2,526 research outputs found
Numerical Method for Shock Front Hugoniot States
We describe a Continuous Hugoniot Method for the efficient simulation of
shock wave fronts. This approach achieves significantly improved efficiency
when the generation of a tightly spaced collection of individual steady-state
shock front states is desired, and allows for the study of shocks as a function
of a continuous shock strength parameter, . This is, to our knowledge, the
first attempt to map the Hugoniot continuously. We apply the method to shock
waves in Lennard-Jonesium along the direction. We obtain very good
agreement with prior simulations, as well as our own benchmark comparison runs.Comment: 4 pages, 3 figures, from Shock Compression of Condensed Matter 200
Dissipative Quantum Dynamics and Optimal Control using Iterative Time Ordering: An Application to Superconducting Qubits
We combine a quantum dynamical propagator that explicitly accounts for
quantum mechanical time ordering with optimal control theory. After analyzing
its performance with a simple model, we apply it to a superconducting circuit
under so-called Pythagorean control. Breakdown of the rotating-wave
approximation is the main source of the very strong time-dependence in this
example. While the propagator that accounts for the time ordering in an
iterative fashion proves its numerical efficiency for the dynamics of the
superconducting circuit, its performance when combined with optimal control
turns out to be rather sensitive to the strength of the time-dependence. We
discuss the kind of quantum gate operations that the superconducting circuit
can implement including their performance bounds in terms of fidelity and
speed.Comment: 16 pages, 11 figure
Patterns in Illinois Educational School Data
We examine Illinois educational data from standardized exams and analyze
primary factors affecting the achievement of public school students. We focus
on the simplest possible models: representation of data through visualizations
and regressions on single variables. Exam scores are shown to depend on school
type, location, and poverty concentration. For most schools in Illinois,
student test scores decline linearly with poverty concentration. However
Chicago must be treated separately. Selective schools in Chicago, as well as
some traditional and charter schools, deviate from this pattern based on
poverty. For any poverty level, Chicago schools perform better than those in
the rest of Illinois. Selective programs for gifted students show high
performance at each grade level, most notably at the high school level, when
compared to other Illinois school types. The case of Chicago charter schools is
more complex. In the last six years, their students' scores overtook those of
students in traditional Chicago high schools.Comment: 9 pages, 6 figure
Dynamic Fracture in Single Crystal Silicon
We have measured the velocity of a running crack in brittle single crystal
silicon as a function of energy flow to the crack tip. The experiments are
designed to permit direct comparison with molecular dynamics simulations;
therefore the experiments provide an indirect but sensitive test of interatomic
potentials. Performing molecular dynamics simulations of brittle crack motion
at the atomic scale we find that experiments and simulations disagree showing
that interatomic potentials are not yet well understood.Comment: 4 pages, 4 figures, 19 reference
Berry phase effect in anomalous thermoelectric transport
We develop a theory of Berry phase effect in anomalous transport in
ferromagnets driven by statistical forces such as the gradient of temperature
or chemical potential. Here a charge Hall current arises from the Berry phase
correction to the orbital magnetization rather than from the anomalous velocity
which does not exist in the absence of a mechanical force. A finite-temperature
formula for the orbital magnetization is derived, which enables us to provide
an explicit expression for the off-diagonal thermoelectric conductivity, to
establish the Mott relation between the anomalous Nernst and Hall effects, and
to reaffirm the Onsager relations between reciprocal thermoelectric
conductivities. A first-principles evaluation of our expression is carried out
for the material CuCrSeBr, obtaining quantitative agreement
with a recent experiment.Comment: Published version in PR
Combining isotonic regression and EM algorithm to predict genetic risk under monotonicity constraint
In certain genetic studies, clinicians and genetic counselors are interested
in estimating the cumulative risk of a disease for individuals with and without
a rare deleterious mutation. Estimating the cumulative risk is difficult,
however, when the estimates are based on family history data. Often, the
genetic mutation status in many family members is unknown; instead, only
estimated probabilities of a patient having a certain mutation status are
available. Also, ages of disease-onset are subject to right censoring. Existing
methods to estimate the cumulative risk using such family-based data only
provide estimation at individual time points, and are not guaranteed to be
monotonic or nonnegative. In this paper, we develop a novel method that
combines Expectation-Maximization and isotonic regression to estimate the
cumulative risk across the entire support. Our estimator is monotonic,
satisfies self-consistent estimating equations and has high power in detecting
differences between the cumulative risks of different populations. Application
of our estimator to a Parkinson's disease (PD) study provides the age-at-onset
distribution of PD in PARK2 mutation carriers and noncarriers, and reveals a
significant difference between the distribution in compound heterozygous
carriers compared to noncarriers, but not between heterozygous carriers and
noncarriers.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS730 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Berry Curvature on the Fermi Surface: Anomalous Hall Effect as a Topological Fermi-Liquid Property
The intrinsic anomalous Hall effect in metallic ferromagnets is shown to be
controlled by Berry phases accumulated by adiabatic motion of quasiparticles on
the Fermi surface, and is purely a Fermi-liquid property, not a ``bulk'' Fermi
sea property like Landau diamagnetism, as has been previously supposed. Berry
phases are a new topological ingredient that must be added to Landau
Fermi-liquid theory in the presence of broken inversion or time-reversal
symmetry.Comment: 4 pages, 0 figures; to appear in Physical Review Letters; cleaner
form of main formula+note added confirming continued validity of result in
interacting Fermi liquids: + improved summary paragraph stating result; final
published version (minor changes
Dynamical stability of the crack front line
Dynamical stability of the crack front line that propagates between two
plates is studied numerically using the simple two-dimensional mass-spring
model. It is demonstrated that the straight front line is unstable for low
speed while it becomes stable for high speed. For the uniform model, the
roughness exponent in the slower speed region is fairly constant around 0.4 and
there seems to be a rough-smooth transition at a certain speed. For the
inhomogeneous case with quenched randomness, the transition is gradual.Comment: 14 pages, 7 figure
Weak localization in mesoscopic hole transport: Berry phases and classical correlations
We consider phase-coherent transport through ballistic and diffusive
two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show
that intrinsic heavy-hole light-hole coupling gives rise to clear-cut
signatures of an associated Berry phase in the weak localization which renders
the magneto-conductance profile distinctly different from electron transport.
Non-universal classical correlations determine the strength of these Berry
phase effects and the effective symmetry class, leading even to
antilocalization-type features for circular quantum dots and Aharonov-Bohm
rings in the absence of additional spin-orbit interaction. Our semiclassical
predictions are quantitatively confirmed by numerical transport calculations
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