Giant thermal Hall conductivity from neutral excitations in the pseudogap phase of cuprates

TL;DR

This study reveals a large negative thermal Hall conductivity in the pseudogap phase of cuprates, likely caused by neutral excitations with spin chirality, suggesting exotic quantum states akin to spin liquids.

Contribution

It provides evidence of a significant thermal Hall effect in the pseudogap phase, indicating the presence of neutral excitations and advancing understanding of this mysterious phase.

Findings

Large negative thermal Hall conductivity appears at the pseudogap onset.

The effect persists in insulating states, ruling out charge carriers.

The phenomenon is attributed to neutral excitations with spin chirality.

Abstract

The nature of the pseudogap phase of cuprates remains a major puzzle. Although there are indications that this phase breaks various symmetries, there is no consensus on its fundamental nature. Although Fermi-surface, transport and thermodynamic signatures of the pseudogap phase are reminiscent of a transition into a phase with antiferromagnetic order, there is no evidence for an associated long-range magnetic order. Here we report measurements of the thermal Hall conductivity $\kappa_{\rm xy}$ in the normal state of four different cuprates (Nd-LSCO, Eu-LSCO, LSCO, and Bi2201) and show that a large negative $\kappa_{\rm xy}$ signal is a property of the pseudogap phase, appearing with the onset of that phase at the critical doping $p^*$. Since it is not due to charge carriers -- as it persists when the material becomes an insulator, at low doping -- or magnons -- as it exists in the absence of magnetic order -- or phonons -- since skew scattering is very weak, we attribute this $\kappa_{\rm xy}$ signal to exotic neutral excitations, presumably with spin chirality. The thermal Hall conductivity in the pseudogap phase of cuprates is reminiscent of that found in insulators with spin-liquid states. In the Mott insulator LCO, it attains the highest known magnitude of any insulator.