Mapping the unoccupied state dispersions in Ta2NiSe5 with resonant inelastic x-ray scattering

Abstract

The transition metal chalcogenide ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ undergoes a second-order phase transition at ${T}_{c}=328\phantom{\rule{0.16em}{0ex}}\text{K}$ 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 ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ 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.