Evolution Of The Electronic Structure In Ta2nise5 Across The Structural Transition Revealed By Resonant Inelastic X-Ray Scattering

We utilized high-energy-resolution resonant inelastic x-ray scattering (RIXS) at both the Ta and Ni L-3 edges to map out element-specific particle-hole excitations in Ta2NiSe5 across the phase transition. Our results reveal a momentum-dependent gaplike feature in the low-energy spectrum, which agrees well with the band gap in element-specific joint density of states calculations based on ab initio estimates of the electronic structure in both the low-temperature monoclinic and high-temperature orthorhombic structures. Below T-c , the RIXS energy-momentum map shows a minimal gap at the Brillouin zone center (similar to 0.16 eV), confirming that Ta2NiSe5 possesses a direct band gap in its low-temperature ground state. However, inside the gap, no signature of anticipated collective modes with an energy scale comparable to the gap size can be identified. Upon increasing the temperature to above T-c , whereas the gap at the zone center closes, the RIXS map at finite momenta still possesses the gross features of the low-temperature map, suggesting a substantial mixing between the Ta and Ni orbits in the conduction and valence bands, which does not change substantially across the phase transition. Our experimental observations and comparison to the theoretical calculations lend further support to the phase transition and the corresponding gap opening in Ta2NiSe5 being largely structural by nature, with a possible minor contribution from the putative exciton condensate.