16,627 research outputs found
Crossed spin-1/2 Heisenberg chains as a quantum impurity problem
Using equivalencies between different models we reduce the model of two
spin-1/2 Heisenberg chains crossed at one point to the model of free fermions.
The spin-spin correlation function is calculated by summing the perturbation
series in the interchain coupling. The result reveals a power law decay with a
nonuniversal exponent.Comment: 3 pages, the background information is adde
Impurity in a granular gas under nonlinear Couette flow
We study in this work the transport properties of an impurity immersed in a
granular gas under stationary nonlinear Couette flow. The starting point is a
kinetic model for low-density granular mixtures recently proposed by the
authors [Vega Reyes F et al. 2007 Phys. Rev. E 75 061306]. Two routes have been
considered. First, a hydrodynamic or normal solution is found by exploiting a
formal mapping between the kinetic equations for the gas particles and for the
impurity. We show that the transport properties of the impurity are
characterized by the ratio between the temperatures of the impurity and gas
particles and by five generalized transport coefficients: three related to the
momentum flux (a nonlinear shear viscosity and two normal stress differences)
and two related to the heat flux (a nonlinear thermal conductivity and a cross
coefficient measuring a component of the heat flux orthogonal to the thermal
gradient). Second, by means of a Monte Carlo simulation method we numerically
solve the kinetic equations and show that our hydrodynamic solution is valid in
the bulk of the fluid when realistic boundary conditions are used. Furthermore,
the hydrodynamic solution applies to arbitrarily (inside the continuum regime)
large values of the shear rate, of the inelasticity, and of the rest of
parameters of the system. Preliminary simulation results of the true Boltzmann
description show the reliability of the nonlinear hydrodynamic solution of the
kinetic model. This shows again the validity of a hydrodynamic description for
granular flows, even under extreme conditions, beyond the Navier-Stokes domain.Comment: 23 pages, 11 figures; v2: Preliminary DSMC results from the Boltzmann
equation included, Fig. 11 is ne
Dynamics of a Heisenberg spin chain in the quantum critical regime: NMR experiment versus effective field theory
A comprehensive comparison between the magnetic field- and
temperature-dependent low frequency spin dynamics in the antiferromagnetic
spin-1/2 Heisenberg chain (AFHC) system copper pyrazine dinitrate, probed via
the 13C-nuclear magnetic resonance (NMR) relaxation rate 1/T1, and the field
theoretical approach in the Luttinger liquid (LL) regime has been performed. We
have found a very good agreement between the experiment and theory in the
investigated temperature and field range. Our results demonstrate how strongly
the quantum critical point affects the spin dynamics of Heisenberg spin chain
compounds.Comment: 5 pages, 3 figure
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