27,971 research outputs found
Statistical Mechanics of Vibration-Induced Compaction of Powders
We propose a theory which describes the density relaxation of loosely packed,
cohesionless granular material under mechanical tapping. Using the compactivity
concept we develope a formalism of statistical mechanics which allows us to
calculate the density of a powder as a function of time and compactivity. A
simple fluctuation-dissipation relation which relates compactivity to the
amplitude and frequency of a tapping is proposed. Experimental data of
E.R.Nowak et al. [{\it Powder Technology} 94, 79 (1997) ] show how density of
initially deposited in a fluffy state powder evolves under carefully controlled
tapping towards a random close packing (RCP) density. Ramping the vibration
amplitude repeatedly up and back down again reveals the existence of reversible
and irreversible branches in the response. In the framework of our approach the
reversible branch (along which the RCP density is obtained) corresponds to the
steady state solution of the Fokker-Planck equation whereas the irreversible
one is represented by a superposition of "excited states" eigenfunctions. These
two regimes of response are analyzed theoretically and a qualitative
explanation of the hysteresis curve is offered.Comment: 11 pages, 2 figures, Latex. Revised tex
A selfconsistent theory of current-induced switching of magnetization
A selfconsistent theory of the current-induced switching of magnetization
using nonequilibrium Keldysh formalism is developed for a junction of two
ferromagnets separated by a nonmagnetic spacer. It is shown that the
spin-transfer torques responsible for current-induced switching of
magnetization can be calculated from first principles in a steady state when
the magnetization of the switching magnet is stationary. The spin-transfer
torque is expressed in terms of one-electron surface Green functions for the
junction cut into two independent parts by a cleavage plane immediately to the
left and right of the switching magnet. The surface Green functions are
calculated using a tight-binding Hamiltonian with parameters determined from a
fit to an {\it ab initio} band structure.This treatment yields the spin
transfer torques taking into account rigorously contributions from all the
parts of the junction. To calculate the hysteresis loops of resistance versus
current, and hence to determine the critical current for switching, the
microscopically calculated spin-transfer torques are used as an input into the
phenomenological Landau-Lifshitz equation with Gilbert damping. The present
calculations for Co/Cu/Co(111) show that the critical current for switching is
, which is in good agreement with experiment.Comment: 23 pages, 16 figure
An exactly solvable model for a beta-hairpin with random interactions
I investigate a disordered version of a simplified model of protein folding,
with binary degrees of freedom, applied to an ideal beta-hairpin structure.
Disorder is introduced by assuming that the contact energies are independent
and identically distributed random variables. The equilibrium free-energy of
the model is studied, performing the exact calculation of its quenched value
and proving the self-averaging feature.Comment: 9 page
High-order harmonic generation from inhomogeneous fields
We present theoretical studies of high-order harmonic generation (HHG)
produced by non-homogeneous fields as resulting from the illumination of
plasmonic nanostructures with a short laser pulse. We show that both the
inhomogeneity of the local fields and the confinement of the electron movement
play an important role in the HHG process and lead to the generation of even
harmonics and a significantly increased cutoff, more pronounced for the longer
wavelengths cases studied. In order to understand and characterize the new HHG
features we employ two different approaches: the numerical solution of the time
dependent Schr\"odinger equation (TDSE) and the semiclassical approach known as
Strong Field Approximation (SFA). Both approaches predict comparable results
and show the new features, but using the semiclassical arguments behind the SFA
and time-frequency analysis tools, we are able to fully understand the reasons
of the cutoff extension.Comment: 25 pages, 12 figure
Local Spin-Gauge Symmetry of the Bose-Einstein Condensates in Atomic Gases
The Bose-Einstein condensates of alkali atomic gases are spinor fields with
local ``spin-gauge" symmetry. This symmetry is manifested by a superfluid
velocity (or gauge field) generated by the Berry phase of the
spin field. In ``static" traps, splits the degeneracy of the
harmonic energy levels, breaks the inversion symmetry of the vortex nucleation
frequency , and can lead to {\em vortex ground states}. The
inversion symmetry of , however, is not broken in ``dynamic"
traps. Rotations of the atom cloud can be generated by adiabatic effects
without physically rotating the entire trap.Comment: Typos in the previous version corrected, thanks to the careful
reading of Daniel L. Cox. 13 pages + 2 Figures in uuencode + gzip for
Quantum transport in noncentrosymmetric superconductors and thermodynamics of ferromagnetic superconductors
We consider a general Hamiltonian describing coexistence of itinerant
ferromagnetism, spin-orbit coupling and mixed spin-singlet/triplet
superconducting pairing in the context of mean-field theory. The Hamiltonian is
diagonalized and exact eigenvalues are obtained, thus allowing us to write down
the coupled gap equations for the different order parameters. Our results may
then be applied to any model describing coexistence of any combination of these
three phenomena. As a specific application of our results, we consider
tunneling between a normal metal and a noncentrosymmetric superconductor with
mixed singlet and triplet gaps. The conductance spectrum reveals information
about these gaps in addition to how the influence of spin-orbit coupling is
manifested. We also consider the coexistence of itinerant ferromagnetism and
triplet superconductivity as a model for recently discovered ferromagnetic
superconductors. The coupled gap equations are solved self-consistently, and we
study the conditions necessary to obtain the coexistent regime of
ferromagnetism and superconductivity. Analytical expressions are presented for
the order parameters, and we provide an analysis of the free energy to identify
the preferred system state. Moreover, we make specific predictions concerning
the heat capacity for a ferromagnetic superconductor. In particular, we report
a nonuniversal relative jump in the specific heat, depending on the
magnetization of the system, at the uppermost superconducting phase transition.
[Shortened abstract due to arXiv submission.]Comment: 19 pages, 15 figures (high quality figures available in published
version). Accepted for publication in Phys. Rev.
Interacting Bose and Fermi gases in low dimensions and the Riemann hypothesis
We apply the S-matrix based finite temperature formalism to non-relativistic
Bose and Fermi gases in 1+1 and 2+1 dimensions. In the 2+1 dimensional case,
the free energy is given in terms of Roger's dilogarithm in a way analagous to
the relativistic 1+1 dimensional case. The 1d fermionic case with a
quasi-periodic 2-body potential provides a physical framework for understanding
the Riemann hypothesis.Comment: version 3: additional appendix explains how the to
duality of Riemann's follows from a special modular
transformation in a massless relativistic theor
The Subpulse Modulation Properties of Pulsars and its Frequency Dependence
A large sample of about two hundred pulsars have been observed to study their
subpulse modulation at an observing wavelength of (when achievable) both 21 and
92 cm using the Westerbork Synthesis Radio Telescope. For 57 pulsars drifting
subpulses are discovered for the first time and are confirmed for many others.
This leads to the conclusion that it could well be that the drifting subpulse
mechanism is an intrinsic property of the emission mechanism itself, although
for some pulsars it is difficult or impossible to detect. It appears that the
youngest pulsars have the most disordered subpulses and the subpulses become
more and more organized into drifting subpulses as the pulsar ages. Drifting
subpulses are in general found at both frequencies and the measured values of
P3 at the two frequencies are highly correlated, showing the broadband nature
of this phenomenon. Also the modulation indices measured at the two frequencies
are clearly correlated, although at 92 cm they are on average possibly higher.
The correlations with the modulation indices are argued to be consistent with
the picture in which the radio emission is composed out of a drifting subpulse
signal plus a quasi-steady signal which becomes, on average, stronger at high
observing frequencies. There is no obvious correlation found between P3 and the
pulsar age (or any other pulsar parameter) contrary to reports in the past.Comment: Proceedings of the 40 Years of Pulsars: Millisecond Pulsars,
Magnetars and More conference in Montrea
Field theory of self-avoiding walks in random media
Based on the analogy with the quantum mechanics of a particle propagating in
a {\em complex} potential, we develop a field-theoretical description of the
statistical properties of a self-avoiding polymer chain in a random
environment. We show that the account of the non-Hermiticity of the quantum
Hamiltonian results in a qualitatively different structure of the effective
action, compared to previous studies. Applying the renormalisation group
analysis, we find a transition between the weak-disorder regime, where the
quenched randomness is irrelevant, and the strong-disorder regime, where the
polymer chain collapses. However, the fact that the renormalised interaction
constants and the chiral symmetry breaking regularisation parameter flow
towards strong coupling raises questions about the applicability of the
perturbative analysis.Comment: RevTeX, 9 pages; accepted for publication in J. Phys.
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