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The excitonic insulator Ta$_2$NiSe$_5$ experiences a first-order structuraltransition under pressure from rippled to flat layer-structure at Ps = 3 GPa,which drives the system from an almost zero-gap semiconductor to a semimetal.The pressure-induced semimetal, with lowering temperature, experiences atransition to another semimetal with a partial-gap of 0.1-0.2 eV, accompaniedwith a monoclinic distortion analogous to that occurs at the excitonictransition below Ps. We argue that the partial-gap originates primarily from asymmetry-allowed hybridization of Ta-conduction and Ni-valence bands due to thelattice distortion, indicative of the importance of electron-lattice coupling.The transition is suppressed with increasing pressure to Pc = 8 GPa.Superconductivity with a maximum Tsc = 1.2 K emerges around Pc, likely mediatedby strongly electron-coupled soft phonons. The electron-lattice coupling is asimportant ingredient as the excitonic instability in Ta2NiSe5.

Hybridization-gap Formation and Superconductivity in the Pressure-induced Semimetallic Phase of the Excitonic Insulator Ta$_2$NiSe$_5$