Semiconductor to Semimetal to Superconductor: An Excitonic Journey

Superconductors comprise a condensate of electron pairs, known as Cooper pairs, capable of electricity conduction without energy loss. Conventional superconductivity involves the formation of electron pairs via the attractive interactions mediated by lattice vibrations. Excitons are the bound states generated via Coulomb attraction between electrons and holes, similar to that for the Cooper pairs in superconductivity. The ineffective screening of Coulomb interactions in narrow-gap semiconductors and semimetals is expected to induce the spontaneous condensation of excitons with a decrease in temperature. Consequently, the system becomes an excitonic insulator.

Superconductivity was initially observed in mercury in 1911; subsequently, various superconducting materials, including high-temperature superconductors, were discovered. Conversely, the theoretical proposal of excitonic insulators over 50 years ago was not subsequently validated by conclusive experimental evidence of the excitonic state.

Ta₂NiSe₅ is a layered chalcogenide located near the semiconductor–semimetal boundary. Recently, this compound has attracted significant attention as a promising candidate for excitonic insulators. Furthermore, there is renewed interest in the formation of a gapped state driven by electron–lattice coupling. The applied pressure is an ideal controlling parameter to tune the electronic states. To the best of our knowledge, the present research was the first to report a high-pressure phase diagram for Ta₂NiSe₅ encompassing the entire range i.e., from the semiconducting to semimetallic region. The investigations revealed pressure-induced superconductivity in the semimetallic phase.

A transition to another semimetal with a partial gap was observed in the pressure-induced semimetallic phase. This was accompanied by lattice distortion analogous to that occurring during excitonic transition in the low-pressure semiconducting phase. An increase in the carrier density facilitated effective screening of the Coulomb interactions between electrons and holes. Therefore, the gap characteristics changed from excitonic dominant to hybridization-gap dominant with an increase in the band overlap under an applied pressure. The results revealed the critical role of electron–lattice coupling in the occurrence of superconductivity and excitonic transition in the low-pressure phase.(Written by Kazuyuki Matsubayashi on behalf of all authors.)