Drag thermopower in nanowires and bulk potassium crystals under the conditions of competition between the boundary and bulk mechanisms of phonon relaxation

Abstract

At low temperatures, the influence of anisotropy of elastic energy on the electron–phonon drag and thermopower in square cross-section single crystals of potassium has been studied. Change in the role of quasi-longitudinal and quasi-transverse phonons in the anisotropy of the drag thermopower under the conditions of transition from nanowires to bulk single crystals has been analyzed. In the regime of the Knudsen phonon gas flow, slow quasi-transverse phonons are shown to predominantly contribute to the drag thermopower in the nanowires but only in the directions close to the focusing of longitudinal phonons. Change in the drag thermopower anisotropy under the conditions of competition between the boundary and bulk phonon relaxation mechanisms has been investigated. As the sample’s cross-section increases, the drag thermopower anisotropy varies non-monotonically. This occurs due to stronger scattering of longitudinal phonons by electrons. When transitioning from the Knudsen phonon gas flow regime to the bulk relaxation mechanisms, the drag thermopower anisotropy first rises, then reaches a maximum and afterward disappears. The contribution of quasi-transverse phonons in bulk potassium crystals exceeds that of longitudinal phonons by an order of magnitude. This result evidences that the model of an isotropic medium cannot properly describe the electron–phonon drag in metals.