Anisotropic Seebeck coefficient of $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ in the incoherent regime

TL;DR

This study investigates the anisotropic Seebeck coefficient of Sr2RuO4 at high temperatures, revealing unexpected anisotropy above 300 K that challenges conventional entropic interpretations of thermoelectric effects in metals.

Contribution

It provides experimental evidence of high-temperature Seebeck anisotropy in Sr2RuO4, aligning with recent theoretical predictions and challenging existing entropic models.

Findings

Seebeck coefficient becomes isotropic near room temperature

Above 300 K, Seebeck coefficient exhibits increasing anisotropy

Experimental results qualitatively agree with ab-initio calculations

Abstract

Intuitive entropic interpretations of the thermoelectric effect in metals predict an isotropic Seebeck coefficient at high temperatures in the incoherent regime even in anisotropic metals since entropy is not directional. $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ is an enigmatic material known for a well characterised anisotropic normal state and unconventional superconductivity. Recent ab-initio transport calculations of $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ that include the effect of strong electronic correlations predicted an enhanced high-temperature anisotropy of the Seebeck coefficient at temperatures above 300 K, but experimental evidence is missing. From measurements on clean $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ single crystals along both crystallographic directions, we find that the Seebeck coefficient becomes increasingly isotropic upon heating towards room temperature as generally expected. Above 300 K, however, $S$ acquires a new anisotropy which rises up to the highest temperatures measured (750 K), in qualitative agreement with calculations. This is a challenge to entropic interpretations and highlights the lack of an intuitive framework to understand the anisotropy of thermopower at high temperatures.