
condensate filling the vacuum in the other). The analogue goes further,
since graphene has a control parameter (the tension of the membrane)
that plays the same role as the bare mass square for the Higgs field,
opening the possibility of investigating the behavior of the Higgs
sector of elementary particle theories at the much smaller energies of a
condensed matter system like graphene.
***
BZ11073
Hours Marked by Neutrons
Small-angle neutron diffraction is a powerful probe for the behavior
of quantized magnetic field lines (and their associated 'vortices' of
screening currents) penetrating into superconductors. Using this
technique, we made the first observation of the so-called 'vortex
lattice' in pure crystals of HgBa2CuO4+d, one of the best
high-temperature superconductors discovered to date. The diffraction
pattern looks exactly like the markings of hours on a clock face
(attached figure), reflecting a simple geometric beauty of nature that
a hexagonal arrangement in two dimensions allows for the largest
nearest-neighbor distance for a given number density -- the vortices
want to stay away from one another, and the presence of two types of
hexagonal domains rotated by 90 degrees relative to each other leads
to the twelve diffraction spots. By further studying the vortex
lattice at various temperatures and magnetic fields, we confirmed a
previously proposed mechanism for long and straight magnetic field
lines to decompose into short segments surrounded by uncorrelated
screening currents (referred to as 'vortex pancakes'), which explains
why the diffraction signal vanishes at high fields. Our work opens the
door for future investigations into the vortex physics in this highly
ideal new family of superconductors.
***
LW12339
Direct observation of superconducting vortex bundles in the critical state
P.W. Anderson proposed in 1962 that motion of flux bundles restores the
critical state in hard superconductors, after a change in temperature or
magnetic field. We now provide first images of vortex bundles, and show how
they accumulate and release stress. We push vortices in a bundle by slightly
increasing the magnetic field, and observe that bundles are not rigid.
Vortex positions inside a bundle can change, accumulating stress, before it
is suddenly released. Data are taken under magnetic fields of some Tesla,
similar to those used in superconducting magnets. Vortex motion in such
devices can, under some conditions, lead to magnet failure. A better
knowledge about the microscopic mechanism behind flux arrangements, directly
imaged in our experiment, could help improving magnet design.
***
LX12217
Jams, Jellies, Creams and Paste: Prediction of long time physical behavior in glasses is now feasible
In our paper we have proposed a procedure to predict a very long time and a very short time physical behavior of glassy materials such as molecular glasses, spin glasses and colloidal glasses (or pastes) by carrying out experiments over practically possible time timescales. We have proposed an effective time approach that adjusts the material clock appropriately to accommodate time dependent change of relaxation time brought about by an aging process in glassy materials. We demonstrate effectiveness of this approach by predicting creep compliance of many soft materials such as aqueous clay suspension, commercial hair gel and acrylic emulsion paint. This Letter demonstrates that the effective time approach successfully predicts a very long and very short time physical behavior of materials showing diverse aging regimes ranging from sub-aging (or slow aging) to hyper-aging (very fast aging) dynamics. This approach can be used to predict behavior of any linear response function in molecular glasses, spin glasses, and variety of soft pasty materials such as pharmaceutical and cosmetic pastes/creams, toothpastes, paints, and variety of high viscosity food materials such as jams and jellies.
NANO-SPIDERS HURRY ALONG TRACKS
We study the movement of nano-scale walkers, called "molecular
spiders" that travel along one-dimensional tracks. Our analysis
reveals a useful transient behavior: the spiders move faster than
ordinary random walkers over significant distances and times. In other
words they travel faster than diffusion. The spiders in our model are
inspired by physical experiments (Pei et al., J. Am. Chem. Soc. 128
12693-12699, 2006; Lund et al., Nature 465 206-210 (2010)), in which
spider molecules are synthesized with a rigid chemically inert body
attached to legs made of single-stranded segments of DNA; they walk on
a surfaces coated with single-stranded DNA complementary to the legs.
The legs attach and detach to the surface strands, causing the spider
to move over the surface. When a leg detaches from a strand for the
first time, it cuts it in half. This change is irreversible and the
legs subsequently bind to cut strands more weakly.
Previous analyses dealt with so-called asymptotic behavior---if time
goes to infinity, the spiders diffuse. Our analysis is based on
stochastic computer simulations, which enable us to predict how the
strand modification affects the motion of spiders in the context of
real experiments that last for a finite time and where spiders cover a
finite distance. This has important applications if spiders walking
along one-dimensional tracks are used as a transport mechanism for molecular
payloads. We hope to use our simulations to help chemists design
faster spiders. In the future spiders might be used for various
medical purposes or to assemble nano-devices built from molecules
which spiders will be able to pick up and carry along tracks.
***
BU11301
Magnetization switched off by crystal vibration
We have connected magnetic phase transition observed in magnetocaloric
MnAs near room temperature (315 K) with particular vibration of crystal
structure. We have shown that excitation of this vibration (with
increasing temperature or stretching the sample) leads to disordering of
magnetic moments and disappearance of total magnetization. Discovery of
such a strong coupling between crystal vibrations and magnetization
enabled us to form a simple explanation of the mechanism of both:
discontinuous magneto-structural phase transition at 315 K as well as
continuous return at 393 K (without magnetic ordering) to the
low-temperature structure. This particular interplay between magnetic
and lattice properties leads to large entropy change around this
discontinuous transition and, consequently, to giant magnetocaloric
effect promising applications in cheap and green refrigeration at room
temperature. Our theoretical predictions agree well with all
experimental findings published earlier.
***
LV12016
SPONTANEOUS RIPPLING OF GRAPHENE MIMICS CONDENSATION OF RELATIVISTIC
HIGGS FIELD
Great part of the current excitement about graphene (the one-atom thick
carbon layer) comes from the fact that it is the only known membrane
(genuine two-dimensional material) with conducting properties. A
remarkable and puzzling property of this material is its strong tendency
to develop ripples (modulations of the vertical displacements) by which
the membrane freezes spontaneously into a corrugated configuration. In
this paper we have shown that the interaction between the membrane
distortions and its conduction electrons places the flat phase of
graphene in unstable equilibrium, closely mimicking the state of the
relativistic Higgs field prior to its decay into a ground-state
condensate filling the space -the mechanism that gives mass to
elementary particles in high-energy physics. We have seen that the
effective potential for both quantum fields (the vertical displacement
in graphene, the Higgs field in relativistic physics) has the same
typical "mexican-hat" shape, which makes the unstable state at the top
to spontaneously decay by rolling down to a lowest-energy state with
broken symmetry (the aggregate of ripples in one case, the Higgs
condensate filling the vacuum in the other). The analogue goes further,
since graphene has a control parameter (the tension of the membrane)
that plays the same role as the bare mass square for the Higgs field,
opening the possibility of investigating the behavior of the Higgs
sector of elementary particle theories at the much smaller energies of a
condensed matter system like graphene.
***
BZ11073
Hours Marked by Neutrons
Small-angle neutron diffraction is a powerful probe for the behavior
of quantized magnetic field lines (and their associated 'vortices' of
screening currents) penetrating into superconductors. Using this
technique, we made the first observation of the so-called 'vortex
lattice' in pure crystals of HgBa2CuO4+d, one of the best
high-temperature superconductors discovered to date. The diffraction
pattern looks exactly like the markings of hours on a clock face
(attached figure), reflecting a simple geometric beauty of nature that
a hexagonal arrangement in two dimensions allows for the largest
nearest-neighbor distance for a given number density -- the vortices
want to stay away from one another, and the presence of two types of
hexagonal domains rotated by 90 degrees relative to each other leads
to the twelve diffraction spots. By further studying the vortex
lattice at various temperatures and magnetic fields, we confirmed a
previously proposed mechanism for long and straight magnetic field
lines to decompose into short segments surrounded by uncorrelated
screening currents (referred to as 'vortex pancakes'), which explains
why the diffraction signal vanishes at high fields. Our work opens the
door for future investigations into the vortex physics in this highly
ideal new family of superconductors.
***
LW12339
Direct observation of superconducting vortex bundles in the critical state
P.W. Anderson proposed in 1962 that motion of flux bundles restores the
critical state in hard superconductors, after a change in temperature or
magnetic field. We now provide first images of vortex bundles, and show how
they accumulate and release stress. We push vortices in a bundle by slightly
increasing the magnetic field, and observe that bundles are not rigid.
Vortex positions inside a bundle can change, accumulating stress, before it
is suddenly released. Data are taken under magnetic fields of some Tesla,
similar to those used in superconducting magnets. Vortex motion in such
devices can, under some conditions, lead to magnet failure. A better
knowledge about the microscopic mechanism behind flux arrangements, directly
imaged in our experiment, could help improving magnet design.
***
LX12217
Jams, Jellies, Creams and Paste: Prediction of long time physical behavior in glasses is now feasible
In our paper we have proposed a procedure to predict a very long time and a very short time physical behavior of glassy materials such as molecular glasses, spin glasses and colloidal glasses (or pastes) by carrying out experiments over practically possible time timescales. We have proposed an effective time approach that adjusts the material clock appropriately to accommodate time dependent change of relaxation time brought about by an aging process in glassy materials. We demonstrate effectiveness of this approach by predicting creep compliance of many soft materials such as aqueous clay suspension, commercial hair gel and acrylic emulsion paint. This Letter demonstrates that the effective time approach successfully predicts a very long and very short time physical behavior of materials showing diverse aging regimes ranging from sub-aging (or slow aging) to hyper-aging (very fast aging) dynamics. This approach can be used to predict behavior of any linear response function in molecular glasses, spin glasses, and variety of soft pasty materials such as pharmaceutical and cosmetic pastes/creams, toothpastes, paints, and variety of high viscosity food materials such as jams and jellies.
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