Researchers at the University of Electro-Communications (UEC) in Tokyo,
led by Professor Ken'ichi Nakagawa, have used the weirdness of quantum
mechanics as a tool to build a new type of ratchet.
A ratchet is a device which converts back and forth movements into
forward motion. Scientists are interested in such behaviour on a
microscopic scale to explain movement in biological systems and also as
a novel means to transport atoms in a desired direction. However, for
atoms, so-called "quantum interference" becomes important, usually
leading to the ratchet effect stopping after a certain time. Now, a new
approach pioneered at UEC has created an atom ratchet which keeps on
going. The trick is to prepare atoms in a so-called "superposition"
where objects may be in two different states at once due to the strange
laws of quantum mechanics. Atoms are initially prepared to have zero and
non-zero speed simultaneously. If light pulses are then applied from
both sides to this state, movement occurs predominantly in one
direction. Unlike previous atom ratchets, in this case quantum effects
allow the motion to continue, rather than eventually stopping it. The
team plans to extend their technique to allow the storage and readout of
information using atoms. LC11385
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An Elastic Pattern Switch
Nature makes extensive use of periodic and layered structures to achieve different properties and attributes. The brilliant coloring of many birds, butterflies, beetles and fish as well as the hydrophobic character of desert beetles and lotus leaves originates in their surface structures. Mimicking the complex periodic forms of the natural world in the manufacture of physical devices is a significant challenge at the required length-scales. The results of our investigation show that the application of a simple load to a periodic structure can trigger an unexpected global pattern switch above a critical point. This enables the possibility of creating prescribed complex patterns on currently available periodic lattices and also switching certain properties on and off with deformation. The effect is both reversible and repeatable. Its origin is an elastic instability so that it occurs over a narrow range of the applied load making precise switching practical.
Our results were obtained in samples at the millimeter-length scale relevant to phononic crystals but the effects should persist at the micro- and nano-scales. At this level, the new elastic pattern switch provides the exciting prospect controlling photonic crystals to manipulate light. LC10939
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Capillary filling - a long-known effect with novel applications
Whether designing a modern ⿿lab-on-a-chip , DNA electrophoresis chips, or
nanowires of 1 nm diameter ⿿ one resorts nowadays to a long-known
phenomenon: if a capillary is inserted into a liquid, the liquid will rapidly
flow into it (fluid imbibition). Described and understood 90 years ago in terms
of well-defined macroscopic quantities such as surface tension, meniscus
curvature, and contact angle of wetting, this phenomenon needs reassessment in
the modern world of tiny (nano)-scale lengths where a nanotube diameter may
span several atom diameters only, and the discrete nature of matter cannot be
ignored. Now, checking the validity of classical laws within the realm of
nanoscales is most efficiently done by computer experiments, most notably by
Molecular Dynamics simulations of model systems.
To this end we simulate both a simple liquid, such as a liquefied noble gas,
and a model of polymer melt, like silicon oil, letting them fill a super narrow
capillary(with diameter of about ten times the atom size), immersed in the
liquid. In both cases, after a transient period (of a few nanoseconds), a
meniscus is observed to move up the tube on a distance which grows as a square
root of the time elapsed, exactly as predicted by Lucas and Washburn nearly a
century ago. As expected, the speed of filling increases as the square root of
the tube diameter. The flow velocity is maximal in the middle of the capillary
and goes gradually down to zero at the capillary wall.
For the polymer melt, however, it turns out that the melt slips with non-zero
velocity even immediately at the wall! The hypothetical distance from the wall,
where the extrapolated flow velocity would have reduced to zero, defines what
is called a slip length, and in our computer experiments this slip length is
comparable in size with the nanotube radius. Therefore, a straightforward
application to nanotubes of the classical Lucas-Washburn law for capillary
filling would not work. Our computer experiments show, however, that a
consistent description of the imbibition process in nanotubes is still possible
upon a simple modification of the Lucas-Washburn law whereby the capillary
radius is incremented by the observed slip length of the flow.
Thus a knowledge, accumulated over decades of scientific research, may now be
readily transferred to modern nano-applications and nano-technology.
Snapshot of a capillary being filled by a simple liquid upon immersion
into a reservoir. The magnifying glass is focused on the meniscus area
and shows the velocities of particle between tube axis and wall against
a density profile of the liquid. LC11834
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Anderson localization goes oscillating
Have you ever thought how much "periodic" can be a random system?
Does natural disorder hide anything strictly periodic? A simple
model of a disordered optical system suggests that its mean
transmission should oscillate with sample length. The model,
supported by numerical experiments, shows for the first time that
the transmission periodically rises and drops as the system gets
thicker. Physicists know since half a century that Anderson
localization is the phenomenon ruling wave transport mechanism in
disordered systems. It is also widely accepted that the transmission
drops exponentially for thicker samples. Now the new results
demonstrate that in a generically produced random mixture of two
different units, e.g. special-sized "black" and "white balls", the
exponential law appears modulated by periodic oscillations.
Responsible for these unexpected oscillations are rarely forming
special states, known as "necklace states", which induce direct
transmission channels even through very thick samples. Such
necklaces periodically appear to be efficiently transmitting each
time the system reaches special thicknesses. The edifying feature of
the effect found adds a new insight into the physics of Anderson
localization. LC11146
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.JPG)
Modified Thermal Emission from Self-Assembly: Opals Meet the Light Bulb
Han et al. predict that if a heated metal is first structured through simple self-assembly techniques, the color of its “glow” can be altered in useful ways. A heated object emits light according to its temperature and optical properties. This phenomenon, known as thermal emission, is utilized in conventional light bulb filaments. Unfortunately, light bulbs emit a large quantity of heat, resulting in inefficiency. Photonic crystals — materials that are structured on a micrometer length scale — are being explored for eliminating this heat, potentially providing more efficient thermal emission sources. However, the study of this effect has been limited due to difficulties in making the appropriate structures. Previously, photonic crystals have been obtained by emulating the natural process that leads to gemstone opals. Unfortunately, experiments on these self-assembled structures suggested that they were ill suited for modifying thermal emission. Han et al. considered the physics of this problem and found small, experimentally realizable changes to the structure that should lead to modified thermal emission. Thus, these new self-assembled photonic crystals, which are easy to prepare, have great potential for modifying thermal emission both for fundamental understanding and applications.
LC11750
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Single-Molecule Force Spectroscopy Reveals Protein Folding Free
Energy Landscapes
Understanding biomolecular interaction is one of the most important
question of biological physics. Atomic force microscopes allow one
to unfold proteins or stretching molecules essentially by hand and,
with the help from modern statistical theory, to reconstruct the
free energy landscape of such interaction.
Kiang's group used a microscopic cantilever attached to the
atomic force microscope tips, to pull a protein while measuring
the protein's reaction force by the bending of the cantilever.
Using the modern nonequilibrium work theorem, the group
mapped the protein's free energy landscape along the entire unfolding
trajectory, which had not been accomplished to date by any other
methods. The new technique can be applied to most any biomolecule,
complex, or molecular interaction. LU10757
***
Magnetic Explosions
We studied the way microscopic spins in magnetic materials move upon
magnetization reversal, such as materials that are used in hard disk
drives. These systems can be designed to have avalanches where only
a few spins are needed to start a cascade, leading to many flipped
spins, for example, as you see with dominoes. However, we show that this
process is much more analogous to an explosion than had been previously
thought. Instead of spins just flipping once from up to down, they
continue to spin around, that is, the spins get hot, and in many materials
they stay hot for a substantial amount of time. This greatly alters the
microscopic physics of the system, which means that a change in one part
of a magnet can affect the behavior much further away. The hot region
forms a growth front and the heat from it recruits neighboring spins
into the explosive region before they eventually cool down. We analyzed
this behavior in detail using simulations and theoretical models and
were able to determine under what circumstances an explosion takes off.
The attached graphics shows a simulation of spins in a film during an
avalanche for: (a) high damping, and (b) low damping. The brightness
is a measure of the amount of spin motion. LR10732
***
From geometry class, we know that we can think about shapes that live
only in a two-dimensional world, but can real materials behave as if
they live in Flatland? New measurements of current flow in a special
type of copper-oxide high-temperature superconductor provide evidence
for two-dimensional superconductivity coexisting with one-dimensional
rivers of charge. The superconducting state, involving resistance-less
flow of electrical current, requires that conduction electrons pair up
and be able to flow in three-dimensional (3D) space. In copper-oxide
superconductors, the interactions that cause the pairing occur in 2D
planes; weak interactions between the planes allow the supercurrents to
flow in 3D. In the special copper-oxide compound considered here,
experiments indicate that the charge carriers tend to segregate into 1D
rivers known as "stripes". It had been common belief that the ordering
of charges into stripes within the Cu-O planes would prevent or compete
with superconductivity. The new measurements of current flow
contradict these expectations and are consistent with theoretical
predictions for superconductivity in Flatland. The observation of 2D
superconductivity within a 3D crystal over an extended temperature
range is extremely unusual, and the coexistence with 1D stripes
indicates the surprising nature of superconductivity in copper-oxide
compounds. LC11292
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Face-to-face with correlated wave functions
The microscopic description of strongly interacting systems is
one of the latest frontiers in condensed matter theory.
Indeed, several materials which show spectacular - and still debated -
properties, like high-temperature superconductivity, superfluidity
and quantum Hall effect, go with a strong repulsion among particles.
The aforementioned phenomena generally manifest themselves at very low
temperatures, where the quantum-mechanical nature of particles comes
into play. As a consequence, all information related to their
collective quantum behavior are encoded into the ground-state
wave function of the system.
Nonetheless, the derivation of the ground-state wave function in presence of
strong correlation is a complicated issue, which so far has been solved in
few cases.
From the experimental point of view, the recent developments on cold atoms
trapped in optical lattices allow one to have a direct control on the
various couplings entering into the game, giving the unique opportunity to
make a close connection with theoretical models and to answer
fundamental questions.
In particular, by tuning the lattice potential depth on trapped-bosonic atoms,
it is possible to control the ratio between the kinetic energy (that tends to
delocalize the particles) and their local repulsion (that instead tends to
localize). It is therefore possible to drive the system from a superfluid to
a Mott insulating state. This behavior, that is also expected in electronic
systems, reflects the dramatic change in the electronic/bosonic arrangement
across a quantum phase transition.
In this work, we tackle the problem by introducing a wave function, which
turns out to be very effective to model a system of interacting bosons in
one, two, and three dimensions. We find that
the essential ingredient in order to capture
the physics of a strongly-interacting system is a two-body correlation term,
known as Jastrow factor.
In presence of this term, particles ''feel each other'' over all possible
distances, making it possible to correct the mean-field wave function and
describe a superfluid-insulator transition.
Remarkably, we find for the first time that the long-range
character of the Jastrow terms is essential in order to correctly reproduce
both the superfluid and the Mott insulating state.
Moreover, our accurate description of the correlated ground state allows us to
explain the peculiar excitation spectrum found in the
experiments of Stoferle and collaborators.
Indeed, the different behavior of the Jastrow correlation factor
across the transition naturally induces high-energy excitations in the
superfluid state in two and three dimensions, but not in one dimension.
These high-energy excitations signal that the superfluid state already contains
the main fingerprints of the correlated insulating state.
This is an interesting and also surprising result, that bears a
lot of similarities with the metal-insulator transition in electronic systems,
but is not accounted for by most accepted theories for bosonic systems.
Moreover, this general scheme indicates that this
class of wave function permit to have access to the Mott
transition in three-dimensional fermionic models, too,
all the more reason when realistic Coulomb interaction is taken
into account. LY10312
***
The addition of randomly scattered waves gives rise to a speckled intensity
pattern. The evolution of such patterns as the frequency scanned is observed
for microwave radiation, however speckle patterns arise for any type of
wave, including light, sound and electrons. Though the pattern is random, it
is not devoid of structure. A network of phase singularities appears about
points of vanishing intensity. We show that the motion of these points of
darkness provides a diagnostic of the nature of wave transport within the
scattering object. The probability distributions of the displacement of
phase singularities together with the variations of phase and intensity
throughout the speckle pattern are governed by a single parameter, which
provides a precise measure of the degree of spatial localization of the wave
within the sample. For exponentially peaked localized waves, the speckle
pattern seems to move in fits and starts as opposed to the smooth
development of speckle for extended waves. This reflects the degree to which
modes within the sample overlap. The paper provides sharp measures of
speckle evolution which can be exploded to monitor motion of hidden objects
and the development of structural defects. LY10404
***
Femtosecond field ion emission by surface optical rectification
Recent experimental evidence from separate fields of laser physics converges into indicating
that ultra short laser pulses may remove atoms from a metal surface within a time as short as
few hundreds of femtoseconds. This effect might lead to interesting applications such as nano
machining of materials with a control of the surface roughness at the atomic level. One
example is the recent advent of the atom probe tomography technique in which laser pulses
are used to remove a tip like shape sample material atom by atom allowing a full three-
dimensional image of the material to be obtained in the real space. Another example is the
possible strong anisotropic ablation of suspended gold nanoparticles induced by 100 fs laser
pulses. Finally a non-thermal ablation regime of planar metal surfaces has been reported. In
all these cases the underlying mechanism of this ultrafast emission is obscure and might be
ascribed to some yet unidentified nonlinear surface optical effect. In this article a possible
explanation for these observations is presented based on the optical rectification of the light
wave at the surface of materials. Moreover, we put forward the working hypothesis that the
same effect may be also at the root of the reported ultrafast ablation phenomena. LB11245
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Unraveling a granular labyrinth
We describe a new pattern formation process where beautiful labyrinth
structures emerge as a mixture of grains and fluid slowly dries out. Key
to the pattern formation is an interesting interaction between granular
and fluidic influences. The granular-fluid system is confined between
two glass plates, and the receding fluid/air meniscus compiles a layer
of compacted grains and becomes unstable, whereby fingers of air invade
the system in a random fashion. We show that a uniform characteristic
length-scale in the pattern develops as a compromise between two
opposing forces: the surface tension of the fluid, and the frictional
stress resulting from grain-grain contact networks within the compacted
granular layer. The granular labyrinth is visually striking, with
characteristic length-scales up to several centimeters. An understanding
of the pattern formation process is developed along three complementary
paths: experiments, simulations and theoretical considerations. LC10922
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Control and dynamics of vortices in a turbulent flow
Vortices and similar structures appear continuously in nature: from
tornadoes to vortices at the tip of plane wings,
from the Jupiter's Great Red spot to the smoke of a cigarette, the fluid
elements tend to form stable structures.
The understanding of their dynamics and stability are important both
from the basic and applied points of view:
alterations of the environmental conditions can produce structural
changes on these flows.
The mechanisms at the origin of these changes and the role played by the
structures themselves are still subject of debate.
That¿s why experiments carried out in controlled environments are so
important: the parameters that regulate
the evolution of the fluid (i.e. pressure, propellers) can be easily
modified, in such a way that the flow can be
destabilized and new structures or configurations can arise. This work
shows that the vortices in a cylindrical
cavity can present a rich dynamics, with random or forced flow
inversions. This dynamics can be explained
using an astonishingly simple toy model. LB11142
Attached figure: Picture of an experimental vortex in the cylindrical cavity
***
New type of electron-nuclear spin polarization
The electron spin in semiconductor quantum dots is currently extensively investigated in the context of quantum information processing. The interaction with the nuclear magnetic moments of the lattice atoms constitutes the major source for the loss of the electron spin. We, for the first time, demonstrate the formation of a strong nonequilibrium dynamical nuclear polarization in semiconductor quantum dots under continuous optical pumping of the electron spin at zero magnetic field. Unlike the vast amount of previous work on semiconductors based on the standard spin cooling regime, the formation time is as short as 20 microseconds. We also uncover a life-time of about 200 microseconds due to nuclear dipole-dipole decay. Specific of our study is the use of quantum dots where the electron spin is coupled to only a few hundred nuclear moments and where the Overhauser field does not exceed markedly the spectral broadening of the Zeeman levels. The nuclear polarization is almost complete and survives up temperatures of about 100 K. In this way, the electron spin is stabilized and the coupled electron-nuclear system exhibits a variety of novel features that might be used for quantum bit implications. LC11325
