Bare Lithium barely understood
Investigating the interactions of electrons in an atomic system is at the
heart of understanding quantum mechanics. We have studied the interaction
of lithium (Li) atoms with soft x-rays (i.e. high-energy light)
coming from a synchrotron light source. At a high enough energy of the
light a rare event can occur in which all 3 electrons of a Li atom are
ejected simultaneously due to the interactions among the electrons leaving
just a Li nucleus behind. We have precisely measured the probability to
remove all 3 electrons by counting the number of Li nuclei at several
different energies of the light. Our measurements cover a significantly
larger energy range and are more precise than previous measurements
allowing us to make stringent tests of different theoretical models even
for high energies. We find that advanced theoretical models are still
having problems to describe that probability over the whole energy range
Our study will hopefully lead to theories that are able to better describe
the interaction of electrons not only in Li but in other systems as well.
***
LM10891B
Quantum simulator for Hubbard model
Our paper proposes a semiconductor device for simulating
a complex quantum many-body problem called the "Hubbard
model", using quantum mechanical effects. Certain
computational problems are by their nature very difficult
to solve. This includes certain optimization problems,
such as the "travelling salesman problem", and also
quantum many-body problems. The Hubbard model is a
particular example of this, and is important in condensed
matter physics, since it describes the way electrons in
solids behave. High temperature superconductors are
thought to be described by the Hubbard model, making its
understanding important. In our paper, we describe a
practical method for making an artificial Hubbard model
in the lab. By measuring the properties of the artificially
created Hubbard model, it should be possible to learn many
things about the model, which are not possible to obtain on
a computer. The difference to the simulation on a computer
is that in our device the electrons behave quantum
mechanically, whereas a computer operates using the
principles of classical physics. Simulating a quantum
mechanical model with quantum mechanical particles makes
for a more efficient simulation, an idea first conceived
by Feynman, and is one of the founding ideas in the field
of quantum computation.
***
BR10717
Quantum Simulations at Micron Scales and Beyond
The ability to perform quantum simulations of materials properties
over length scales that are relevant to experiments represents a
grand challenge in computational materials science. If one could
treat multi-millions or billions of electrons at a length scale of
microns or beyond, such first-principle quantum simulations could
revolutionize materials research and pave the way to the
computational design of advanced materials. In this paper, we
propose a multiscale approach that is entirely based on an exact quantum
mechanical theory for many electrons - the density functional theory
(DFT),
and allows prediction of materials properties at micron scales and
beyond,
without empirical or experimental input. The method, termed
QCDFT, combines two novel developments in multiscale modeling: a
systematic coarse graining of degrees of freedom through finite-
element interpolation and embedding a smaller region
in a larger environment quantum mechanically. The QCDFT method
has been applied successfully to a nanoindentation study of an Al
thin film in which more than 60 million electrons are simulated; more
applications are under way.
***
BR10995
Understanding electrical noise in carbon nanotube films
Carbon nanotube films, which consist of a mesh of single-walled
carbon nanotubes, have been demonstrated as a transparent,
conductive, and flexible material for applications in
photovoltaics, optoelectronics, and sensing. However, nanotube
films have high 1/f noise, a specific type of electrical noise
present at low frequencies that determines the detection limit of
any sensor. In this work, we use Monte Carlo simulations to
understand the sources of 1/f noise in carbon nanotube films and
to analyze the parameters that affect this noise, such as device
dimensions and film resistivity. We simulate the film by
generating nanotubes with random orientations in a 3D volume. By
comparing the simulation results with the published experimental
data, we find that tube-tube junctions, and not the nanotubes
themselves, dominate the 1/f noise. These simulations not only
provide important fundamental physical insights into the complex
transport mechanisms in nanotube films, but also help improve the
performance of these nanomaterials in potential device
applications where noise is an important figure of merit.
***
LQ11413
Onset of ferrielectricity and the hidden nature of nanoscale polarization
in ferroelectric thin films
Using calculations from first principles, we have elucidated the nanoscale
organization and local polarization in ferroelectric thin films. The
spontaneous
electric polarization exhibited by particular classes of materials, where
individual crystal dipoles orient in a fixed direction, is a property known
as ferroelectricity and it is at the basis of a broad range of modern
technological applications. Although ferroelectricity has been understood
for a long time in bulk crystals, the recent drive towards miniaturization
of electronic devices has unveiled novel characteristics when the size of
the material is scaled to the microscopic dimensions of thin films. Our
results unveil a peculiar spatial pattern of the local microscopic dipoles
where individual atomic layers acquire uncompensated opposing polarizations
in what was originally thought to be a simple domain of homogeneous
orientation. This behavior arises as consequence of the complex energetic
competition between the interface effects, the internal electric fields due
to uncompensated interface charges, and the orientation and mutual
interaction of the layer dipoles. Moreover, as the thickness of the film is
varied, we discovered the appearance of a dielectric phase transition that
we believe is a universal feature of ferroelectric materials at the
nanoscale.
***
AH10250
Atomic collisions from basic research to new technologies
In this work, we studied collisions between atoms of the alkali-metal cesium and argon gas. Such studies offer significant insights into the
atomic collisional dynamics and energy transfers important to astrophysics, the atmospheric sciences, plasma processing of semiconductor devices,
combustion diagnostics, and radiation therapy. First, a circularly polarized laser created an anisotropy in the angular momentum of the cesium
atoms-that is, the momentum now varied along specific directions. Then, a second circularly polarized laser further excited these atoms and
sensed the anisotropy. We collected the light emitted by the atoms as they decayed back to a lower energy state. This procedure was
repeated, only this time introducing the argon gas at differing pressures. In this way, destruction of the anisotropy could be studied by
slowly injecting these gas perturbers into the system. Our work demonstrated how a pump-probe pulse laser technique can be used to study
such processes, providing a deeper understanding of the collisional dynamics of excited alkali atoms.
***
LV10442B
Electron conductivity peaks at "magic angles" in organic molecular crystals
In molecular crystals of the (TMTSF)2X family, big organic molecules form
an array of parallel conducting chains. Electron conductivity across the
chains exhibits sharp peaks, when an external magnetic field is oriented
along some special "magic angles". This effect was first predicted by
Andrei Lebed in 1986, and its theoretical understanding has been developing
since then. One material shows only 3 magic angles, while another material
shows as many as 21 magic angles, but only with the odd values, whereas yet
another material shows only even magic angle. A comprehensive theory
presented in our paper explains the difference between the odd and even,
many and few magic angles in different materials. These effects result
from quantum interference in the electron motion, which is affected by the
orientation of the magnetic field. Curiously, the same mathematical theory
also describes quantum interference in a very different system: a
superconducting qubit driven by a radio-frequency electric field. The
similarity demonstrates that electrons in organic molecular crystals have
as high quantum coherence as in superconducting qubits, which are the
candidates for building blocks of a quantum computer.
***
ER10389
Converting Genetic Network Oscillation into Embryonic Backbone
Converting genetic network oscillation into embryonic somite formation
(the repeated backbone and associated musculature structure in
vertebrate embryos) is a well studied case of multicellular
morphogenesis in which complex spatial and temporal distributions of
cells are produced by genetic networks. The spatially periodic nature of
somite formation suggests that the genetic network involved must
display intracellular oscillations that interact with a longitudinal
positional information gradient, called determination front, down the
axis of vertebrate embryos to create this spatial patterning. Many of
the elements of the network have been identified, but their interaction
with the determination front remains poorly understood. In this paper we
propose a mathematical model for the genetic network that takes into
account the interaction of the oscillation clock with the determination
front to obtain a growing approximately spatially periodic sequence of
somites using the known network dynamics for the zebrafish embryo. We
simulated the transient manipulation of the wavefront in zebrafish
embryos and also propose a way to perturb somitic pattern by changing
segmentation clock period as an experimental prediction of our model
that can be used in some future experiment to test its validity.
***
LS11827ER
Exciting Hard Spheres: The Billiards Break Shot as an Explosion
It is fascinating to watch what happens during the initial "break shot" in
billiards. What happens in the ideal setting of no dissipation and an
infinite billiard table? In this work, we investigated the kinetics that
ensues due to a break shot on a d-dimensional infinite homogeneous gas of
perfectly elastic stationary billiard particles. Just as in real billiards,
progressively more particles become mobile as the collision cascade
develops.
In spite of the asymmetry associated with the initial break, the expanding
region of moving particles is nearly spherically symmetric about the initial
point of collision. Using elementary kinetic theory, we determined the time
dependence of the number of moving particles and the total number of
collisions in the cascade. The properties of this cascade are closely
analogous to those for a continuum shock wave that emanates from a point
explosion in a gas in the infinite Mach number limit. Strikingly, for an
initial break shot on a semi-infinite gas, all the initial energy is
ultimately reflected into the initially empty half-space by back-scattered
particles .
***
LM11412
Novel composite nanostructures consisting of metals and semiconductors
have exciting new properties which may allow for realizing entirely
novel nanophotonic devices such as surface plasmon polariton lasers
(SPASERs) or ultrafast switches. So far, however, designing such devices
has been hindered by a very limited understanding of interaction between
surface plasmon polaritons, the elementary optical excitations of metal
nanostructures, and excitons, the optical excitations of semiconductors.
In our paper, we for the first time demonstrate and
quantitatively explain the energy transfer processes between excitons
and surface plasmon polaritons in a prototypical composite structure
made out of a quantum well and a nano-sized metal grating. Our
experiments reveal the formation of new coupled exciton-plasmon modes
having properties which are a mixture of those of the two original
excitations.
We anticipate that our results will play a key role in designing new
metal semiconductor hybrid nanostructures which will be at the core of
novel devices like nanometer-sized laser sources or which will enable
the transfer of quantum information over mesoscopic distances.
Fig. Schematic of the prototype hybrid structruce made out of a quantum
well and a nano-sized metal grating
