Scattering dominated high-temperature phase of 1T-TiSe2: an optical conductivity study

TL;DR

This study investigates the high-temperature phase of 1T-TiSe2 using optical conductivity measurements, revealing complex scattering behavior and the role of intervalley scattering in its electronic properties.

Contribution

It provides a detailed analysis of the optical response of 1T-TiSe2, clarifying the nature of its high-temperature phase and identifying the softening of a collective mode.

Findings

Scattering rates are near the Ioffe-Regel limit for both electrons and holes.

Near the transition, electrons and holes contribute equally to conductivity.

Identified a low-frequency peak as a softening collective mode.

Abstract

The controversy regarding the precise nature of the high-temperature phase of 1T-TiSe2 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-TiSe2 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.