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 $H \neq 0$. 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 NiCl2−4SC(NH2)2NiCl_2-4SC(NH_2)_2

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