Terahertz Spectroscopy of Osmate Double Perovskites

Double perovskites containing 5d transition metal elements allow study of the interplay of spin-orbit coupling and electronic correlations due to the heavy nuclei and large electronic wavefunctions. Here we have studied polycrystalline Sr$_{\mathrm{2}}$MOsO$_{\mathrm{6\thinspace }}$(M $=$ Mg, Fe, Co; with Os electronic configuration of d$^{\mathrm{2}}$, d$^{\mathrm{3}}$, d$^{\mathrm{2}}$, respectively) with time-domain terahertz spectroscopy. Terahertz electrodynamics seem to be decoupled from observed magnetic and structural phase transitions in M$=$Mg, Co. A strong absorption is measured in M$=$Mg, Co around 1.5 THz, which softens with temperature, as expected for an optical phonon. The effectiveness of the variable-range hopping model and the origin of higher temperature conductivity are examined. Work at OSU supported by the NSF MRSEC Center for Emergent Materials under Grant DMR-1420451.