291 research outputs found
Anomalous scaling of conductivity in integrable fermion systems
We analyze the high-temperature conductivity in one-dimensional integrable
models of interacting fermions: the t-V model (anisotropic Heisenberg spin
chain) and the Hubbard model, at half-filling in the regime corresponding to
insulating ground state. A microcanonical Lanczos method study for finite size
systems reveals anomalously large finite-size effects at low frequencies while
a frequency-moment analysis indicates a finite d.c. conductivity. This
phenomenon also appears in a prototype integrable quantum system of
impenetrable particles, representing a strong-coupling limit of both models. In
the thermodynamic limit, the two results could converge to a finite d.c.
conductivity rather than an ideal conductor or insulator scenario.Comment: 6 pages, 3 figures. Submitted to PR
On the nonlinear response of a particle interacting with fermions in a 1D lattice
By the Bethe ansatz method we study the energy dispersion of a particle
interacting by a local interaction with fermions (or hard core bosons) of equal
mass in a one dimensional lattice. We focus on the period of the Bloch
oscillations which turns out to be related to the Fermi wavevector of the Fermi
sea and in particular on how this dispersion emerges as a collective effect in
the thermodynamic limit. We show by symmetry that the dispersion is temperature
independent for a half-filled system. We also discuss the adiabatic coherent
collective response of the particle to an applied field.Comment: 4 pages, 4 figure
Finite temperature Drude weight of the one dimensional spin 1/2 Heisenberg model}
Using the Bethe ansatz method, the zero frequency contribution (Drude weight)
to the spin current correlations is analyzed for the easy plane
antiferromagnetic Heisenberg model. The Drude weight is a monotonically
decreasing function of temperature for all 0<Delta< 1, it approaches the zero
temperature value with a power law and it appears to vanish for all finite
temperatures at the isotropic Delta=1 point.Comment: 5 pages, 2 Postscript figure
Thermal transport in a spin-1/2 Heisenberg chain coupled to a (non) magnetic impurity
We explore the effect of a (non) magnetic impurity on the thermal transport
of the spin-1/2 Heisenberg chain model. This unique system allows to probe
Kondo-type phenomena in a prototype strongly correlated system. Using numerical
diagonalization techniques we study the scaling of the frequency dependent
thermal conductivity with system size and host-impurity coupling strength as
well as the dependence on temperature. We focus in particular on the analysis
of cutting-healing of weak links or a magnetic impurity by the host chain via
Kondo-like screening as the temperature is lowered.Comment: 7 pages, 12 figure
Comment on "Spin Transport properties of the quantum one-dimensional non-linear sigma model"
In a recent preprint (cond-mat/9905415), Fujimoto has used the Bethe ansatz
to compute the finite temperature, zero frequency Drude weight of spin
transport in the quantum O(3) non-linear sigma model in a magnetic field . We show here that, contrary to his claims, the results are in accord
with earlier semiclassical results (Sachdev and Damle, cond-mat/9610115). We
also comment on his 1/N expansion, and show that it does not properly describe
the long-time correlations.Comment: 4 page
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
Low-temperature transport in Heisenberg chains
A technique to determine accurately transport properties of integrable and
non-integrable quantum-spin chains at finite temperatures by Quantum
Monte-Carlo is presented. The reduction of the Drude weight by interactions in
the integrable gapless regime is evaluated. Evidence for the absence of a Drude
weight in the gapless regime of a non-integrable system with longer-ranged
interactions is presented. We estimate the effect of the non-integrability on
the transport properties and compare with recent experiments on one-dimensional
quantum-spin chains.Comment: accepted for publication (PRL
Magnetic excitations in the spin-1 anisotropic antiferromagnet
The spin-1 anisotropic antiferromagnet NiCl_2-4SC(NH2)_2 exhibits a
field-induced quantum phase transition that is formally analogous to
Bose-Einstein condensation. Here we present results of systematic high-field
electron spin resonance (ESR) experimental and theoretical studies of this
compound with a special emphasis on single-ion two-magnon bound states. In
order to clarify some remaining discrepancies between theory and experiment,
the frequency-field dependence of magnetic excitations in this material is
reanalyzed. In particular, a more comprehensive interpretation of the
experimental signature of single-ion two-magnon bound states is shown to be
fully consistent with theoretical results. We also clarify the structure of the
ESR spectrum in the so-called intermediate phase.Comment: 9 pages, 10 figure
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