Thermal Hall conductivity near field-suppressed magnetic order in a Kitaev-Heisenberg model

TL;DR

This study examines the thermal Hall conductivity in a Kitaev-Heisenberg model under magnetic fields, revealing that half-quantized values are due to fine-tuning rather than topological Majorana states.

Contribution

It provides a comprehensive analysis using tensor network methods, showing that half-quantized thermal Hall effects near field-suppressed magnetic order are not indicative of Ising topological order.

Findings

Half-quantized $_{xy}/T$ decreases monotonously with field.

Significant $_{xy}/T$ observed near field-suppressed magnetic order.

Half-quantization is a fine-tuning effect, not a topological Majorana state.

Abstract

We investigate thermal Hall conductivity $\kappa_{xy}$ of a $J$-$K$ Kitaev-Heisenberg model with a Zeeman field in the $(111)$ direction in the light of the recent debate surrounding the possible re-emergence of Ising topological order (ITO) and half-quantized $\kappa_{xy}/T$ upon field-suppression of long-range magnetic order in Kitaev materials. We use the purification-based finite temperature Tensor Network approach making no prior assumptions about the nature of the excitations: Majorana, visons or spin waves. For purely Kitaev interactions and fields $h/K \gtrsim 0.02$ sufficient to degrade ITO, the peak $\kappa_{xy}/T$ monotonously decreases from half-quantization associated with lower fields - a behavior reminiscent of vison fluctuation corrections. For higher fields $h/K\gtrsim 0.1,$ we find the results qualitatively consistent with a spin-wave treatment. In our $J$-$K$ model (with ferro-$K$ and antiferro-$J$), in the vicinity of field-suppressed magnetic order, we found $\kappa_{xy}/T$ to be significant, with peak magnitudes exceeding half-quantization followed by a monotonous decrease with increasing $h.$ We thus conclude that half-quantized thermal Hall effect in the vicinity of field suppressed magnetic order in our model, is a fine-tuning effect and is not associated with a Majorana Hall state with ITO.