Optical properties of the delafossite PdCoO₂

The delafossite PdCoO₂ is remarkable for its extremely low in-plane residual resistivity at low temperature [1]. The in-plane optical properties have been determined over a wide frequency range at a variety of temperatures. At 295 K the reflectance of this material is extremely high (>99%) before encountering a sharp plasma edge at ~ 6000 cm^-1, above which it decreases rapidly. The real part of the optical conductivity reveals that the free-carrier (intraband) response falls well below the interband excitations, allowing the plasma frequency to be determined from the f-sum rule, ω_p ~ 33500 cm^-1; this value is in good agreement with first principle results. The Drude model for the optical properties of a metal may be used to estimate the free-carrier scattering rate, 1/τ ~ 80 cm^-1 at 5 K, which is well above the transport estimate of 1/τ < 0.2 cm^-1; this suggests that the scattering rate is strongly renormalized with frequency, and that the optical value is an average of these scattering rates. Structure is observed in the region of the expected infrared-active modes; however, the location and strength of these features suggests that some are not simple vibrations.

[1] C. W. Hicks et al., Phys. Rev. Lett. 109, 116401 (2012).