Evidence for an Excitonic Insulator State in Ta2Pd3Te5

The excitonic insulator (EI) is an exotic ground state of narrow -gap semiconductors and semimetals arising from spontaneous condensation of electron -hole pairs bound by attractive Coulomb interaction. Despite research on EIs dating back to half a century ago, their existence in real materials remains a subject of ongoing debate. In this study, through systematic experimental and theoretical investigations, we provide evidence for the existence of an EI ground state in a van der Waals compound Ta2Pd3Te5. Densityfunctional -theory calculations suggest that it is a semimetal with a small band overlap, whereas various experiments exhibit an insulating ground state with a clear band gap. Upon incorporating electron -hole Coulomb interaction into our calculations, we obtain an EI phase where the electronic symmetry breaking opens a many -body gap. Angle -resolved photoemission spectroscopy measurements exhibit that the band gap is closed with a significant change in the dispersions as the number of thermally excited charge carriers becomes sufficiently large in both equilibrium and nonequilibrium states. Structural measurements reveal a slight breaking of crystal symmetry with exceptionally small lattice distortion in the insulating state, which cannot account for the significant gap opening. Therefore, we attribute the insulating ground state with a gap opening in Ta2Pd3Te5 to exciton condensation, where the coupling to the symmetry -breaking electronic state induces a subtle change in the crystal structure.