Mapping the unoccupied state dispersions in Ta$_2$NiSe$_5$ with resonant inelastic x-ray scattering

TL;DR

This study uses resonant inelastic x-ray scattering to map the electron-hole excitation dispersions in Ta$_2$NiSe$_5$, revealing detailed electronic structure changes across its phase transition.

Contribution

It provides the first detailed mapping of unoccupied state dispersions in Ta$_2$NiSe$_5$ using RIXS, combining experimental and theoretical analysis.

Findings

Identification of interband transitions as Raman-like peaks

Mapping of electron-hole excitation dispersions

Determination of the conduction band effective mass

Abstract

The transition metal chalcogenide Ta$_2$NiSe$_5$ undergoes a second-order phase transition at $T_c=328$ K involving a small lattice distortion. Below $T_c$, a band gap at the center of its Brillouin zone increases up to about 0.35 eV. In this work, we study the electronic structure of Ta$_2$NiSe$_5$ in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ni $L_3$-edge. In addition to a weak fluorescence response, we observe a collection of intense Raman-like peaks that we attribute to electron-hole excitations. Using density functional theory calculations of its electronic band structure, we identify the main Raman-like peaks as interband transitions between valence and conduction bands. By performing angle-dependent RIXS measurements, we uncover the dispersion of these electron-hole excitations that allows us to extract the low-energy boundary of the electron-hole continuum. From the dispersion of the valence band measured by angle-resolved photoemission spectroscopy, we derive the effective mass of the lowest unoccupied conduction band.