Scientists uncover new insights into transport of probes in crowded matrices
Scientists from University of Texas at Austin have uncovered that the
motion of small probes in crowded matrices can happen in more ways
than previously envisioned. Many critical biological processes rely
on the movement of units such as proteins and viruses through crowded
environments such as in the cytoplasm and nucleus of the cells. A
fundamental question is "What is the relationship between the
transport characteristics of such (small) units and the properties of
the medium?" Conventionally, it has been viewed that entities smaller
than the pores of the permeating matrix move by sneaking through the
static pores of the matrix, whereas moieties larger than the pores of
the matrix have been assumed to behave similar to particles moving in
thick, gooey fluids. Drs. Pryamitsyn and Ganesan at The University of
Texas at Austin have used computer simulations on a model system to
show that a novel, intervening regime can occur in the transport of
small probes in crowded matrices. They demonstrate that for probes
larger than the pore sizes but smaller than the size of the network
polymers, the motion of the matrix environment itself may open and
close spaces and facilitate the movement of the probes. Many earlier
experiments have indeed observed that transport properties of probes
may not always be rationalized within the two conventional regimes.
Drs. Pryamitsyn and Ganesan hope that their findings would shed light
on some of those results and even possibly change the way the motion
of small units are viewed in relationship to the properties of the
crowded environments they move in.
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LL11317
The transfer of electrons in or out of the molecule, the heart of an oxidation-reduction process, modifies not only the electronic properties of the molecule, but can also drastically influence its magnetism.
In our letter we show that it is possible to manipulate the magnetization direction in organic magnetic molecules by changing their oxidation state: a basic mechanism, which is common for the biomolecular world.
The oscillatory behavior of the easy axis of magnetization as a function of the oxidation state of the molecule predicted by our calculations could lead to revolutionary technological applications, e.g. it would allow combining the spin-single electron transistor principles with the magnetoresistive biosensor. In such a device an additional control over the transport mechanism can be achieved by using interplay between charge state and the magnetization direction in the channel.
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LG11339
Doping Atomic Wires for Band Gap Engineering
Doping is the key technique in electronic and optical devices. Although this technique is also crucial for any nano device fabrication, its application into nano-scale materials has been challenging due both to fundamental and technical reasons. In this letter, we successfully demonstrate an extreme application of doping even beyond the conventional nano-scale materials, that is, doping atomic-scale wires of only one-nanometer width. The atomic wires are self-assembled on a silicon surface by gold atoms and extra silicon atoms are deposited on as electron doping adsorbates. The gold atomic wires are imbedded into the surface silicon layer to have a robust structure and a well defined one-dimensional metallic band structure with a strong dispersion. The one-dimensional band of gold atomic wires, measured by angle-resolved photoemission, changes from a fully metallic to semiconducting one with its band gap linearly tunable up to about 0.5 eV as the silicon dopant density increases. This paves a way to create novel atomic scale devices based on wires with engineered band structure.
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LM11144
Terahertz Generation from Optical Pulses in a Nonlinear Negative
Refractive Index Metamaterial
"In this paper, the authors have theoretically discovered that terahertz
waves can be generated from optical pulses in a nonlinear negative
refractive index medium/metamaterial. Terahertz radiation has a variety
of important applications including imaging, sensing, security and
spectroscopy. The nonlinear phenomenon that enables this conversion is
long wave short wave resonance and occurs when the group velocity of a
short wave (e.g. optical) is equal to the phase velocity of a long wave
(e.g. terahertz). This resonance phenomenon was first studied in fluids
and plasmas over 30 years ago. In negative index metamaterials, it is
possible to satisfy the resonance condition and generate long waves when
the short wave lies in the region of negative index. In addition to the
significant application of optical to terahertz conversion, other
phenomena that may be realized are solitary waves and photonic
turbulence. Long wave short wave resonance brings a new horizon to the
research and applications of the exciting area of metamaterials."
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LB11396
Stable liquid Hydrogen at High pressure,
discovered by generating noise with the computer.
The fundamental laws of physics and chemistry can be now put in a computer
for a realistic simulation of matter, and for the discovery of new phases that
are not always accessible by experiments, as to go inside the
inner core of Jupiter where the pressure can be up to 300Gpa, namely
three million times the standard atmospheric pressure.
In this work we show that, by generating fictitious
noisy numbers (pseudo-random numbers) with common computers,
we can perform accurate simulations,
by means of the so called quantum Monte Carlo technique,
and exploit very important correlation effects between electrons.
A quantum many body system of up to 128 Hydrogen atoms and 128 electrons
interacting by means of Coulomb forces
and for a time of about 2ps (1ps=10^{-12} sec), has been
simulated at 400K and 300GPa.
Though 2ps may appear a very short time,
it is meaningful for a computer simulation of condensed matter phases.
Indeed in our calculations we have been able to melt
the most plausible high-pressure solid phases, strongly supporting
the thermodynamic stability of the liquid phase in high pressure Hydrogen.
Before this work it was not possible to follow the faithful dynamical
evolution of the quantum mechanical electronic wave function with so many
electrons, or without doing ''mean-field'' like approximations where
each electron see the effect of the other ones only on average,
namely wthout feeling their correlation, seemingly very important
at this very high pressure.
We believe that our work open a new frontier for the discovery of novel
phases of matter with realistic computer simulations.
***
LH11026
Cloaking Goes Multi-Band!
There has been a great deal of interest in the subject of electromagnetic cloaking in the scientific communities and news media in the past few years. The currently available techniques for cloaking have so far been limited to a given fixed frequency and narrow bandwidth of operation. Now, in their upcoming theory paper in Phys. Rev. Lett, researchers from the University of Pennsylvania have proposed a technique to obtain the cloaking effects simultaneously at two (and in principle more) frequencies. Their method, which is derived from their earlier approach to cloaking [Phys. Rev. E 72, 016623 (2005)], is based on the mechanism of scattering cancellation by metamaterials and plasmonic media, and it is achieved by utilizing layered plasmonic covers. These cloaks may be isotropic and homogenous, and the effect is relatively robust to variations in the object and cloak parameters. This may pave the way to multi-band cloaking, with various exciting possibilities and applications.
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LJ11439
A scalable method to detect quantum critical points
Phase transitions describe sudden changes in
the properties of a physical system when an external control
parameter changes through some critical value. If the system under
consideration is a quantum mechanical system in its ground state,
i.e. at zero temperature, and the phase transition occurs as a
function of a non-thermal control parameter, we speak of quantum
phase transitions (QPTs). In this paper we demonstrate a technique for studying QPTs
by coupling the system to a probe qubit, i.e., a two-state system.
It uses directly the increased sensibility of the quantum system
to perturbations when it is close to a critical point.
Using a nuclear magnetic resonance (NMR) quantum simulator, we demonstrate this measurement
technique for the QPTs in an Ising
spin chain. Only one qubit is measured for the detection of the critical points,
independent of the size of the simulated quantum system. Hence this
method scales very favorably with the size of the system
***
LJ10838
FRACTALS: THE ROLE OF THE UNDERLYING GEOMETRY
Fractals are geometric entities of fractional dimension. While our common perception shows us two-dimensional objects such as planes or three-dimensional bodies in the everyday experience, these entities can be found everywhere in the physical world. Examples of fractals are the coasts of islands and continents, crystal surfaces, tumors, fire fronts, and even abstract paintings. Studying the geometrical properties of these systems sheds light on their physical properties, and helps us understanding important phenomena such as crystal and tumor growth. Fractals are “rough” versions of objects of lower integer dimension, for instance, a fractal of fractional dimension 1.1 is a rough counterpart of a line. What we have shown in this article is that the geometry of this lower dimensional space affects dramatically the dynamical properties of fractals. Curved spaces induce a completely different behavior than the extensively studied planar ones. These considerations affect strongly the way of analyzing semiconductor surfaces, due to the presence of circular atom or vacancy islands formed on them, or growing tumor spheroids, just to cite a couple of examples of a large practical importance. The long time goal is to be able to understand the basic growth mechanisms and even to control them.
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LK11469
"Periodic electron structures in gases: a fluid model of the 'Window' phenomenon"
The seminal experiment of Franck and Hertz carried out almost one hundred years ago helped lay the foundations of modern quantum and atomic physics but surprisingly, what really happens in the experiment remains poorly understood and is sometimes even misrepresented in the physics literature at large. Current is past through a gas, and for the experiment to work as planned, periodic structures reflecting quantization of the atoms must develop. They are observed to do so only in a certain range ('window') of well-defined voltages and gas pressures. This paper explains in the simplest possible terms, consistent with physical rigor, the origin of this 'window' , and provides the theoretical machinery for dealing with similar structures which occur in modern day low temperature plasmas.
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LM11709 and LM11738
In a two papers that appear back-to-back in Phys. Rev. Lett, physicists
from The University of Texas at Austin report the realization of general
methods for trapping and cooling of atoms and molecules. These methods
will be applicable to most of the periodic table as well as many molecules.
To date, cooling atoms near the Absolute Zero has been accomplished using
laser cooling. Despite its enormous success, laser cooling has been
limited to a small set of atoms in the periodic table due to the
requirement for a closed cycling transition that is accessible with lasers..
The Texas group, led by Professor Mark Raizen, stopped atoms by passing a
supersonic beam through an "atomic coilgun" and cooled atoms using
"single-photon cooling".
The starting point for the work is the supersonic beam, a source of cold
atoms that are also moving very fast in the laboratory frame. The beam is
mostly comprised of noble gas atoms that can be seeded with any species
that has a permanent magnetic moment. These paramagnetic atoms are then
stopped by a coilgun, a series of 64 electromagnetic coils that create
large pulsed magnetic fields. The coilgun stops atoms by making them climb
a magnetic hill which is removed before the atoms have time to roll off and
regain speed. Key to the success of the coilgun is the use of supersonic
beam technology developed by Raizen's collaborator, Professor Uzi Even,
from Tel-Aviv University. This method will work on any paramagnetic atom
or molecule which can then be held in a magnetic trap. To further cool the
trapped atoms or molecules, the Raizen group developed the method of
"single-photon cooling". This is based on the construction of a "one-way
wall of light" proposed by the same group in earlier publications. An
ensemble of atoms is cooled from a magnetic trap into an optical tweezer,
where each atom scatters on average only a single-photon. The method does
not require a closed cycling transition, and hence is completely
general. It is also a direct experimental realization of the concept of
information cooling as introduced by Leo Szilard in 1929 to resolve the
paradox of Maxwell's demon.
This combination of general methods opens many new directions in physics
and chemistry. The Raizen group plans to focus on trapping and cooling of
atomic hydrogen isotopes, primarily atomic deuterium and atomic
tritium. The latter is particularly important towards determination of the
neutrino rest mass, one of the most pressing questions in physics
today. On the other extreme, this work opens the possibility for trapping
and cooling of molecules, which will enable the study of ultracold
chemistry and precision molecular spectroscopy.
***
LM11265
De Sitter Universe goes Quantum
When well-known Dutch physicist and astronomer Willem de Sitter in 1917
came up with a new solution to Einstein's recently discovered equations of
general relativity, he could hardly have anticipated its enduring appeal
as a model of the real, expanding universe. But what would have been
impossible for him to even contemplate was that his de Sitter universe -
as it is now known - would one day be shown to arise from the most basic
principles of quantum theory, a theory not even developed at the time.
This is exactly what has been achieved recently by a team of researchers
from Denmark, the Netherlands and Poland. In their model of the universe,
tiny building blocks, which represent the "atoms" of space-time and
interact according to the laws of quantum theory and gravity, magically
organize themselves into a big lump of space-time which on macroscopic
scales looks just like de Sitter's universe. In an area of science often
regarded as esoteric, this work provides tangible evidence that not only
the material world, but also the structure of space and time can
ultimately be traced to fundamental quantum origins.
