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

TL;DR

This study investigates how pressure induces a semimetallic phase and superconductivity in Ta$_2$NiSe$_5$, highlighting the roles of hybridization, electron-lattice coupling, and structural transitions.

Contribution

It reveals the interplay of hybridization, lattice distortion, and electron-phonon interactions in pressure-driven phase transitions and superconductivity in Ta$_2$NiSe$_5$.

Findings

Pressure induces a transition from semiconductor to semimetal.

Superconductivity emerges around 8 GPa with Tsc up to 1.2 K.

Hybridization and electron-lattice coupling are key to the phase behavior.

Abstract

The excitonic insulator Ta$_2$NiSe$_5$ experiences a first-order structural transition 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 a transition to another semimetal with a partial-gap of 0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Ni-valence bands due to the lattice 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 mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5.