Epitaxially Strained Na_(0.7)coo_2 Thin Films On Srtio_3 Buffer Layer

摘要

We prepared Na_(0.7)CoO_2 thin films on (0 01) Al_2O_3 substrates that showed a negligibly strained structure and a metallic behavior similar to Na_(0.7)CoO_2 single crystal. In particular, a Na_(0.7)CoO_2 thin film on a (111) SrTiO_3/(0 01) Al_2O_3 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 SrTiO_3. From spectroscopic ellipsometry analysis, we obtained optical conductivities for the Na_(0.7)CoO_2 thin films on the Al_2O_3 and SrTiO_3/Al_2O_3 substrates. The Na_(0.7)CoO_2 thin film on the SrTiO_3/ Al_2O_3 substrate exhibited higher energy splitting between e_g and a_(1g) than the Na_(0.7)CoO_2 thin film on the Al_2O_3 substrate, resulting from the c-lattice elongation.