11 research outputs found
Thermodynamics of the spin-flop transition in a quantum XYZ chain
A special limit of an antiferromagnetic XYZ chain was recently shown to
exhibit interesting bulk as well as surface spin-flop transitions at T=0. Here
we provide a complete calculation of the thermodynamics of the bulk transition
using a transfer-matrix-renormalization-group (TMRG) method that addresses
directly the thermodynamic limit of quantum spin chains. We also shed some
light on certain spinwave anomalies at low temperature predicted earlier by
Johnson and Bonner.Comment: 4 pages, 6 Postscript figure
Diffusive transport in spin-1 chains at high temperatures
We present a numerical study on the spin and thermal conductivities of the
spin-1 Heisenberg chain in the high temperature limit, in particular of the
Drude weight contribution and frequency dependence. We use the Exact
Diagonalization and the recently developed microcanonical Lanczos method; it
allows us a finite size scaling analysis by the study of significantly larger
lattices. This work, pointing to a diffusive rather than ballistic behavior is
discussed with respect to other recent theoretical and experimental studies
Nonlinear excitations in CsNiF3 in magnetic fields perpendicular to the easy plane
Experimental and numerical studies of the magnetic field dependence of the
specific heat and magnetization of single crystals of CsNiF3 have been
performed at 2.4 K, 2.9 K, and 4.2 K in magnetic fields up to 9 T oriented
perpendicular to the easy plane. The experimental results confirm the presence
of the theoretically predicted double peak structure in the specific heat
arising from the formation of nonlinear spin modes. The demagnetizing effects
are found to be negligible, and the overall agreement between the data and
numerical predictions is better than reported for the case when the magnetic
field was oriented in the easy plane. Demagnetizing effects might play a role
in generating the difference observed between theory and experiment in previous
work analyzing the excess specific heat using the sine-Gordon model.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
Magnon dispersion and thermodynamics in CsNiF_3
We present an accurate transfer matrix renormalization group calculation of
the thermodynamics in a quantum spin-1 planar ferromagnetic chain. We also
calculate the field dependence of the magnon gap and confirm the accuracy of
the magnon dispersion derived earlier through an 1/n expansion. We are thus
able to examine the validity of a number of previous calculations and further
analyze a wide range of experiments on CsNiF_3 concerning the magnon
dispersion, magnetization, susceptibility, and specific heat. Although it is
not possible to account for all data with a single set of parameters, the
overall qualitative agreement is good and the remaining discrepancies may
reflect departure from ideal quasi-one-dimensional model behavior. Finally, we
present some indirect evidence to the effect that the popular interpretation of
the excess specific heat in terms of sine-Gordon solitons may not be
appropriate.Comment: 9 pages 10 figure
Thermomagnetic Power and Figure of Merit for Spin-1/2 Heisenberg Chain
Transport properties in the presence of magnetic fields are numerically
studied for the spin-1/2 Heisenberg XXZ chain. The breakdown of the
spin-reversal symmetry due to the magnetic field induces the magnetothermal
effect. In analogy with the thermoelectric effect in electron systems, the
thermomagnetic power (magnetic Seebeck coefficient) is provided, and is
numerically evaluated by the exact diagonalization for wide ranges of
temperatures and various magnetic fields. For the antiferromagnetic regime, we
find the magnetic Seebeck coefficient changes sign at certain temperatures,
which is interpreted as an effect of strong correlations. We also compute the
thermomagnetic figure of merit determining the efficiency of the thermomagnetic
devices for cooling or power generation.Comment: 8 page
Thermal conductivity via magnetic excitations in spin-chain materials
We discuss the recent progress and the current status of experimental
investigations of spin-mediated energy transport in spin-chain and spin-ladder
materials with antiferromagnetic coupling. We briefly outline the central
results of theoretical studies on the subject but focus mainly on recent
experimental results that were obtained on materials which may be regarded as
adequate physical realizations of the idealized theoretical model systems. Some
open questions and unsettled issues are also addressed.Comment: 17 pages, 4 figure
Spin transport in the Néel and collinear antiferromagnetic phase of the two dimensional spatial and spin anisotropic Heisenberg model on a square lattice
We analyze and compare the effect of spatial and spin anisotropy on spin
conductivity in a two dimensional S=1/2 Heisenberg quantum magnet on a square
lattice. We explore the model in both the Neel antiferromagnetic (AF) phase and
the collinear antiferromagnetic (CAF) phase. We find that in contrast to the
effects of spin anisotropy in the Heisenberg model, spatial anisotropy in the
AF phase does not suppress the zero temperature regular part of the spin
conductivity in the zero frequency limit - rather it enhances it. We also
explore the finite temperature effects on the Drude weight in the AF phase for
various spatial and spin anisotropy parameters. We find that the Drude weight
goes to zero as the temperature approaches zero. At finite temperatures (within
the collision less approximation) enhancing spatial anisotropy increases the
Drude weight value and increasing spin anisotropy decreases the Drude weight
value. In the CAF phase (within the non-interacting approximation) the zero
frequency spin conductivity has a finite value for non-zero values of the
spatial anisotropy parameter. In the CAF phase increasing the spatial
anisotropy parameter suppresses the regular part of the spin conductivity
response at zero frequency. Furthermore, we find that the CAF phase displays a
spike in the spin conductivity not seen in the AF phase. Inclusion of the
smallest amount of spin anisotropy causes a gap to develop in the spin
conductivity response of both the AF and CAF phase. Based on these studies we
conclude that materials with spatial anisotropy are better spin conductors than
those with spin anisotropy both at zero and finite temperatures. We utilize
exchange parameter ratios for real material systems as inputs to the
computation of spin conductivity.Comment: 10 pages, 8 figure