Phonon anomaly at the charge ordering transition in 1T-TaS2

TL;DR

This study investigates the phonon behavior in 1T-TaS2 across its charge ordering transition, revealing an anomalous phonon softening and hysteresis linked to structural changes during the metal-insulator transition.

Contribution

The paper provides detailed infrared spectroscopic analysis showing phonon anomalies and hysteresis at the charge ordering transition in 1T-TaS2, highlighting the connection to structural changes.

Findings

Phonon mode at 208cm^-1 softens below 180K

Hysteresis observed in phonon frequency and linewidth

Development of a gap at 800cm^-1 during transition

Abstract

The infrared reflectance of the transition metal chalcogenide 1T-TaS2 has been measured at temperatures from 30K to 360K over 30-45,000cm^-1 (4meV-5.5eV). The optical conductivity was obtained by Kramers-Kronig analysis. At 360K only modest traces of the phonon lines are noticeable. The phonon modes are followed by a pseudogap-like increase of the optical conductivity, with direct optical transitions observed at frequencies above 1eV. As the temperature decreases, the low frequency conductivity also decreases, phonon modes become more pronounced and pseudogap develops into a gap at 800cm^-1 (100meV). We observe an anomalous frequency dependence of the 208cm^-1 infrared-active phonon mode. This mode demonstrates softening as the temperature decreases below the 180K metal-to-insulator 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 associated with the metal-to-insulator transition.