[25] Epitaxially strained Na_0.7CoO₂ thin films on SrTiO₃ buffer layer

We prepared Na_0.7CoO₂ 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 Na_0.7CoO₂ thin film on a (111) SrTiO₃/(001) 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 Na_0.7CoO₂ thin films on the Al₂O₃ and SrTiO₃/Al₂O₃ substrates. The Na_0.7CoO₂ thin film on the SrTiO₃/Al₂O₃ substrate exhibited higher energy splitting between eg and a1g than the Na_0.7CoO₂2 thin film on the Al₂O₃ substrate, resulting from the c-lattice elongation.