10,355 research outputs found
Comparison of 2D melting criteria in a colloidal system
We use super-paramagnetic spherical particles which are arranged in a
two-dimensional monolayer at a water/air interface to investigate the crystal
to liquid phase transition. According to the KTHNY theory a crystal melts in
thermal equilibrium by two continuous phase transitions into the isotropic
liquid state with an intermediate phase, commonly known as hexatic phase. We
verify the significance of several criteria based on dynamical and structural
properties to identify the crystal - hexatic and hexatic - isotropic liquid
phase transition for the same experimental data of the given setup. Those
criteria are the bond orientational correlation function, the Larson-Grier
criterion, 2D dynamic Lindemann parameter, the bond-orientational
susceptibility, the 2D Hansen-Verlet rule, the L\"{o}wen-Palberg-Simon
criterion as well as a criterion based on the shape factor of Voronoi cells and
Minkowski functionals. For our system with long range repulsion, the bond order
correlation function and bond order susceptibility works best to identify the
hexatic - isotropic liquid transition and the 2D dynamic Lindemann parameter
identifies unambiguously the hexatic - crystalline transition.Comment: 20 pages, 13 figure
DD-AMG on QPACE 3
We describe our experience porting the Regensburg implementation of the
DD-AMG solver from QPACE 2 to QPACE 3. We first review how the code was
ported from the first generation Intel Xeon Phi processor (Knights Corner) to
its successor (Knights Landing). We then describe the modifications in the
communication library necessitated by the switch from InfiniBand to Omni-Path.
Finally, we present the performance of the code on a single processor as well
as the scaling on many nodes, where in both cases the speedup factor is close
to the theoretical expectations.Comment: 12 pages, 6 figures, Proceedings of Lattice 201
Discontinuous shear modulus determines the glass transition temperature
A solid - amorphous or crystalline - is defined by a finite shear modulus
while a fluid lacks such. We thus experimentally investigate the elastic
properties of a colloidal glass former near the glass transition: spectroscopy
of vibrational excitations yields the dispersion relations of longitudinal and
transverse phonons in the glassy state. From the long wavelength limit of the
dispersion relation we extract the bulk and the shear modulus. As expected, the
latter disappear in a fluid and we measure a clearly resolved discontinuous
behaviour of the elastic moduli at the glass transition. This not only
determines the transition temperature T_G of the system but also directly
addresses recent discussions about elasticity during vitrification. We show
that low frequency excitations in our system are plane waves such that
continuum elasticity theory can be used to describe the macroscopic behaviour.Comment: 8 pages, 6 figure
Properties of principal component methods for functional and longitudinal data analysis
The use of principal component methods to analyze functional data is
appropriate in a wide range of different settings. In studies of ``functional
data analysis,'' it has often been assumed that a sample of random functions is
observed precisely, in the continuum and without noise. While this has been the
traditional setting for functional data analysis, in the context of
longitudinal data analysis a random function typically represents a patient, or
subject, who is observed at only a small number of randomly distributed points,
with nonnegligible measurement error. Nevertheless, essentially the same
methods can be used in both these cases, as well as in the vast number of
settings that lie between them. How is performance affected by the sampling
plan? In this paper we answer that question. We show that if there is a sample
of functions, or subjects, then estimation of eigenvalues is a
semiparametric problem, with root- consistent estimators, even if only a few
observations are made of each function, and if each observation is encumbered
by noise. However, estimation of eigenfunctions becomes a nonparametric problem
when observations are sparse. The optimal convergence rates in this case are
those which pertain to more familiar function-estimation settings. We also
describe the effects of sampling at regularly spaced points, as opposed to
random points. In particular, it is shown that there are often advantages in
sampling randomly. However, even in the case of noisy data there is a threshold
sampling rate (depending on the number of functions treated) above which the
rate of sampling (either randomly or regularly) has negligible impact on
estimator performance, no matter whether eigenfunctions or eigenvectors are
being estimated.Comment: Published at http://dx.doi.org/10.1214/009053606000000272 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Entanglement and Disentanglement in Circuit QED Architectures
We propose a protocol for creating entanglement within a dissipative circuit
QED network architecture that consists of two electromagnetic circuits
(cavities) and two superconducting qubits. The system interacts with a quantum
environment, giving rise to decoherence and dissipation. We discuss the
preparation of two separate entangled cavity-qubit states via Landau-Zener
sweeps, after which the cavities interact via a tunable "quantum switch" which
is realized with an ancilla qubit. Moreover, we discuss the decay of the
resulting entangled two-cavity state due to the influence of the environment,
where we focus on the entanglement decay.Comment: 7 pages, 5 figure
Specific heat in two-dimensional melting
We report the specific heat around the melting transition(s) of
micrometer-sized superparamagnetic particles confined in two dimensions,
calculated from fluctuations of positions and internal energy, and
corresponding Monte Carlo simulations. Since colloidal systems provide single
particle resolution, they offer the unique possibility to compare the
experimental temperatures of peak position of and symmetry breaking,
respectively. While order parameter correlation functions confirm the
Kosterlitz-Thouless-Halperin-Nelson-Young melting scenario where translational
and orientational order symmetries are broken at different temperatures with an
intermediate so called hexatic phase, we observe a single peak of the specific
heat within the hexatic phase, with excellent agreement between experiment and
simulation. Thus, the peak is not associated with broken symmetries but can be
explained with the total defect density, which correlates with the maximum
increase of isolated dislocations. The absence of a latent heat strongly
supports the continuous character of both transitions
Experimental evidence for a partially dissociated water bilayer on Ru{0001}
Core-level photoelectron spectra, in excellent agreement with ab initio calculations, confirm that the stable wetting layer of water on Ru{0001} contains O-H and H2O in roughly 3:5 proportion, for OHx coverages between 0.25 and 0.7 ML, and T<170 K. Proton disorder explains why the wetting structure looks to low energy electron diffraction (LEED) to be an ordered p(root3xroot3)R30degrees adlayer, even though approximate to3/8 of its molecules are dissociated. Complete dissociation to atomic oxygen starts near 190 K. Low photon flux in the synchrotron experiments ensured that the diagnosis of the nature of the wetting structure quantified by LEED is free of beam-induced damage
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