Ta2NiSe5: excitonic or not?

B. Chatterjee1, J. Mravlje,2,3 D. Golež2,3

1Faculty of Physics, University of Duisburg-Essen, 47057 Duisburg, Germany

2Jozef Stefan Institute, SI-1000 Ljubljana, Slovenia

3FMF, University of Ljubljana, SI-1000 Ljubljana, Slovenia

Ta2NiSe5 has long been considered as a prominent candidate for realization of the excitonic condensation that was invoked to explain the opening of the gap in the photoemission and to rationalize the existence of short time scales in pump-probe experiments. A structural transition that coincides with the putative excitonic transition leads to eternal chicken-egg debate addressing the dominant mechanism of the transition. We consider a realistic 6 orbital model and discuss its instabilities in Hartree-Fock calculations including the relevant B2g Raman active phonon. The model realizes excitonic transition with experimentally expected symmetry only provided the electron-phonon coupling is taken into account. From time-dependent calculations we evaluate also two-particle response corresponding to Raman and optical spectra. The key feature of the calculated response is a prominent phase mode, and the extent to which it survives in the structurally distorted ground state may help settling the debate.