Low-dimensional correlations under thermal fluctuations

Abstract

We study the correlation functions of quantum spin 1/2 two-leg ladders at finite temperature, under a magnetic field, in the gapless phase at various relevant temperatures T = 0, momenta q, and frequencies ω. We compute those quantities using the time-dependent density-matrix renormalization group (T-DMRG) in an optimal numerical scheme. We compare these correlations with the ones of dimerized quantum spin chains and simple spin chains, that we compute by a similar technique. We analyze the intermediate energy modes and show that the effect of temperature leads to the formation of an essentially dispersive mode corresponding to the propagation of a triplet mode in an incoherent background, with a dispersion quite different from the one occurring at very low temperatures. We compare the low-energy part of the spectrum with the predictions of the Tomonaga-Luttinger liquid field theory at finite temperature. We show that the field theory describes in a remarkably robust way the low-energy correlations for frequencies or temperatures up to the natural cutoff (the effective dispersion) of the system. We discuss how our results could be tested in, e.g., neutron-scattering experiments. arXiv:1901.04339v2 [cond-mat.str-el]

Possibility to detect the bound state of the Heisenberg ferromagnetic chain at intermediate temperature

“…The computation of the DSF at finite temperature can be performed by using a time-dependent DMRG algorithm on a thermal ensemble (denoted as ρβ ) pioneered by Barthel et al [18,19] and Kestin et al [20]. The thermal ensemble is a mixed state and the matrix product density operator (MPDO) ansatz is better suited for simulating mixed states.…”