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

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.