Unusual Transport and Strongly Anisotropic Thermopower in ${\mathrm{PtCoO}}_2$ and ${\mathrm{PdCoO}}_2$

We show, using Boltzmann transport calculations and analysis of experimental data, that hexagonal ${\mathrm{PdCoO}}_2$ and ${\mathrm{PtCoO}}_2$ have a highly unusual metallic transport. The in-plane transport is typical of a very good metal, with high conductivity and low positive thermopower. The $c$-axis transport is completely different, with 2 orders of magnitude lower, but still coherent, conductivity and remarkably a very large negative thermopower. This large anisotropy of the thermopower provides an opportunity for investigating transport in a highly unusual regime using bulk materials.

Figure 1

Calculated band structure of ${\mathrm{PtCoO}}_2$ around $E_F$, which is at 0 eV, including spin orbit.

Figure 2

Calculated Fermi surface of ${\mathrm{PtCoO}}_2$ including spin orbit. $\Gamma$ is at the center.

Figure 3

Constant scattering time $S(T)$ for ${\mathrm{PtCoO}}_2$ and ${\mathrm{PdCoO}}_2$ in-plane and along $c$. Note the different scales for the two directions.

Figure 4

Fermi surface of ${\mathrm{PtCoO}}_2$ as in Fig. 2 but with $E_F$ increased by 100 meV. Note the enhanced three dimensionality along the corners of the hexagonal cylinders.

Figure 5

Calculated conductivity anisotropy ${\sigma }_{zz}/{\sigma }_{xx}$ at 260 K as a function of chemical potential, where zero is the stoichiometric value for the compound.