Optical properties of Na0.7CoO2
The discovery of superconductivity at 5 K in hydrated sodium cobaltate has attracted considerable attention. How water inclusion triggers superconductivity in NaxCoO2 is not fully understood yet. The investigation of non-hydrated sample is therefore of relevance and a considerable research effort has been devoted to NaxCoO2 specimens with x ranging between 0.3 and 0.85. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator for x = 0.5, to a Curie-Weiss metal around 0.7, and finally to a weak-moment magnetically ordered state for x > 0.75. This latter phase is supposed to be equivalent to a so called spin-density-wave (SDW) metal. Several recent investigations, based on magnetic, thermal and transport properties as well as muon spin spectroscopy, indicate the formation of a SDW metallic state for x = 0.75.
Optical reflectivity experiments (Fig. 1.7a) are well-known tools in order to achieve information about the electrodynamic response of the investigated system, and to shed light on its electronic structure. The capability to cover the energy range from the far-infrared up to the ultraviolet allows us to perform reliable Kramers-Kronig transformation, in order to obtain the absorption spectrum (i.e. the complex optical conductivity). To the complex optical conductivity we apply the generalized Drude model, extracting the frequency dependence of the scattering rate Γ (Fig. 1.8) and effective mass m* (not shown here) of the itinerant charge carriers.
We have provided the complete absorption spectrum of Na0.7CoO2 (Fig.1.7b) We have established that Γ(ω) ~ ω at low temperatures (Fig. 1.8). Ruvalds and Virosztek proposed a while ago a Fermi-surface nesting scenario for describing the optical properties of superconducting oxides. They showed that Fermi-surface nesting modifies the electron-electron scattering and therefore yields an unusual variation of the optical reflectivity. Within this scenario, also applicable for the charge- and spin-density-wave state where nesting is an essential ingredient, the effective Drude component is characterized by a relaxation rate that is linear in frequency for ω > T and the reflectivity is also linear in frequency in a broad spectral range. Our data agree with the theoretical predictions. Therefore, Na0.7CoO2 seems to be in the proximity of a spin-density-wave metallic state.