Spin dynamics and transport in gapped one-dimensional Heisenberg antiferromagnets at nonzero temperatures

TL;DR

This paper develops a semiclassical theory for spin dynamics and transport in gapped one-dimensional Heisenberg antiferromagnets at nonzero temperatures, providing universal expressions and comparing with experiments.

Contribution

It introduces a universal semiclassical framework for understanding spin dynamics and transport in gapped 1D Heisenberg antiferromagnets at finite temperatures, with exact results and experimental validation.

Findings

Universal expressions for thermal broadening of INS peaks

Good agreement with NMR relaxation rate experiments

Identification of a second peak due to bound states

Abstract

We present the theory of nonzero temperature ($T$) spin dynamics and transport in one-dimensional Heisenberg antiferromagnets with an energy gap $\Delta$. For $T << \Delta$, we develop a semiclassical picture of thermally excited particles. Multiple inelastic collisions between the particles are crucial, and are described by a two-particle S-matrix which has a super-universal form at low momenta. This is established by computations on the O(3) $\sigma$-model, and strong and weak coupling expansions (the latter using a Majorana fermion representation) for the two-leg S=1/2 Heisenberg antiferromagnetic ladder. As an aside, we note that the strong-coupling calculation reveals a S=1, two particle bound state which leads to the presence of a second peak in the T=0 inelastic neutron scattering (INS) cross-section for a range of values of momentum transfer. We obtain exact, or numerically exact, universal expressions for the thermal broadening of the quasi-particle peak in the INS cross-section, for the magnetization transport, and for the field dependence of the NMR relaxation rate $1/T_1$ of the effective semiclassical model: these are expected to be asymptotically exact for the quantum antiferromagnets. The results for $1/T_1$ are compared with the experimental findings of Takigawa et al and the agreement is quite good. In the regime $\Delta < T < (a typical microscopic exchange)$ we argue that a complementary description in terms of semiclassical waves applies, and give some exact results for the thermodynamics and dynamics.