9,860 research outputs found

### Zero-temperature transition and correlation-length exponent of the frustrated XY model on a honeycomb lattice

Phase coherence and vortex order in the fully frustrated XY model on a
two-dimensional honeycomb lattice are studied by extensive Monte Carlo
simulations using the parallel tempering method and finite-size scaling. No
evidence is found for an equilibrium order-disorder or a spin/vortex-glass
transition, suggested in previous simulation works. Instead, the scaling
analysis of correlations of phase and vortex variables in the full equilibrated
system is consistent with a phase transition where the critical temperature
vanishes and the correlation lengths diverge as a power-law with decreasing
temperatures and corresponding critical exponents $\nu_{ph}$ and $\nu_{v}$.
This behavior and the near agreement of the critical exponents suggest a
zero-temperature transition scenario where phase and vortex variables remain
coupled on large length scales.Comment: 8 pages, 10 figure

### A New Technique for Heterodyne Spectroscopy: Least-Squares Frequency Switching (LSFS)

We describe a new technique for heterodyne spectroscopy, which we call
Least-Squares Frequency Switching, or LSFS. This technique avoids the need for
a traditional reference spectrum, which--when combined with the on-source
spectrum--introduces both noise and systematic artifacts such as ``baseline
wiggles''. In contrast, LSFS derives the spectrum directly, and in addition the
instrumental gain profile. The resulting spectrum retains nearly the full
theoretical sensitivity and introduces no systematic artifacts.
Here we discuss mathematical details of the technique and use numerical
experiments to explore optimum observing schemas. We outline a modification
suitable for computationally difficult cases as the number of spectral channels
grows beyond several thousand. We illustrate the method with three real-life
examples. In one of practical interest, we created a large contiguous bandwidth
aligning three smaller bandwidths end-to-end; radio astronomers are often faced
with the need for a larger contiguous bandwidth than is provided with the
available correlator.Comment: 37 pages, 8 figure

### Geometrical properties of the potential energy of the soft-sphere binary mixture

We report a detailed study of the stationary points (zero-force points) of
the potential energy surface (PES) of a model structural glassformer. We
compare stationary points found with two different algorithms (eigenvector
following and square gradient minimization), and show that the mapping between
instantaneous configuration and stationary points defined by those algorithms
is as different as to strongly influence the instability index K vs.
temperature plot, which relevance in analyzing the liquid dynamics is thus
questioned. On the other hand, the plot of K vs. energy is much less sensitive
to the algorithm employed, showing that the energy is the good variable to
discuss geometric properties of the PES. We find new evidence of a geometric
transition between a minima-dominated phase and a saddle-point-dominated one.
We analyze the distances between instantaneous configurations and stationary
points, and find that above the glass transition, the system is closer to
saddle points than to minima

### Fermion Masses from SO(10) Hermitian Matrices

Masses of fermions in the SO(10) 16-plet are constructed using only the 10,
120 and 126 scalar multiplets. The mass matrices are restricted to be hermitian
and the theory is constructed to have certain assumed quark masses, charged
lepton masses and CKM matrix in accord with data. The remaining free parameters
are found by fitting to light neutrino masses and MSN matrices result as
predictions.Comment: 23 pages. Small textual additions for clarification; formalism and
results unchanged. Version to appear in Phys. Rev.

### Simulation of complete many-body quantum dynamics using controlled quantum-semiclassical hybrids

A controlled hybridization between full quantum dynamics and semiclassical
approaches (mean-field and truncated Wigner) is implemented for interacting
many-boson systems. It is then demonstrated how simulating the resulting hybrid
evolution equations allows one to obtain the full quantum dynamics for much
longer times than is possible using an exact treatment directly. A collision of
sodium BECs with 1.x10^5 atoms is simulated, in a regime that is difficult to
describe semiclassically. The uncertainty of physical quantities depends on the
statistics of the full quantum prediction. Cutoffs are minimised to a
discretization of the Hamiltonian. The technique presented is quite general and
extension to other systems is considered.Comment: Published version. Broader background and discussion, slightly
shortened, less figures in epaps. Research part unchanged. Article + epaps
(4+4 pages), 8 figure

### Quantum heat transfer in harmonic chains with self consistent reservoirs: Exact numerical simulations

We describe a numerical scheme for exactly simulating the heat current
behavior in a quantum harmonic chain with self-consistent reservoirs.
Numerically-exact results are compared to classical simulations and to the
quantum behavior under the linear response approximation. In the classical
limit or for small temperature biases our results coincide with previous
calculations. At large bias and for low temperatures the quantum dynamics of
the system fundamentally differs from the close-to-equilibrium behavior,
revealing in particular the effect of thermal rectification for asymmetric
chains. Since this effect is absent in the classical analog of our model, we
conclude that in the quantum model studied here thermal rectification is a
purely quantum phenomenon, rooted in the quantum statistics

### Determination of the zeta potential for highly charged colloidal suspensions

We compute the electrostatic potential at the surface, or zeta potential
$\zeta$, of a charged particle embedded in a colloidal suspension using a
hybrid mesoscopic model. We show that for weakly perturbing electric fields,
the value of $\zeta$ obtained at steady state during electrophoresis is
statistically indistinguishable from $\zeta$ in thermodynamic equilibrium. We
quantify the effect of counterions concentration on $\zeta$. We also evaluate
the relevance of the lattice resolution for the calculation of $\zeta$ and
discuss how to identify the effective electrostatic radius.Comment: 8 pages, 3 figures with 2 panel

### Regularization of fields for self-force problems in curved spacetime: foundations and a time-domain application

We propose an approach for the calculation of self-forces, energy fluxes and
waveforms arising from moving point charges in curved spacetimes. As opposed to
mode-sum schemes that regularize the self-force derived from the singular
retarded field, this approach regularizes the retarded field itself. The
singular part of the retarded field is first analytically identified and
removed, yielding a finite, differentiable remainder from which the self-force
is easily calculated. This regular remainder solves a wave equation which
enjoys the benefit of having a non-singular source. Solving this wave equation
for the remainder completely avoids the calculation of the singular retarded
field along with the attendant difficulties associated with numerically
modeling a delta function source. From this differentiable remainder one may
compute the self-force, the energy flux, and also a waveform which reflects the
effects of the self-force. As a test of principle, we implement this method
using a 4th-order (1+1) code, and calculate the self-force for the simple case
of a scalar charge moving in a circular orbit around a Schwarzschild black
hole. We achieve agreement with frequency-domain results to ~ 0.1% or better.Comment: 15 pages, 12 figures, 1 table. More figures, extended summar

### Statistical Tests for Scaling in the Inter-Event Times of Earthquakes in California

We explore in depth the validity of a recently proposed scaling law for
earthquake interevent time distributions in the case of the Southern
California, using the waveform cross-correlation catalog of Shearer et al. Two
statistical tests are used: on the one hand, the standard two-sample
Kolmogorov-Smirnov test is in agreement with the scaling of the distributions.
On the other hand, the one-sample Kolmogorov-Smirnov statistic complemented
with Monte Carlo simulation of the inter-event times, as done by Clauset et
al., supports the validity of the gamma distribution as a simple model of the
scaling function appearing on the scaling law, for rescaled inter-event times
above 0.01, except for the largest data set (magnitude greater than 2). A
discussion of these results is provided.Comment: proceedings of Erice conference, 200

### The motion of the freely falling chain tip

The dynamics of the tip of the falling chain is analyzed. Results of
laboratory experiments are presented and compared with results of numerical
simulations. Time dependences of the velocity and the acceleration of the chain
tip for a number of different initial conformations of the chain are
determined. A simple analytical model of the system is also considered.Comment: 29 pages, 13 figure

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