Scattering-dominated high-temperature phase of1T−TiSe2: An optical conductivity study

The controversy regarding the precise nature of the high-temperature phase of $1T\text{\ensuremath{-}}\mathrm{TiS}{\mathrm{e}}₂$ lasts for decades. It has intensified in recent times when new evidence for the excitonic origin of the low-temperature charge-density wave state started to unveil. Here we address the problem of the high-temperature phase through precise measurements and detailed analysis of the optical response of $1T\text{\ensuremath{-}}\mathrm{TiS}{\mathrm{e}}₂$ single crystals. The separate responses of electron and hole subsystems are identified and followed in temperature. We show that neither semiconductor nor semimetal pictures can be applied in their generic forms as the scattering for both types of carriers is in the vicinity of the Ioffe-Regel limit with decay rates being comparable to or larger than the offsets of band extrema. The nonmetallic temperature dependence of transport properties comes from the anomalous temperature dependence of scattering rates. Near the transition temperature the heavy electrons and the light holes contribute equally to the conductivity; this surprising coincidence is regarded as the consequence of dominant intervalley scattering that precedes the transition. The low-frequency peak in the optical spectra is identified and attributed to the critical softening of the $L$-point collective mode.