Exciting Rydberg atoms in Bose-Einstein condensates
In the third of a series of Letters on coherent Rydberg excitation of
dense ultracold atoms, the Pfau group in Stuttgart reports on the
investigation of Rydberg atoms in a Bose-Einstein condensate (Heidemann
et al., PRL in press). Due to the strong interactions between the
Rydberg atoms up to 10000 atoms team up as a so called "super atom" to
share a single Rydberg excitation in a coherent way (Heidemann et al.,
PRL *99*, 163601 (2007)). In another experiment they prove the coherence
of the excitation despite strong interactions and investigated dephasing
processes (Raitzsch et al., PRL in press).
Rydberg atoms are several thousands of times larger than normal atoms.
The loosely bound electron is extremely sensitive to electric fields and
to other nearby Rydberg atoms. Therefore Rydberg atoms can talk to each
other on distances of around 5 micrometers. This is 50 times their own
size and half the size of a human red blood cell - huge distances for
atoms. At the same time Rydberg atoms can store quantum information by a
well protected nuclear spin. That's why they are discussed as a possible
system for the implementation of quantum algorithms (Lukin et al. PRL
*87*, 037901 (2001)). Besides this Rydberg atoms in a BEC will be able
to model coherent many body physics far from equilibrium and impurities
in a quantum gas.
***
LK11168
Smectic vortex phase at high fields in optimally-doped high-temperature
superconductor
A long-standing debate about the existence of a smectic vortex phase in
"low anisotropy" high-temperature superconductors is settled by a study
of optimally-doped YBa(2)Cu(3)O(7) films at pulsed fields larger than 40T.
Deep inside the superconducting state when a magnetic field penetrates a
superconductor, vortex matter can exist in solid or liquid phases. The
nature of these phases is given by that of the pinning centers holding
the vortices in place. Pinning centers enable films to carry current
without dissipation, which is extremely important for applications of
superconductivity. When magnetic field or temperature increases, the
vortex lattice crosses the melting line and becomes a liquid with the
concomitant increase of the electrical resistance.
In this letter we report electrical resistivity measurements as a
function of angle, field, and temperature. We confirm predictions for
layered superconductors by finding that near 80 K, when the magnetic
field (H>40T) is aligned with the layers, the melting line turns upward
and the critical exponent becomes similar to that of the
nematic-smectic-A (liquid-crystal) transition. Also we observe that up
to the highest fields measured (50T) correlated defects arrest the
motion of vortices well into the liquid phase.
***
LJ11359
Spin-flips in electron quantum fluids now come in pairs
When exposed to certain strong magnetic fields, a system of electrons confined in a very thin layer of a semiconductor at temperatures just a few hundredths of a degree above absolute zero can condense into a remarkable state called a fractional quantum Hall (FQH) liquid. In this work, we discover a fundamental excitation in a FQH system in which two electronic magnetic moments (spins) are simultaneously flipped, in contrast to the already known single spin-flip processes. The excitation is selectively induced by laser illumination. We find that, unlike other excitations of the system, the two-spin excitation becomes more predominant as the temperature is increased. This suggests that in the system there are in fact two distinct but coexisting phases of electrons that support different kinds of excitations. These results offer new insights into FQH systems, particularly on how this state is destroyed as the temperature increases. Indeed, two characteristics of such systems are the absence of electrical resistance in the direction of current flow, and a strong alignment or polarization of the spins. These two properties are lost as the sample is warmed up, and the two-spin excitation may have an important role in these processes.
***
LD11333
The discovery of two energy-scales in under-doped cuprate materials sheds
light on the mechanism of High-Temperature Superconductivity
Spectroscopy and photoemission experiments have recently brought into
focus the concept of two energy-gaps with opposite doping dependence in
cuprate-based superconductors, reviving the experimental and theoretical
debate on the mechanism of high-temperature superconductivity. A theory
for superconductivity in the cuprates is extremely difficult to develop,
as the strongly-interacting many-body nature of high-temperature
superconductors does not allow employing standard theoretical tools. In
this work we study the superconducting phase of a two-dimensional model of
electrons, using new powerful theoretical tools (a cluster-extension of
Dynamical Mean Field Theory). Our study shows that, in the region of the
phase diagram close to the superconductor-insulator transition (driven by
varying doping), the gap observed in photoemission displays indeed two
components, which have distinct behavior as a function of doping. A first
component has origin from superconductivity and decreases with decreasing
doping. A second component has an additional contribution from a
“pseudo-gap” already existing in the normal-state and increases with
decreasing doping. Our result shows that the two-component gap is a key
feature of the superconducting phase, in agreement with recent experiments
on low-doped superconducting cuprates.
***
LL11226
How Sacred are the Laws of Thermodynamics?
Einstein believed that thermodynamics is the only physical theory which
"will never be overthrown".
While the first law -- the conservation of energy -- is universally
understood and accepted, the conditions
of applicability of the second law are much more subtle. The second law
prohibits the existence of perpetual motion
machines. It also requires that for an isolated system,
the degree of disorder -- entropy -- be maximum.
One of the consequences of the second law
is that the temperature of coexisting thermodynamic
phases -- such as liquid and its vapor --
must be the same. There is, however, an important class of systems
which seems
to violate the second law of thermodynamics. Consider a plasma (gas of
charged particles such
as electrons) confined by a potential (in the paper we studied a
specific example of charged
particle beam, confined by a magnetic potential). Under very broad
conditions, the plasma will
separate into coexisting liquid-like (high-density) and gas-like
(low-density) phases. However,
unlike for normal fluids, the two coexisting phases are at very
different temperatures -- the
high-density core is very cold, while the low-density halo is very hot!
From the practical point of view,
the halo production is very detrimental to the efficient propagation of
intense charged particle beams
and must be minimized as much as possible.
The theory presented in the paper explains how
this unusual phase transition can be predicted theoretically and how it
can be reconciled with the
second law of thermodynamics.
***
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LG11576
Biological physics of snake hearing
A novel theoretical analysis has provided the first explicit model for
the biophysical and neuronal bases of hearing in snakes. The authors
trace surface vibrations in the sand on which a snake lives, through the
two sides of the lower jaw and middle ear, detail the conversion of
vibrations into neuronal signals at the cochlea, and explore how these
stereo signals can lead to accurate azimuthal localization of the
stimulus. One of the key methods used by the authors is to treat the
movements of the substrate, viz., dry sand, as water waves and apply
nautical engineering to model both sides of the lower jaw as "boats"
moving with these waves. The work builds on behavioral studies of snake
hearing showing high sensitivity, and supports an earlier assertion that
snakes have re-evolved the ear. This "new" hearing system is proposed to
be based on a Jeffress-like system of interaural time differences (ITDs)
that are integrated to achieve azimuthal localization. If supported
experimentally, this would provide additional evidence for the repeated
evolution of a Jeffress system.
***
LA11373
Visualizing Rapidly Migrating Hydrogen by Molecular Fireworks
We have demonstrated the visualization of hydrogen atoms rapidly
migrating within a molecule by using ultrashort intense laser
pulses. Highly charged molecular ions formed by such laser pulses
subsequently explode into fragments due to the strong electrostatic
repulsion among the positive charges. The structure of the molecules
at the instance of the laser irradiation can be studied by measuring
precisely the momenta of the fragments. This can be compared to
fireworks exploded in the night sky, which reflect how the “stars”
containing sparklers are embedded in the shells. In this paper, we
studied the location of hydrogen atoms in deuterated acetylene
dication, by igniting the “molecular fireworks” at different times
after the creation of dication. The migration is found to proceed in
a recurrent manner: One of the hydrogen atoms shifts from carbon site
to the other in 90 fs (1 fs = 10^-15 s) and then migrates back to the
original site by 280 fs. Hydrogen migration plays important roles in
various chemical reactions such as the synthesis of vitamin D in
skin. The direct visualization demonstrated here will provide a
deeper understanding of such chemical reactions as well as new
prospects for their control.
***
LK11295
The motions of graphite under the light
When a pencil traces a line, small two-dimensional flakes of graphite (termed graphene) are detached from its tip and deposited on paper. The weakness of the bond keeping together graphite's two-dimensional layers is responsible for this behavior. When such a weak bond is destabilized by an ultrashort laser pulse, graphene layer to layer distance shrinks, while common materials immediately react to a temperature jump with an expansion of the unit cell. Depending on the intensity of the laser pulse, contraction can be followed by an expansion able to damage the surface of the crystal.
We perturb the material with ultrashort laser pulses which, in graphite, create a charge distribution able to selectively destabilize the bond perpendicular to the graphene layers. The reaction of the material to this perturbation is observed by capturing subsequent snapshots of the crystal structure with ultrashort electron pulses. The simultaneous use of photons and electrons, which thanks to their very short associated wavelength provide spatial atomic resolution, allowed us for the first time to produce a movie of the non thermal motions of graphite`s lattice. These motions are responsible for many mechanical behaviors of graphite, a material widely used in nuclear reactors or electronic technology, such as its damage under radiation or its dry lubricant properties.
***
LF11567
Drawing Graph of Graphene with Large Spin
The unexpected emergence of magnetism in carbon is now explained with a graph theory argument, which also suggests a venue of building spintronics circuits by sculpting graphene, a single sheet of carbon atoms. Traditional magnetism has been relying on metals such as iron for thousands of years. Recent experiments however reported magnetic order in carbon, a central element for life and organic chemistry. Such carbon magnetism is still under intensive debate because the underlying mechanism of the magnetic order remains illusive. In this work, we demonstrate through a topological argument and ab-initio calculation that large spin naturally arise in the honeycomb network of carbon atoms through a mechanism called topological frustration, i.e., all the pi bond between nearest-neighbor atoms can not be satisfied simultaneously in a random broken hexagonal network. This principle is facilitated by wisdom in graph theory revealing the connection between singularities of a hexagonal graph and its maximum nonadjacent vertices. Based on this principle, one may introduce large spin and desired spin distribution at room temperature by sculpting graphene, a recently discovered material touted as the basis for next generation electronic and spintronics devices. The potential application of these sculpted graphene circuits is tantalizing for its low spin-orbital coupling, flexibility, light weight, and biocompatibility.
***
LG11625
Researchers predict that electric current can inhibit stress-induced cracking on metal surfaces
Stressed crystalline solids are particularly important materials in applications ranging from aerospace and nuclear engineering to microelectronics and nanofabrication. These materials, however, are known to undergo surface morphological instabilities, which may introduce various catastrophic defects, such as cracks, into the components or devices made of these materials. In a recent theoretical study that has been accepted for publication in Physical Review Letters, Tomar, Gungor, and Maroudas have shown how such surface morphological instabilities can be inhibited through the action of an external force on the solid material simultaneously with the mechanical stress. Specifically, these researchers have demonstrated that the simultaneous action of a sufficiently strong electric field on an electrically conducting stressed solid (such as a metal) can inhibit stress-induced crack-like surface instabilities. Therefore, electric current can stabilize surfaces of stressed solids that are otherwise vulnerable to cracking. This is facilitated through the mass transport phenomenon of surface electromigration. Their work has broader implications for the stabilization of materials structure and morphology through the simultaneous application of multiple external forces.
***
LG11546
Compressing stars
In this work, purely stellar matter was compressed for the first time.
"Pieces of star"» are exotic beam of 56 Nickel, created in laboratory.
Thanks to a revolutionary gaseous target called MAYA, they reach an
excited state, the giant monopole resonance, in which nuclei " breath"
through a compression-dilatation mode. This breakthrough, which is also
valid for neutron-rich exotic nuclei, paves the way to the exploration
of the equation of state of asymmetric nuclear matter, which plays a
pivotal role in compression-explosion scenarios of supernovae, or in the
structure of pulsars and neutrons stars compressed by the gravitational
attraction.
***
LJ11334
Decompressing emulsion droplets favors coalescence
An emulsion is a widespread liquid system used in many industrial
areas like food-processing (ice-cream, mayonnaise, …), cosmetics,
paints as well as oil recovery. It is a metastable mixing of two
immiscible liquids, one being dispersed in droplets in the second
one, and surfactants as stabilizing agents. Taking advantage of the
recent development of microfluidics, we investigated the
destabilization process of an emulsion under flow in a microfluidic
device. The experimental approach enables us to generate a periodic
train of droplet pairs, and thus to isolate and analyze the basic
step of the destabilization, namely the coalescence of two droplets
that collide. We demonstrate a counter intuitive phenomenon:
coalescence occurs during the separation phase and not during the
impact. Separation induces the formation of two facing nipples in the
contact area that hastens the connection of the interfaces prior to
fusion. We note also that the destabilization mechanism is
responsible of a cascade of coalescence events in a compact system of
droplets, a phenomenon that may explain rapid destruction of
macroscopic emulsions.
