Anisotropic thermopower and magnetothermopower in a misfit-layered calcium cobaltite

An unusual anisotropy of thermopower and magnetothermopower has been observed in the powerful thermoelectric Ca$_3$Co$_4$O$_{9+\delta}$ single crystal. The in-plane thermopower is about twice as big as the out-of-plane thermopower. Combining {\em ab initio} band structure calculation with semi-classical model analysis, we understand this anisotropy with band structure effects and especially with anisotropic Fermi surface. We find that a strong anisotropy in the topology of Fermi surface leads to the anisotropy of (magneto)thermopower. This study may also shed light on anisotropic properties of other layered cobalt oxides.