Thermoelectric anisotropy in Ba(Fe1-xCox)2As2 iron-based superconductor

TL;DR

This study investigates the in-plane thermoelectric anisotropy in Ba(Fe1-xCox)2As2 superconductors, revealing that nematicity persists above magnetic transition temperatures and varies with doping, indicating a non-magnetic origin.

Contribution

It provides detailed measurements of Seebeck and Nernst anisotropies across different doping levels, highlighting the complex nature of nematicity in iron-based superconductors.

Findings

Nernst anisotropy appears above magnetic transition temperatures in underdoped samples.

Transport properties show in-plane anisotropy even without magnetic order in optimally doped samples.

Anisotropy signs differ between underdoped and optimally doped samples, suggesting different nematicity origins.

Abstract

We report the in-plane anisotropy of the Seebeck and Nernst coefficients as well as of the electrical resistivity determined for the series of the strain-detwinned single crystals of Ba(Fe1-xCox)2As2. Two underdoped samples (x = 0.024, 0.045) exhibiting the transition from the tetragonal paramagnetic phase to the orthorhombic spin density wave (SDW) phase (at Ttr = 100 and 60 K, respectively) show an onset of the Nernst anisotropy at temperatures above 200 K, which is significantly higher than Ttr. In the optimally doped sample (x = 0.06) the transport properties also appear to be in-plane anisotropic below T = 120 K, despite the fact that this particular composition does not show any evidence of long-range magnetic order. However, the anisotropy observed in the optimally doped crystal is rather small and for the Seebeck and Nernst coefficients the difference between values measured along and across the uniaxial strain has opposite sign to those observed for underdoped crystals with x = 0.024 and 0.045. For these two samples, insensitivity of the Nernst anisotropy to the SDW transition suggests that the nematicity might be of other than magnetic origin.