Epitaxially strained Na0.7CoO2 thin films on SrTiO3 buffer layer

We prepared Na0.7CoO2 thin films on (0 0 1) Al2O3 substrates that showed a negligibly strained structure and a metallic behavior similar to Na0.7CoO2 single crystal. In particular, a Na0.7CoO2 thin film on a (1 1 1) SrTiO3/(0 0 1) Al2O3 substrate exhibited an elongated c-lattice constant as well as an anomaly in the electric transporting behavior: a transition from an insulating state to a metallic state at 105 K. This change is caused by the structural symmetry break near the cubic-to-tetragonal transition temperature of SrTiO3. From spectroscopic ellipsometry analysis, we obtained optical conductivities for the Na0.7CoO2 thin films on the Al2O3 and SrTiO3/Al2O3 substrates. The Na0.7CoO2 thin film on the SrTiO3/Al2O3 substrate exhibited higher energy splitting between eg and a1g than the Na0.7CoO2 thin film on the Al2O3 substrate, resulting from the c-lattice elongation.