Resolving the Thermal Paradox: Many-body localization or fractionalization?

TL;DR

This paper investigates the 'thermal paradox' in insulators and superconductors, proposing that long thermal relaxation times caused by either many-body localization or neutral Fermi surfaces explain the observed phenomena.

Contribution

The authors reinterpret thermal conductivity data by modeling thermal relaxation as RC circuits, providing a new framework to distinguish between many-body localization and neutral Fermi surfaces.

Findings

Thermal relaxation times can be extremely long due to a thermal bottleneck.

Mapping thermal conductivity to RC circuits offers a new interpretative approach.

Re-evaluation of experimental data suggests ways to differentiate between the two theories.

Abstract

Thermal measurements of heat capacity and thermal conductivity in a wide range of insulators and superconductors exhibit a ``thermal paradox": a large linear specific heat reminiscent of neutral Fermi surfaces in samples that exhibit no corresponding linear temperature coefficient to the thermal conductivity. At first sight, these observations appear to support the formation of a continuum of thermally localized many-body excitations, a form of many-body localization that would be fascinating in its own right. Here, by mapping thermal conductivity measurements onto thermal RC circuits, we argue that the development of extremely long thermal relaxation times, a ``thermal bottleneck," is likely in systems with either many-body localization or neutral Fermi surfaces due to the large ratio between the electron and phonon specific heat capacities. We present a re-evaluation of thermal conductivity measurements in materials exhibiting a thermal paradox that can be used in future experiments to deliberate between these two exciting alternatives.