Searching for the Slater transition in the pyrochlore Cd 2 Os 2 O 7 with infrared spectroscopy

Infrared reflectance measurements were made on the single crystal pyrochlore ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ in order to examine the transformations of the electronic structure and crystal lattice across the boundary of the metal-insulator transition (MIT) at ${T}_{\mathrm{MIT}}=226 \mathrm{K}.$ All predicted IR active phonons are observed in the conductivity over all temperatures and the oscillator strength is found to be temperature independent. These results indicate that charge ordering plays only a minor role in the MIT and that the transition is strictly electronic in nature. The conductivity shows the clear opening of a gap with $2\ensuremath{\Delta}{=5.2k}_{B}{T}_{\mathrm{MIT}}.$ The gap opens continuously, with a temperature dependence similar to that of Bardeen-Cooper-Schrieffer (BCS) superconductors, and the gap edge having a distinct $\ensuremath{\sigma}(\ensuremath{\omega})\ensuremath{\sim}{\ensuremath{\omega}}^{1/2}$ dependence. All of these observables support the suggestion of a Slater transition in ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}.$