Carrier filtering effect for enhanced thermopower in a body-centered tetragonal ruthenate

TL;DR

This study investigates the carrier filtering effect in Sr₂RuO₄, revealing enhanced out-of-plane thermopower likely due to hole filtering, which could be a universal feature in body-centered tetragonal materials.

Contribution

It provides experimental evidence of carrier filtering enhancing thermopower in a layered ruthenate, highlighting the role of crystal structure in thermoelectric properties.

Findings

Out-of-plane thermopower increases with temperature.

In-plane thermopower remains constant, consistent with Heikes formula.

Carrier filtering effect may be universal in body-centered tetragonal systems.

Abstract

Charged carriers in solids diffuse from hot to cold sides under temperature gradient to induce the thermoelectric voltage. Carrier filtering effect, which only passes either electrons or holes for the conduction process, is an efficient method to enhance such voltage, although it is challenging to experimentally realize it especially in conventional metals with weak energy dependence of the density of states near the Fermi level. Here we measure the in-plane and out-of-plane thermopower of the layered perovskite Sr$_2$RuO$_4$ single crystals above room temperature, and find that the out-of-plane thermopower is largely enhanced with increasing temperature, while the in-plane one seems to remain a temperature-independent constant value which is expected from the Heikes formula. The observed large out-of-plane thermopower may originate from the recently proposed intriguing hole filtering effect in the body-centered tetragonal system, in which the carrier hopping through the centered atom is essential. Thus, the present carrier filtering effect may be a universal property to be applicable in various materials belonging to such crystal system.