1,206 research outputs found
Inertial modes in stratified rotating neutron stars : An evolutionary description
With (non-barotropic) equations of state valid even when the neutron, proton
and electron content of neutron star cores is not in beta equilibrium, we study
inertial and composition gravity modes of relativistic rotating neutron stars.
We solve the relativistic Euler equations in the time domain with a three
dimensional numerical code based on spectral methods, in the slow rotation,
relativistic Cowling and anelastic approximations. Principally, after a short
description of the gravity modes due to smooth composition gradients, we focus
our analysis on the question of how the inertial modes are affected by
non-barotropicity of the nuclear matter. In our study, the deviation with
respect to barotropicity results from the frozen composition of non-superfluid
matter composed of neutrons, protons and electrons, when beta equilibrium is
broken by millisecond oscillations. We show that already for moderatly fast
rotating stars the increasing coupling between polar and axial modes makes
those two cases less different than for very slowly rotating stars. In
addition, as we directly solve the Euler equations, without coupling only a few
number of spherical harmonics, we always found, for the models that we use, a
discrete spectrum for the inertial mode. Finally, we find that, for
non-barotropic stars, the frequency of this mode, which is our main focus,
decreases in a non-negligible way, whereas the time dependence of the energy
transfer between polar and axial modes is substantially different due to the
existence of low-frequencies gravity modes.Comment: 34 pages, 24 figures, published versio
Study of Chirality in the Two-Dimensional XY Spin Glass
We study the chirality in the Villain form of the XY spin glass in
two--dimensions by Monte Carlo simulations. We calculate the chiral-glass
correlation length exponent and find that
in reasonable agreement with
earlier studies. This indicates that the chiral and phase variables are
decoupled on long length scales and diverge as with {\em different}
exponents, since the spin-glass correlation length exponent was found, in
earlier studies, to be about 1.0.Comment: 4 pages. Latex file and 4 embedded postscript files are included in a
self-unpacking compressed tar file. A postscript version is available at
ftp://chopin.ucsc.edu/pub/xysg.p
Simplex solid states of SU(N) quantum antiferromagnets
I define a set of wavefunctions for SU(N) lattice antiferromagnets, analogous
to the valence bond solid states of Affleck, Kennedy, Lieb, and Tasaki (AKLT),
in which the singlets are extended over N-site simplices. As with the valence
bond solids, the new simplex solid (SS) states are extinguished by certain
local projection operators, allowing us to construct Hamiltonians with local
interactions which render the SS states exact ground states. Using a coherent
state representation, we show that the quantum correlations in each SS state
are calculable as the finite temperature correlations of an associated
classical model, with N-spin interactions, on the same lattice. In three and
higher dimensions, the SS states can spontaneously break SU(N) and exhibit
N-sublattice long-ranged order, as a function of a discrete parameter which
fixes the local representation of SU(N). I analyze this transition using a
classical mean field approach. For N>2 the ordered state is selected via an
"order by disorder" mechanism. As in the AKLT case, the bulk representations
fractionalize at an edge, and the ground state entropy is proportional to the
volume of the boundary.Comment: 14 pages, 8 figures, minor typos correcte
Controlling high-harmonic generation and above-threshold ionization with an attosecond-pulse train
We perform a detailed analysis of how high-order harmonic generation (HHG)
and above-threshold ionization (ATI) can be controlled by a time-delayed
attosecond-pulse train superposed to a strong, near-infrared laser field. In
particular we show that the high-harmonic and photoelectron intensities, the
high-harmonic plateau structure and cutoff energies, and the ATI angular
distributions can be manipulated by changing this delay. This is a direct
consequence of the fact that the attosecond pulse train can be employed as a
tool for constraining the instant an electronic wave packet is ejected in the
continuum. A change in such initial conditions strongly affects its subsequent
motion in the laser field, and thus HHG and ATI. In our studies, we employ the
Strong-Field Approximation and explain the features observed in terms of
interference effects between various electron quantum orbits. Our results are
in agreement with recent experimental findings and theoretical studies
employing purely numerical methods.Comment: 10 pages revtex and 6 figures (eps files
Finite-size effects on the Hamiltonian dynamics of the XY-model
The dynamical properties of the finite-size magnetization M in the critical
region T<T_{KTB} of the planar rotor model on a L x L square lattice are
analyzed by means of microcanonical simulations . The behavior of the q=0
structure factor at high frequencies is consistent with field-theoretical
results, but new additional features occur at lower frequencies. The motion of
M determines a region of spectral lines and the presence of a central peak,
which we attribute to phase diffusion. Near T_{KTB} the diffusion constant
scales with system size as D ~ L^{-1.6(3)}.Comment: To be published in Europhysics Letter
Non-equilibrium beta processes in superfluid neutron star cores
The influence of nucleons superfluidity on the beta relaxation time of
degenerate neutron star cores, composed of neutrons, protons and electrons, is
investigated. We numerically calculate the implied reduction factors for both
direct and modified Urca reactions, with isotropic pairing of protons or
anisotropic pairing of neutrons. We find that due to the non-zero value of the
temperature and/or to the vanishing of anisotropic gaps in some directions of
the phase-space, superfluidity does not always completely inhibit beta
relaxation, allowing for some reactions if the superfluid gap amplitude is not
too large in respect to both the typical thermal energy and the chemical
potential mismatch. We even observe that if the ratio between the critical
temperature and the actual temperature is very small, a suprathermal regime is
reached for which superfluidity is almost irrelevant. On the contrary, if the
gap is large enough, the composition of the nuclear matter can stay frozen for
very long durations, unless the departure from beta equilibrium is at least as
important as the gap amplitude. These results are crucial for precise
estimation of the superfluidity effect on the cooling/slowing-down of pulsars
and we provide online subroutines to be implemented in codes for simulating
such evolutions.Comment: 11 pages, 6 Figs., published, minor changes, subroutines can be found
on line at http://luth2.obspm.fr/~etu/villain/Micro/Resolution.htm
Conserved Growth on Vicinal Surfaces
A crystal surface which is miscut with respect to a high symmetry plane
exhibits steps with a characteristic distance. It is argued that the continuum
description of growth on such a surface, when desorption can be neglected, is
given by the anisotropic version of the conserved KPZ equation (T. Sun, H. Guo,
and M. Grant, Phys. Rev. A 40, 6763 (1989)) with non-conserved noise. A
one--loop dynamical renormalization group calculation yields the values of the
dynamical exponent and the roughness exponent which are shown to be the same as
in the isotropic case. The results presented here should apply in particular to
growth under conditions which are typical for molecular beam epitaxy.Comment: 10 pages, uses revte
Phase diagram of an anisotropic frustrated ferromagnetic spin-1/2 chain in a magnetic field: a density matrix renormalization group study
We study the phase diagram of a frustrated spin-1/2 ferromagnetic chain with
anisotropic exchange interactions in an external magnetic field, using the
density matrix renormalization group method. We show that an easy-axis
anisotropy enhances the tendency towards multimagnon bound states, while an
easy-plane anisotropy favors chirally ordered phases. In particular, a moderate
easy-plane anisotropy gives rise to a quantum phase transition at intermediate
magnetization. We argue that this transition is related to the finite-field
phase transition experimentally observed in the spin-1/2 compound LiCuVO_4.Comment: The final published versio
A simulation study of energy transport in the Hamiltonian XY-model
The transport properties of the planar rotator model on a square lattice are
analyzed by means of microcanonical and non--equilibrium simulations. Well
below the Kosterlitz--Thouless--Berezinskii transition temperature, both
approaches consistently indicate that the energy current autocorrelation
displays a long--time tail decaying as t^{-1}. This yields a thermal
conductivity coefficient which diverges logarithmically with the lattice size.
Conversely, conductivity is found to be finite in the high--temperature
disordered phase. Simulations close to the transition temperature are insted
limited by slow convergence that is presumably due to the slow kinetics of
vortex pairs.Comment: Submitted to Journal of Statistical Mechanics: theory and experimen
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