High temperature thermoelectric properties of Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y}$ thin films grown by Pulsed Laser Deposition

Misfit cobaltates are strongly correlated materials showing exceptional thermoelectric properties. Sodium cobaltate is the model system for this class of materials, which show high thermopower in the limit of large carrier concentration. Even though the exact origin of enhanced thermoelectricity is debatable, there is a general consensus that strong correlation plays a huge role in this effect. In order to better understand the nature of these materials, we chose to investigate Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y}$, which is one of the least studied materials in thin film form. We grew c-axis oriented thin films of Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y}$ on c-plane sapphire substrates by Pulsed Laser Deposition method. The thermoelectric properties of the films namely thermopower, electrical and thermal conductivities were measured over a temperature range of 300-800 K. Magnetic measurements, Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR) were performed to determine the valence and spin states of Cobalt and the validity of Heikes formula is checked for this compound.