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The transition metal chalcogenide Ta$_2$NiSe$_5$ undergoes a second-orderphase 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 about0.35 eV. In this work, we study the electronic structure of Ta$_2$NiSe$_5$ inits low-temperature semiconducting phase, using resonant inelastic x-rayscattering (RIXS) at the Ni $L_3$-edge. In addition to a weak fluorescenceresponse, we observe a collection of intense Raman-like peaks that we attributeto electron-hole excitations. Using density functional theory calculations ofits electronic band structure, we identify the main Raman-like peaks asinterband transitions between valence and conduction bands. By performingangle-dependent RIXS measurements, we uncover the dispersion of theseelectron-hole excitations that allows us to extract the low-energy boundary ofthe electron-hole continuum. From the dispersion of the valence band measuredby angle-resolved photoemission spectroscopy, we derive the effective mass ofthe lowest unoccupied conduction band.

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