Apparent nonreciprocal transport in FeSe bulk crystals

TL;DR

This study investigates apparent nonreciprocal transport in bulk FeSe crystals, finding that observed signals are due to thermoelectric effects rather than genuine nonreciprocal charge transport.

Contribution

The paper clarifies that nonreciprocal signals in FeSe bulk crystals are caused by thermoelectric effects, not intrinsic nonreciprocal transport, contrasting with previous claims of a superconducting diode effect.

Findings

Second-harmonic resistance observed in some samples.

Thermoelectric effects, not genuine nonreciprocal transport, explain the signals.

Consistent with recent preprints on FeSe flakes.

Abstract

We performed low-frequency ac first- and second-harmonic resistance measurements and dc $I-V$ measurements on bulk FeSe crystals in a temperature range between 1.8 and 150 K and in magnetic field up to 14 T. We observed considerable second-harmonic resistance, indicative of nonreciprocal charge transport, in some samples. By examining correlation between contact resistances and second-harmonic signals, we concluded that the second-harmonic resistance was not due to the genuine nonreciprocal transport effect but was caused by joule heating at a current contact through the thermoelectric effect. Our conclusion is consistent with a recent preprint (Nagata \textit{et al.}, arXiv:2409.01715), in which the authors reported a zero-field superconducting diode effect in devices fabricated with FeSe flakes and attributed it to the thermoelectric effect.