Fabrication and optical conductivities of strained epitaxial NaxCoO2 thin films: x=0.5, 0.7

Epitaxial γ phase-NaxCoO2 thin films were deposited on (001) sapphire by the pulsed laser deposition method. To fabricate epitaxial Na0.5CoO2 thin films, we used a solution of iodine-dissolved acetonitrile and obtained an epitaxial Na0.5CoO2 thin film with a high crystallinity because of Na deintercalation of epitaxial Na0.7CoO2. From the spectroscopic ellipsometry analysis, we obtained the optical constants as well as the optical conductivities for the Na0.5CoO2 and Na0.7CoO2 thin films. The energy splitting between eg and a1g increased because of the structural strain of the Na0.7CoO2 thin film. It is inferred that the structural strain is the source for the lower resistivity and the preservation of the strongly correlated system up to 200 K for the Na0.7CoO2 thin film. On the other hand, the strain in the Na0.5CoO2 thin film was not affected, and the charge-ordering state and the Na content (x=0.5) only cause the charge-ordering state.