Mapping the dispersion of the occupied and unoccupied band structure in photoexcited 1T-TiSe$_2$

TL;DR

This study uses time- and angle-resolved photoelectron spectroscopy to visualize band structure changes in photoexcited 1T-TiSe$_2$, revealing transient effective mass variations and lattice contributions to charge density wave formation.

Contribution

First direct visualization of unoccupied band structure dynamics in 1T-TiSe$_2$ post-photoexcitation, linking lattice vibrations to CDW phase changes.

Findings

Transient effective mass change in Se$_{4p}$ valence band

Reduction of CDW phase strength after photoexcitation

Oscillation at A$_{1g}$ phonon frequency

Abstract

Charge density waves (CDW) are states of broken symmetry with a periodic modulation of charge and lattice typically leading to the opening of a gap in the band structure. In the model CDW system 1T-TiSe$_2$ such a gap opens up between its Se$_{4p}$ valence and Ti$_{3d}$ conduction band, accompanied by a change of dispersion. These changes are crucial in understanding the CDW phase, as they provide a measure of the Se$_{4p}$-Ti$_{3d}$ hybridization strength and characteristic mechanistic features. Using time- and angle-resolved photoelectron spectroscopy (trARPES), the unoccupied band structure is populated with near-infrared (NIR) pump pulses which allows to to directly visualize the parabolically-shaped Ti$_{3d}$ conduction band. Furthermore, we observe a transient change of effective mass in the Se$_{4p}$ valence band following photoexcitation. This occurs alongside an overall reduction due to weakening of the CDW phase and is accompanied by an oscillating component with the frequency of the characteristic A$_{1g}$ phonon. These observations, enabled by trAPRES, highlight the importance of the lattice contributions in establishing the CDW order in 1T-TiSe$_2$.