P. Prelovsek

Many-body localization in the Heisenberg XXZ magnet in a random field

We numerically investigate Heisenberg XXZ spin-1/2 chain in a spatially random static magnetic field. We find that tDMRG simulations of time evolution can be performed efficiently, namely the dimension of matrices needed to efficiently represent the time-evolution increases linearly with time and entanglement entropies for typical chain bipartitions increase logarithmically. As a result, we show that for large enough random fields infinite temperature spin-spin correlation function displays exponential localization in space indicating insulating behavior of the model.

Transport and conservation laws

We study the effect of conservation laws on the finite-temperature transport properties in one-dimensional integrable quantum many-body systems. We show that the energy current is closely related to the first conservation law in these systems and therefore the thermal transport coefficients are anomalous. Using an inequality on the time decay of current correlations we show how the existence of conserved quantities implies a finite charge stiffness (weight of the zero-frequency component of the conductivity) and so ideal conductivity at finite temperatures.

Integrability and Ideal Conductance at Finite Temperatures

We analyse the finite temperature charge stiffness D(T>0), by a generalization of Kohns method, for the problem of a particle interacting with a fermionic bath in one dimension. We present analytical evidence, using the Bethe ansatz method, that D(T>0) is finite in the integrable case where the mass of the particle equals the mass of the fermions and numerical evidence that it vanishes in the nonintegrable one of unequal masses. We conjecture that a finite D(T>0) is a generic property of integrable systems.

EVIDENCE FOR IDEAL INSULATING OR CONDUCTING STATE IN A ONE-DIMENSIONAL INTEGRABLE SYSTEM

Using numerical diagonalization techniques we analyze the finite temperature/frequency conductance of a one dimensional model of interacting spinless fermions. Depending on the interaction, the observed finite temperature charge stiffness and low frequency conductance indicate a fundamental difference between integrable and non-integrable cases. The integrable systems behave as ideal conductors in the metallic regime and as ideal insulators in the insulating one. The non-integrable systems are, as expected, generic conductors in the metallic regime and activated ones in the insulating regime.

Magnetic fluctuations and resonant peak in cuprates: Towards a microscopic theory

Magnetic fluctuations and evolution of the resonant peak with doping in superconducting cuprates are studied within the planar t-J model. The analysis is based on the equations of motion for spins and the memory-function approach to dynamics of magnetic response where the main damping of the low-energy spin collective mode comes from the decay into fermionic degrees of freedom. In general the normal-state damping is large, leading to a overdamped collective mode. At an intermediate doping in the superconducting phase, a d-wave gap leads to a sharp resonant peak with reduced intensity and downward dispersion. At low doping the damping function is closely related to the c-axis optical conductivity, and the resonant-peak behavior is determined by two energy scales: the pseudogap and the coherent superconducting gap.

Thermodynamic Properties of the Planar t − J Model

Several thermodynamic quantities within the planar

Scaling of the magnetic response in doped antiferromagnets.

t-J

Open Problems in Strongly Correlated Electron Systems

model are calculated using the

Breakdown of the generalized Gibbs ensemble for current-generating quenches.

T>0

Eigenstate thermalization within isolated spin-chain systems

Lanczos method on clusters of up to 26 sites. Hole density