Spatially Indirect Exciton Condensation in Two-Dimensional Strongly Correlated Semimetals

TL;DR

This study explores how strong electron-electron interactions in two-dimensional semimetals influence spatially indirect exciton condensation, revealing suppression effects on transition temperature and competition among pairing channels.

Contribution

It demonstrates the impact of on-site Coulomb interactions on exciton condensation and uncovers orbital-selective pairing states in strongly correlated 2D semimetals.

Findings

On-site Hubbard U suppresses condensation temperature T_c.

Higher electron-hole pair density reduces T_c when U is strong.

Multiple pairing channels compete, affecting T_c and pairing symmetry.

Abstract

Identifying materials hosting an excitonic insulator ground state has been one of the major pursuits in condensed matter physics in recent years. Promising candidates in transition metal chalcogenide compounds (TMC), including $1T-\mathrm{TiSe_2}$, $\mathrm{Ta_2Pd_3Te_5}$, and $\mathrm{Ta_2NiSe_5}$, share a crucial common characteristic: their low-energy physics is governed by electrons in $d-$ orbitals subject to strong on-site Coulomb interactions. In this work, we investigate spatially indirect exciton condensation in two-dimensional semimetals on triangular lattice. Using a combination of dynamical mean-field theory and the determinant quantum Monte Carlo method, we study two- and three-orbital Hubbard models incorporating strong on-site ($U$) and inter-orbital interactions ($V$). Our results demonstrate that on-site Hubbard $U$ can strongly suppress the condensation temperature $T_c$, an effect that is particularly pronounced at higher electron-hole pair densities. This behavior contrasts sharply with the case without on-site $U$, where $T_c$ grows with pair density at fixed $V$. Moreover, we uncover competition among multiple electron-hole pairing channels in the three-orbital model, which also acts to suppress $T_c$ of exciton condensation. An orbital-selective electron-hole pairing state is identified. These findings may help explain the large discrepancy between strong binding-energy and relative low transition temperature for indirect excitons in TMCs materials, offering important insights for understanding and engineering exciton condensation in materials with strongly correlated $d-$ shell electrons.