Anisotropic thermopower and planar Nernst effect in Ga1-xMnxAs

We have carried out the thermopower and magneto-thermopower measurements on three Ga1-xMnxAs~ (x=0.039, 0.049, and 0.059) samples above and below their~Curie temperatures. Below the Curie temperature, we have observed magnetic field dependence in both parallel (longitudinal) and perpendicular (transverse) directions to the temperature gradient.~ The magnetic field dependence does not depend on the relative orientation between the temperature gradient and crystallographic direction of the GaAs.~ The transverse thermopower or the planar Nernst effect,~shows abrupt jumps as the magnetization switches between the four-fold symmetry axes, which resembles the giant planar Hall effect found in similar materials.~ Although the longitudinal thermopower is not sensitive to the same switchings of the magnetization, it has a strong magnetic field dependence as the magnetization rotates.~ We have measured both effects vs. the magnetic field orientation with respect to the temperature gradient.~~Similar to the origin of the giant planar Hall effect, we attribute the longitudinal and transverse effects to the anistropic magneto-thermopower.~ We interpret the both effects~in the context of~the~anisotropic transport.