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

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.

arXiv comment6 pages, 4 figures

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• Pressure suppression of the excitonic insulator state in Ta2NiSe5 observed by optical conductivity
• Third-harmonic generation in excitonic insulators
• Electronic band structure in pristine and Sulfur-doped Ta${}_2$NiSe${}_5$
• Introduction to High-Temperature Superconductivity for Solid State Chemists