[PCP1] Physics and Chemistry1

[PCP1-06] Excitonic phase diagram in the electron-hole two-band Hubbard model based on the dynamical mean-field theory

*Yusuke Inokuma¹, Shota Masuzawa², Kento Sasaki², Yoshiaki Ono² (1. Japan Atomic Energy Agency (Japan), 2. Niigata Univ. (Japan))

Keywords:

excitonic insulator,transition-metal chalcogenide

Ta₂NiSe₅undergoes a structural phase transition from an orthorhombic to a monoclinic semiconducting phase at Ts = 328 K. Below Ts, angle-resolved photoemission spectroscopy (ARPES) reveals a flattening of the valence band top, suggesting the formation of an excitonic phase. Theoretical studies based on a three-chain Hubbard model and BCS-type mean-field approximation successfully reproduce this transition as a second-order excitonic insulator transition. However, time-resolved ARPES (trARPES) experiments show that the bare electronic state in the excitonic phase becomes semimetallic after photoexcitation, highlighting the importance of strong correlation and strong coupling effects beyond mean-field theory. Furthermore, superconductivity emerges under applied pressure, raising interest in its possible link to the excitonic phase. In this study, we determine the excitonic transition temperature Tc using the dynamical mean-field theory (DMFT), which can accurately account for strong coupling and correlation effects. We employ a simplified two-band Hubbard model with semi-elliptic density of states (bandwidth is 4) and analyze Tc as a function of interband Coulomb interaction V and band splitting Δ. Our results show that the normal state is semimetallic in the weak-coupling regime and semiconducting in the strong-coupling regime, consistent with ambient-pressure experiments. We will discuss the temperature dependence of the spectral function near the Fermi level and the potential for superconductivity in this system.