Anisotropy of the Seebeck and Nernst coefficients in BaFe₂As₂ and CaFe₂As₂

In-plane anisotropic longitudinal and transverse thermoelectric phenomena in the parent compounds of iron-based superconductors are studied. Namely, the Seebeck (S) and Nernst (ν) coefficients were measured in detwinned BaFe₂As₂ and CaFe₂As₂ single crystals in the temperature range 10 – 300 K. The thermoelectric response shows sizeable anisotropy in the spin density wave (SDW) state for both compounds, while dissimilarities in the vicinity of the SDW transition can be attributed to different order of the phase transition in BaFe₂As₂ and CaFe₂As₂. Temperature dependences of S and ν can be manifestations of an influence of highly mobile, perhaps Dirac, electrons in the two-band model. The Dirac band seems to be rather isotropic, whereas most of the anisotropy in transport phenomena could be attributed to “regular” hole-like charge carriers. We also observe that the off-diagonal element of the Peltier tensor α_xy differs between a and b orthorhombic axis, which indicates that the widely used Mott formula is not applicable to the SDW state of iron-based superconductors.