Large anisotropic thermal conductivity of the intrinsically two-dimensional metallic oxide PdCoO2

The highly conductive layered metallic oxide PdCoO2 is a near-perfect analog to an alkali metal in two dimensions. It is distinguished from other two-dimensional (2D) electron systems where the Fermi surface does not reach the Brillouin zone boundary by a high planar electron density exceeding 10(15) cm(-2). The simple single-band quasi-2D electronic structure results in strongly anisotropic transport properties and limits the effectiveness of electron-phonon scattering. Measurements on single crystals in the temperature range from 10 to 300 K show that the thermal conductivity is much more weakly anisotropic than the electrical resistivity, as a result of significant phonon heat transport. The in-plane thermoelectric power is linear in temperature at 300 K and displays a purity-dependent peak around 50 K. Given the extreme simplicity of the band structure, it is possible to identify this peak with phonon drag driven by normal electron-phonon scattering processes.