Resonant Ultrasound Studies of Quasi-2D Na_xCoO₂

Sodium cobaltate possesses all the important traits needed in a material for potential thermoelectric applications yet it shows little regard for the traditional physics of thermoelectrics. As existing theories regarding the phenomenon in Na_xCoO₂ are incomplete, we have set out to elucidate some of the issues by looking into the elastic response of the system. To this effect, we have employed Resonant Ultrasound Spectroscopy to obtain the elastic tensor at various temperatures. As the Na_0.75CoO₂ stoichiometry seemed to potentially offer the most rewards due to a pronounced ground state and anomalous thermopower at higher temperatures, this composition was chosen for study. Polycrystal samples and floating zone grown single crystal samples were studied in temperatures ranging from 5K to 300K and in various magnetic fields from 0T to 5T.

Elastic tensors were obtained for various samples whereas the absolute values of the moduli show insufficient agreement to be conclusive. The temperature dependence however is very consistent and displays two regions of interest (30K-60K temperature range and 250K- 300K range). In both regions, the material appears to stiffen with increasing temperature before relaxing back to its lower initial state. Anomalous activities at these temperatures have not been observed by most existing studies, thus have not been the subject of much discussion to date. As neither our data nor that of thermal conductivity measurements has displayed any signs of the 22K low temperature transition observed by magnetic and electronic transport measurements, it is our belief that the transition into the low temperature ground state is characterized by a rather weak coupling between phonons and the charge carriers that drive the transition. The natures of the high and low temperature anomalies are currently unclear.