Formation of the density wave energy gap in Na2Ti2Sb2O: An optical spectroscopy study

We performed an optical spectroscopy measurement on single crystals of Na2Ti2Sb2O, which is a parent compound for the newly discovered Ti-based superconductors. The study reveals a significant spectral change across the phase transition at 114 K and formation of a density wave type energy gap. The opening of the gap removes most of the free carrier spectral weight and causes a dramatic reduction in the carrier scattering rate. The ratio of 2 Delta/k(B)T(s) approximate to 14 is considerably larger than the mean-field value based on the weak-coupling BCS theory. The study also reveals a weak correlation effect in the titanium oxypnictides. DOI: 10.1103/PhysRevB.87.100507