  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 Ta$_2$Pd$_3$Te$_5$.
Density-functional-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 Ta$_2$Pd$_3$Te$_5$ to exciton
condensation, where the coupling to the symmetry-breaking electronic state
induces a subtle change in the crystal structure.
