Hybridization-Gap Formation and Superconductivity in the Pressure-Induced Semimetallic Phase of the Excitonic Insulator Ta<sub>2</sub>NiSe<sub>5</sub>
Kazuyuki Matsubayashi, Hidekazu Okamura, Takashi Mizokawa, Naoyuki Katayama, Akitoshi Nakano, Hiroshi Sawa, Tatsuya Kaneko, Tatsuya Toriyama, Takehisa Konishi, Yukinori Ohta, Hiroto Arima, Rina Yamanaka, Akihiko Hisada, Taku Okada, Yuka Ikemoto, Taro Moriwaki, Koji Munakata, Akiko Nakao, Minoru Nohara, Yangfan Lu, Hidenori Takagi, Yoshiya Uwatoko
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.