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