Memories that Consume No Power
Single molecular magnets have long been regarded as one of the
promising candidates for next generation of magnetic random access
memories, because of their bi-stable magnetic orientation as well as
their nanometer size. A problem is that when using a current to
individually access such small molecules integrated at large scale,
the total current density could be enormously large and may lead to
many spurious effects. In this paper, we propose that the
magnetization of a magnetic molecule can be switched reversibly,
without the help of a magnetic field or a spin-polarized electric
current. Instead we suggest employing the so-called spin bias, which
can drive the same amount of spin-up and spin-down electrons
travelling in and out of a molecule simultaneously. While angular
momenta can be transferred to the molecule, much like a
current-induced spin torque, the two opposite spin flows cancel with
each other, yielding no net electric current. This idea can be applied
to any nanoscale magnetic storage units, to fabricate memories that
consume no power.
***
AC10669
A Universal Quantum Computer Exists
In 1936, Alan Turing formulated the model of classical programmable
computers, the Universal Turing Machine (UTM). Essentially all modern
day computers are based on the UTM. Less than a century later,
classical computers have become ubiquitous in modern life and
virtually all technology is ultimately dependent upon them. In 1985,
David Deutsch formulated the model of a quantum version of the UTM,
the Universal Quantum Turing Machine (UQTM). Since then, researchers
have shown that quantum computers can solve certain problems
significantly faster than classical computers ever will. However,
researchers also raised questions about the validity of some aspects
of the UQTM. Thus, the existence of a programmable quantum computer
became an open question. In this paper, we present an explicit
theoretical construction of such a computer, the Universal Quantum
Computer (UQC), addressing the open questions that were raised about
the UQTM. Our machine can emulate a UTM and can execute any quantum
computing algorithm. In addition, the UQC supports conditional
branching and execution, a feature that greatly aids computer
programming but that has not been directly possible in previous
quantum computing frameworks. We thereby show that a universal
quantum computer exists, settling the question about the validity of
the UQTM. The UQC serves as a prototypical model for general-purpose
programmable quantum computation in much the same way that the UTM
serves for classical computation. The UQC should find uses in the
development and analysis of quantum algorithms and complexity and
paves the way for the physical construction of a programmable quantum
computer some day.
***
LA12116
World’s smallest incandescent lamp
In this paper we report imaging the smallest incandescent lamp ever
constructed. The filament, visible to the unaided eye when lit,
appears as a tiny point of light since its length is comparable to the
wavelength of the incandescent light it emits. Even our modern
optical microscope barely resolves the filament’s non-zero length,
since the light’s wavelength places a fundamental limit on the
achievable optical resolution. To “see” the filament, a single carbon
nanotube, with atomic resolution we use an electron microscope (as
opposed to a light microscope). In this way we can correlate the
properties of the light emitted with the detailed molecular structure
of the filament, which is only about 100 atoms wide. Our goal is to
understand how Planck’s blackbody radiation law gets modified at small
length scales. Planck’s law dates from 1900 and describes radiation
from large, hot objects, such as a toaster, the Sun, or a light bulb.
Some such objects are of fundamental and current scientific interest;
for instance, the thermal radiation left over from the Big Bang
(called the cosmic microwave background) is described by Planck’s law.
The carbon nanotube makes an ideal filament for this experiment,
since it has both the requisite smallness, and the extraordinary
temperature stability of carbon. In fact, Thomas Edison’s original
light bulbs had carbon filaments. Our light bulb is very similar,
except that the filament is 100,000 times narrower and 10,000 times
shorter, for a total volume only one one-hundred-trillionth of that of
Edison’s version.
***
LB12061BR
Approaching reality by ab-initio calculations: symmetry is lost!
Symmetry is the invariance of an object to a set of changes, i.e. transformations.
and it is central to our understanding and description of natural phenomena.
However, the world we observe is often unsymmetrical, from daily life to
microscopic word, like atoms governed by quantum mechanics. The consequences
of broken symmetry can be dramatic!
It's a great challenge to describe the microscopic world by using ab-initio calculations,
i.e. just define your atomic systems and put everything in a computer, as much as close
to reality. We have achieved this in our work, and surprisingly we lost some symmetry,
as Nature requires for our system.
Example: imagine to insert a magnetic atom, like Manganese,
in a non magnetic semiconductor, like GaN. The magnetic atom carries a spin density,
which is an unbalance of spin-up and spin-down charge density. Remember that the spin is a
quantum mechanical property of the electron, like an up- or down-arrow carried by
the electrons in Mn. So far, approximate calculations describe this spin density as a
symmetric object: in the figure, left part, if you rotate by 120 degree, you end up with the same
situation before rotation. Our calculations, which describe better the interactions among
the electrons, clearly show that this is not the case: in the figure, right part, you lost
a 120 rotation symmetry, and your object (spin density) is no longer symmetric.
Experiments confirm our calculations: Nature prefers an unsymmetrical description, although
you start from symmetric quantum laws (spontaneous symmetric breaking)!
***
LX11701
A VOID PHASE IN DIPOLAR COLLOIDS
The spontaneous self assembly of nanoparticles into linear aggregates such
as nanowires is a process of much interest in applied physics and material
science, but one that is not well understood. Dipole-dipole interactions
are thought to be the driving force for this process.
In the current work, a Brownian colloidal system with a controllable
dipolar interaction is seen to produce linear aggregates (chains) that
further aggregate to form an very low density network structure at
particle packing fractions between 0.07% and 4%. We follow the real-space
structure and the long-time dynamics of the structure formation by
fluorescence confocal microscopy, and find that this structure is a an
equilibrium network-forming phase characterized by cellular particulate
structures surrounding large particle-free voids. We call this a void
phase.
Dipolar interactions are attractive at short range and repulsive at large
distances. We observe that the aggregates at network walls are not
compact. This might result from a shallow attractive interaction, possibly
as a result of a combination of dipolar and van der Waals interactions.
Ultra-low density equilibrium cellular structures also have potential
applications as structured macroporous materials. The void phase reported
here is 10-100 times emptier than typical low-density cellular materials.
***
LX11217
Atomic-scale imaging with ultrasound
In Medicine, ultrasound is successfully used as a non-invasive
tool to image an unborn baby in the mother's womb. As its
counterpart in nanotechnology, we introduce the non-invasive
Damping Force Spectroscopy (DFS) technique capable of imaging
subsurface structures and vibrational modes on the atomic scale by
observing the damping of an oscillating atomic force microscope
(AFM) tip in the "non-contact" regime.
We apply DFS to peapods, consisting of carbon nanotubes filled
with metallofullerenes (hollow C_82 'buckyballs' containing a Dy
atom inside). Spatial maps of the damping signal show atomic-scale
features superior to state-of-the-art topographic AMF images. Not
only can DFS clearly distinguish between empty and filled peapods,
but can also reveal the location and packing of the
metallofullerenes in nanotubes of different diameter as well as
changes of the local vibrational modes.
We trace back the microscopic origin of the damping signal to a
hysteresis in the interaction between the AFM tip and the elastic
peapod. First principles total energy and molecular dynamics
calculations allow us to provide a quantitative interpretation of
the DFS signal by identifying which vibrational modes may be
excited by the AFM tip at a particular location.
***
LC11988
Polarons under the microscope: electron meets lattice vibration
When an electron is injected into a nanometer-sized semiconductor
crystal - so-called quantum dot (QD) - the charge of the electron causes
a distortion of the crystal lattice. The combination of the electron and
the surrounding lattice deformation is known as a polaron in the physics
jargon. This process is the one of the most important factors governing
electron transport and energy relaxation in nanostructured materials and
has not been previously quantitatively investigated. We present the
first direct, clear-cut, and quantitative experimental determination of
the coupling between injected electrons and lattice vibrations (phonons)
in semiconductor QDs.
Many of the proposed optoelectronic applications of colloidal QDs, such
as infra-red lasers and solar cells, require that energy relaxation of
excited electrons can be slowed down. The electron-phonon coupling is
the fundamental process that ultimately determines the rate of energy
relaxation in colloidal QDs once competing relaxation processes (e.g.
coupling of the excited electron to vibrations of the ligand molecules)
have been minimized by nanocrystal engineering.
We measure the electron-phonon coupling strength in a single-electron
transport experiment by low-temperature scanning tunneling microscopy
(STM) and spectroscopy (STS). The quantum mechanically coupled
electron-phonon states are directly observed by STS and we explore how
the coupling strength depends on the nanocrystal size and shape and the
symmetry of the electronic orbital.
***
AB10501
Sub-Planckian physics in molecular system
The Heisenberg's uncertainty principle is a fundamental in quantum physics. This implies that the product of uncertainties of
position and momentum must always be greater than Planck constant. The sub-Planck scale structures have phase space dimension smaller
than Planck constant. Hence, one might be tempted to dismiss its existence even thought it appears in Wigner quasi-probability
distribution. These do not violate the uncertainty principle, since it only tells about the phase space area, not the quantum
uncertainty product of the system. These structures are first reported by Zurek (Nature 412, 712 (2001)) and showed that in a
chaotic system they are most sensitive against decoherence.
At present, no doubt about the existence of these structures in phase space. Importantly, sub-Planck scale structures can be found in a realistic system of vibrating diatomic molecule (Phys. Rev. A 73, 013411 (2006)). Superposition of few vibrational energy levels creates a wave
packet and its temporal dynamics reveal the signature of this smallest structure in phase space. In reality, every system is
coupled with its surrounding, called environment, which results in the decoherence. Now the question is whether these structures are
really sensitive to decoherence in the quantum system of diatomic molecule. In this work, for the first time, we show the
decoherence-sensitivity of these structures in a diatomic molecular system. The time evolution of a vibrational wave packet (coherent state)
is investigated under the influence of a bosonic environment describing either photonic or phononic excitations. These
structures, being smallest in quantum regime, are found to be most sensitive against decoherence.
Moreover, it is important to verify the decoherence-sensitivity of the sub-Planck scale structures due to the variation of
physical parameters like environment coupling, temperature and the system evolution time. Our quantitative analysis provides an
exponential decay of the amplitude of the quantum interference structures as a function of the coupling with the environment, in
agreement with usual predictions. We find that the sub-Planck structures decay fastest among the other interference structures.
Influence of the environment temperature on the decoherence is also investigated. This obeys the well known Bose-distribution
law, where we predict the value of the critical temperature for this realistic system. Sensitivity study upto a longer time again
shows that these structures are most sensitive compare to their original counterparts. In a nutshell, if a very small disturbance
acts on a diatomic molecule, these structures are the most convenient to be used as a quantum switch for detection.
Figure Caption:
The sub-Planck scale structures appear in the central region of the phase space as localized maxima (red spot) and minima (black spot). These are the result of superposition of two diagonal cat-state interferences. Due to the environment coupling, they are most sensitive against decoherence and disappear much faster compare to their counterparts (four coherent states).
***
ER10385
Can information travel faster than light?
This is an old, delicate, and as yet unanswered question. Previous
studies claimed that, under certain circumstances, a signal can cover
a distance in less time than that required for the same distance in a
vacuum. Electromagnetic tunneling is one way to observe this kind of
phenomenon. In return, just as many studies criticized this claim as
an unexpected possibility. Detractors argue that it is difficult to
locate a point of the signal as a reference for a correct measurement
of its velocity, the signal does not travel but simply sojourns in
the distance and is subjected to deformation, narrowing, reshaping.
In other words, the "faster than light" behavior should be considered
only apparent.
By using a set-up based on waveguide, that detects only the
transmitted components of an electromagnetic signal, the present
study provides a novel, unambiguous method for measuring the
tunneling time of subsequent wave packets. It shows that the signal
is not degraded and is delivered in a short, superluminal time, that
is independent of the tunneled distance.
(Figure caption) A tunneled, evanescent wave packet appears at the
end of the barrier in less time than that required for traversing the
same distance in a vacuum.
***
BB11176
Positrons as probes of free volumes in materials with complex defect
structure
A model is presented which paths the way to apply the powerful
defect-sensitive method of positron annihilation even in such complex
situations of fine-grained materials where point defects and interfaces
coexist. Positrons, the antiparticles of electrons, are versatile probes
for the study of atom-scale sized free volumes in condensed matter -- a
class of important defects which hardly can be detected by other
microscopic techniques. A focus of application is on the study of point
defects (such as single missing atoms in a perfect lattice), but there
is an increasing demand to employ this method in the study of interfaces
of polycrystalline materials and nanophase materials. The paper
addresses theoretically a physical situation where the positron probes
see both the point defects in the crystallites and the interfaces
between the the crystallites. Closed-form solutions are obtained which
can be conveniently applied for the analysis of experimental data. The
model is not only essential for studies which aim at issues of interface
physics or nanoscaled material, but is also of relevance for studies of
point defects in polycrystalline materials when grain sizes are in the
micrometer range.
***
LC11962
Breaking of an emulsion under ac electric field
An emulsion is a widespread liquid system used in many industrial
areas like food-processing (ice-cream, mayonnaise, …), cosmetics,
paints, ... The coalescence of emulsion droplets induced by the
presence of an ac electric field can be a beneficial phenomenon used
for enhancing the destruction of water-in-oil emulsions, as for
example in oil recovery technologies, or more recently for
controlling the fusion of individual droplets in digital microfluidic
applications.
In this letter, by using microfluidic technology, we investigate the
stability regarding coalescence of droplet pairs under electric field
as a function of drop separation and ac field intensity. Three
different regimes are found: stable, coalescence and partial merging.
From this, we identify the two breaking scenarii of a one
dimensional train of droplets: in one case the coalescence front
propagates, in the other case, which for pairs corresponds to the
partial merging regime, the coalescence front can become
heterogeneous. From these findings, we can propose a destruction
mechanism for a macroscopic emulsion, which includes the packing
condition for which total and immediate destruction is effective.
***
LX11244
From JET new experimental findings on ion heat transport in tokamaks
On the JET tokamak novel experiments using radio-frequency ion
heating and a powerful ion temperaure diagnostic have allowed the
determination of the threshold for on-set of ion turbulence and of
the level of rigidity of ion temperature profiles. Both are basic
ingredients of theoretical models of turbulent core ion heat
transport, which had so far escaped detailed experimental validation.
They play a key role in determining the core ion temperature value
achievable in a fusion device for a given (first wall compatible)
edge temperature. and influence as a consequence the fusion power
production. The non-linear threshold predictions have been found
optimistic with respect to this experiment, which is rather close to
the lower linear predictions. The major observation is that the
rigidity of ion temperature profiles in the core is very high in low
rotation plasmas, in accordance with theory, but decreases
significantly at high rotation, the latter result presently not
predicted by theory. This means that rotation is highly beneficial in
a fusion device, allowing a steeper ion temperature profile and
therefore high core ion temperature in the presence of a lower edge
temperature. It also implies though that rotationless devices would
be constrained by turbulence to ion temperature profiles close to
marginal stability, no matter the heating power level. This
observation further motivates studies of novel means of inducing
rotation in future fusion devices.
***
LU11039B
Quantum hurricane forecast for a nanoscopic globe
Sometimes physics at large and small scales unexpectedly meet,
demonstrating remarkable universalities. In a recent paper in Physical
Review B, a striking similarity is revealed between the behavior of
hurricanes in the atmosphere of a planet, like the earth, and the behavior
of electric currents on a spherical shell of superconducting material just
a few hundred nanometers accross. In this "superconducting nanoshell",
hurricane-like patterns of current, called "vortices", are present and
their dynamics under the influence of magnetic fields is elucidated. The
curvature of the superconducting nanoshell leads to a vortex-free zone
(the "Meissner belt") at the equator of the nanoparticle; it is also here
at the equator where the vortices originate and migrate towards the poles,
where they aggregate in regular lattices. Also in a 3 degree band near the
earth's equator, hurricanes are rare; they originate only at the edge of
this safe zone and move towards the poles. On earth, these "classical",
large-scale vortices don't reach the poles since they dissipate over
unfortunate lands and cold water, but on planets like Saturn vortex-like
patterns were recently observed at the poles by the NASA & ESA Cassini
mission. In the nanoscopic counterpart, this mechanism can actually be of
technical use since it allows superconductivity to survive in stronger
magnetic fields.
