Anisotropy of the Seebeck and Nernst coefficients in parent compounds of the iron-based superconductors

TL;DR

This study investigates the anisotropic thermoelectric properties of parent iron-based superconductors BaFe2As2 and CaFe2As2, revealing significant anisotropy in the spin density wave state and insights into the electronic contributions affecting transport phenomena.

Contribution

It provides the first detailed analysis of anisotropic Seebeck and Nernst coefficients in detwinned parent compounds, highlighting the role of Dirac and hole-like carriers in thermoelectric behavior.

Findings

Significant thermoelectric anisotropy in SDW state

Presence of highly mobile Dirac electrons affecting transport

Mott formula not applicable to SDW state

Abstract

In-plane longitudinal and transverse thermoelectric phenomena in two parent compounds of iron-based superconductors are studied. Namely, the Seebeck (S) and Nernst (n) coefficients were measured in the temperature range 10 - 300 K for BaFe2As2 and CaFe2As2 single crystals that were detwinned in-situ. The thermoelectric response shows sizeable anisotropy in the spin density wave state (SDW) for both compounds, while some dissimilarities in the vicinity of the SDW transition can be attributed to the different nature of the phase change in BaFe2As2 and CaFe2As2. Temperature dependences of S and n can be described within a two-band model that contains a contribution from highly mobile, probably Dirac, electrons. The Dirac band seems to be rather isotropic, whereas most of the anisotropy in the transport phenomena could be attributed to "regular" hole-like charge carriers. We also observe that the off-diagonal element of the Peltier tensor axy is not the same for the a and b orthorhombic axes, which indicates that the widely used Mott formula is not applicable to the SDW state of iron-based superconductors.