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

摘要

In-plane longitudinal and transverse thermoelectric phenomena in two parent compounds of iron-based superconductors are studied. Namely, the Seebeck (S) and Nernst (v) coefficients were measured in the temperature range 10-300 K for BaFe_2As_2 and CaFe_2As_2 single crystals that were detwinned in situ. The thermoelectric response shows sizable anisotropy in the spin density wave (SDW) state 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 BaFe_2As_2 and CaFe_2As_2. Temperature dependences of S and v 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" holelike charge carriers. We also observe that the off-diagonal element of the Peltier tensor α_(xy) 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.