Phonon focusing and anisotropy of drag thermopower in potassium thin films at low temperatures

Abstract

We have investigated the influence of elastic energy anisotropy on the dependences of drag thermopower in potassium thin films on the orientation of the film planes, directions of heat flux, and their geometric parameters. It is shown that the drag thermopower in square-shaped potassium films with orientations {100} and {111} is isotropic in the film planes. Their anisotropy is determined by the orientation of the film plane. As for square-shaped films with {110} orientation, the drag thermopower dependences appear ellipsoidal, with a long axis that points towards the [100] direction. As the transverse parameters of potassium films with {100} and {111} orientations are increased, the drag thermopower anisotropy of the films in the [110] direction varies in a non-monotonic manner. This effect is due to the competition between the contributions of slow quasi-transverse modes and longitudinal phonons. The relaxation rate of the latter by electrons has proved to be 30 times higher than that for the slow mode. The drag thermopower and phonon mean free paths in potassium films with lengths much larger than their widths in the boundary scattering regime are found to become anisotropic. However, accounting for the electron–phonon relaxation leads to isotropization of these quantities in thin films with {100} and {111} film plane orientations, with the orientation anisotropy being retained. For long films with {110} orientation, the shape of the drag thermopower dependences remains ellipsoidal with a long axis along the [100] direction.