Formation of the density wave energy gap in Na$_2$Ti$_2$Sb$_2$O: an optical spectroscopy study

TL;DR

This optical spectroscopy study on Na$_2$Ti$_2$Sb$_2$O uncovers a density wave energy gap formation at 114 K, showing strong coupling effects and weak correlation in this Ti-based compound.

Contribution

First optical spectroscopy investigation revealing the density wave gap and strong coupling behavior in Na$_2$Ti$_2$Sb$_2$O.

Findings

Significant spectral change across 114 K phase transition

Formation of a density-wave energy gap

Large gap ratio indicating strong coupling

Abstract

We performed optical spectroscopy measurement on single crystals of Na$_2$Ti$_2$Sb$_2$O, which is the parent compound for the newly discovered Ti-based superconductors. The study reveals 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 part of the free carrier spectral weight and causes a dramatic reduction of the carrier scattering rate. The ratio of 2$\Delta/k_BT_s\approx$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.