Phonon anomaly at the charge ordering transition in1T−TaS2

Abstract

The infrared reflectance of the transition-metal chalcogenide $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ has been measured at temperatures from 30 K to 360 K over $30--45000{\mathrm{cm}}^{\ensuremath{-}1}$ (4 meV--5.5 eV). The optical conductivity was obtained by Kramers-Kronig analysis. At 360 K only modest traces of the phonon lines are noticeable. The phonon modes are followed by a pseudo-gap-like increase of the optical conductivity, with direct optical transitions observed at frequencies above 1 eV. As the temperature decreases, the low-frequency conductivity also decreases, phonon modes become more pronounced, and a pseudogap develops into a gap at $800{\mathrm{cm}}^{\ensuremath{-}1}$ (100 meV). We observe an anomalous frequency dependence of the $208{\mathrm{cm}}^{\ensuremath{-}1}$ infrared-active phonon mode. This mode demonstrates softening as the temperature decreases below the 180-K transition. The same mode demonstrates strong hysteresis of the frequency and linewidth changes, similar in its temperature behavior to the hysteresis in the dc resistivity. We discuss a possible relation of the observed softening of the mode to the structural changes and changes in electronic properties associated with the 180-K transition.