Heat transport of clean spin-ladders coupled to phonons: Umklapp scattering and drag

TL;DR

This paper investigates how weakly broken conservation laws and pseudo-momenta in clean spin-ladder compounds influence low-temperature heat transport, highlighting the role of spin-phonon drag and symmetry considerations.

Contribution

It introduces a theoretical framework linking approximate symmetries to large heat conductivities and analyzes the magnetic contribution's behavior depending on system parameters.

Findings

Magnetic heat conductivity can be positive or negative depending on parameters.

Spin-phonon drag dominates heat transport in most regimes.

Approximate symmetries explain large heat conductivities in clean spin ladders.

Abstract

We study the low-temperature heat transport in clean two-leg spin ladder compounds coupled to three-dimensional phonons. We argue that the very large heat conductivities observed in such systems can be traced back to the existence of approximate symmetries and corresponding weakly violated conservation laws of the effective (gapful) low--energy model, namely pseudo-momenta. Depending on the ratios of spin gaps and Debye energy and on the temperature, the magnetic contribution to the heat conductivity can be positive or negative, and exhibit an activated or anti-activated behavior. In most regimes, the magnetic heat conductivity is dominated by the spin-phonon drag: the excitations of the two subsystems have almost the same drift velocity, and this allows for an estimate of the ratio of the magnetic and phononic contributions to the heat conductivity.