493 research outputs found
Interplanetary magnetic fields, their fluctuations, and cosmic ray variations
The cause of Forbush decreases is examined using neutron monitor data and measurements of the interplanetary magnetic field. It is found that for the period examined (Dec. 15, 1965 to April 23, 1966) large enhancements of the interplanetary magnetic field correlate well with decreases in cosmic ray intensity, while various parameters connected with the fluctuations in the field do not display such good correlation. The inference is drawn that Forbush decreases are not related to the turbulence or random motions in the field but to the large scale features of the field
Hahn echo and criticality in spin-chain systems
We establish a relation between Hahn spin-echo of a spin-
particle and quantum phase transition in a spin-chain, which couples to the
particle. The Hahn echo is calculated and discussed at zero as well as at
finite temperatures. On the example of XY model, we show that the critical
points of the chain are marked by the extremal values in the Hahn echo, and
influence the Hahn echo in surprising high temperature. An explanation for the
relation between the echo and criticality is also presented.Comment: 5 pages, 6 figure
A solvable model of a random spin-1/2 XY chain
The paper presents exact calculations of thermodynamic quantities for the
spin-1/2 isotropic XY chain with random lorentzian intersite interaction and
transverse field that depends linearly on the surrounding intersite
interactions.Comment: 14 pages (Latex), 2 tables, 13 ps-figures included, (accepted for
publication in Phys.Rev.B
Relaxation in the XX quantum chain
We present the results obtained on the magnetisation relaxation properties of
an XX quantum chain in a transverse magnetic field. We first consider an
initial thermal kink-like state where half of the chain is initially
thermalized at a very high temperature while the remaining half, called
the system, is put at a lower temperature . From this initial state, we
derive analytically the Green function associated to the dynamical behaviour of
the transverse magnetisation. Depending on the strength of the magnetic field
and on the temperature of the system, different regimes are obtained for the
magnetic relaxation. In particular, with an initial droplet-like state, that is
a cold subsystem of finite size in contact at both ends with an infinite
temperature environnement, we derive analytically the behaviour of the
time-dependent system magnetisation
Formfactors and functional form of correlators in the XX spin chain
We present the new expressions for the formfactors of local operators for the
XX - quantum spin chain as a Cauchy determinants. Using the known functional
form of the correlator at large distances we propose the new expression for the
constant for the asymptotics of the correlator as a Cauchy determinant. We
calculate the momentum distribution for the general case of the XXZ spin chain
and point out that it is completely different from the Luttinger model (the
system of fermions). For the XX chain we compare numerically the value of the
lowest formfactor and the expectation value of momentum- zero operators which
is determined by the functional form of the correlator.Comment: LaTex, 18 page
Nonmonotonical crossover of the effective susceptibility exponent
We have numerically determined the behavior of the magnetic susceptibility
upon approach of the critical point in two-dimensional spin systems with an
interaction range that was varied over nearly two orders of magnitude. The full
crossover from classical to Ising-like critical behavior, spanning several
decades in the reduced temperature, could be observed. Our results convincingly
show that the effective susceptibility exponent gamma_eff changes
nonmonotonically from its classical to its Ising value when approaching the
critical point in the ordered phase. In the disordered phase the behavior is
monotonic. Furthermore the hypothesis that the crossover function is universal
is supported.Comment: 4 pages RevTeX 3.0/3.1, 5 Encapsulated PostScript figures. Uses
epsf.sty. Accepted for publication in Physical Review Letters. Also available
as PostScript and PDF file at http://www.tn.tudelft.nl/tn/erikpubs.htm
Exact solution of Markovian master equations for quadratic fermi systems: thermal baths, open XY spin chains, and non-equilibrium phase transition
We generalize the method of third quantization to a unified exact treatment
of Redfield and Lindblad master equations for open quadratic systems of n
fermions in terms of diagonalization of 4n x 4n matrix. Non-equilibrium thermal
driving in terms of the Redfield equation is analyzed in detail. We explain how
to compute all physically relevant quantities, such as non-equilibrium
expectation values of local observables, various entropies or information
measures, or time evolution and properties of relaxation. We also discuss how
to exactly treat explicitly time dependent problems. The general formalism is
then applied to study a thermally driven open XY spin 1/2 chain. We find that
recently proposed non-equilibrium quantum phase transition in the open XY chain
survives the thermal driving within the Redfield model. In particular, the
phase of long-range magnetic correlations can be characterized by
hypersensitivity of the non-equilibrium-steady state to external (bath or bulk)
parameters. Studying the heat transport we find negative thermal conductance
for sufficiently strong thermal driving, as well as non-monotonic dependence of
the heat current on the strength of the bath coupling.Comment: 24 pages, 12 figures, submitted to New Journal of Physics, Focus
issue "Quantum Information and Many-Body Theory
Entanglement in a simple quantum phase transition
What entanglement is present in naturally occurring physical systems at
thermal equilibrium? Most such systems are intractable and it is desirable to
study simple but realistic systems which can be solved. An example of such a
system is the 1D infinite-lattice anisotropic XY model. This model is exactly
solvable using the Jordan-Wigner transform, and it is possible to calculate the
two-site reduced density matrix for all pairs of sites. Using the two-site
density matrix, the entanglement of formation between any two sites is
calculated for all parameter values and temperatures. We also study the
entanglement in the transverse Ising model, a special case of the XY model,
which exhibits a quantum phase transition. It is found that the next-nearest
neighbour entanglement (though not the nearest-neighbour entanglement) is a
maximum at the critical point. Furthermore, we show that the critical point in
the transverse Ising model corresponds to a transition in the behaviour of the
entanglement between a single site and the remainder of the lattice.Comment: 14 pages, 7 eps figure
Lattice Boltzmann Method for Electromagnetic Wave Propagation
We present a new Lattice Boltzmann (LB) formulation to solve the Maxwell
equations for electromagnetic (EM) waves propagating in a heterogeneous medium.
By using a pseudo-vector discrete Boltzmann distribution, the scheme is shown
to reproduce the continuum Maxwell equations. The technique compares well with
a pseudo-spectral method at solving for two-dimensional wave propagation in a
heterogeneous medium, which by design contains substantial contrasts in the
refractive index. The extension to three dimensions follows naturally and,
owing to the recognized efficiency of LB schemes for parallel computation in
irregular geometries, it gives a powerful method to numerically simulate a wide
range of problems involving EM wave propagation in complex media.Comment: 6 pages, 3 figures, accepted Europhysics letter
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