Spatially Indirect Exciton Condensation in Two-Dimensional Strongly Correlated Semimetals

summary

The gist

The gist: The critical temperature for indirect exciton condensation can be strongly suppressed by intra-orbital Coulomb repulsion U of the d-orbitals, and competition among multiple pairing channels

In short

The study investigates indirect exciton condensation in 2D triangular lattices using a two- and three-orbital Hubbard model with strong Coulomb interactions (U and V). Results show that on-site repulsion U strongly suppresses the condensation temperature Tc, especially at high carrier densities, contrasting with the U=0 case. Competition between different pairing channels in the three-orbital model further reduces Tc, revealing an orbital-selective pairing state.

Key concepts

Hubbard Model: This is a theoretical model used to describe strongly interacting electrons in materials. It includes terms for electron hopping (movement between sites), on-site repulsion (U, the energy cost of two electrons occupying the same site), and inter-orbital interactions (V). It helps capture the complex quantum behavior of d-electrons.
Exciton Condensation Temperature ($ ext{T}_c$): This is a critical temperature indicating when indirect excitons—bound electron-hole pairs formed across different orbitals or layers—begin to condense into a collective state. The paper finds that this temperature can be significantly lowered by strong Coulomb repulsion (U) and competition between different pairing types.
Orbital-Selective Pairing: This phenomenon means that the electron-hole pairing state is not uniform across all orbitals. In the three-orbital model, the system favors a specific pairing between the d and p orbitals ($ ext{G}_{ ext{ac}}$) while ignoring pairing between two d orbitals ($ ext{G}_{ ext{ab}}$), which is influenced by Hubbard U hindering inter-orbital exciton formation.

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This episode discusses

The paper

Spatially Indirect Exciton Condensation in Two-Dimensional Strongly Correlated Semimetals · Read on arXiv

Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices · School of Physics, Sun Yat-sen University

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-TiSe 2, Ta 2Pd 3Te 5, and 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.

DOI: 10.1007/s11433-026-3047-7

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Spatially Indirect Exciton Condensation in Two-Dimensional Strongly Correlated Semimetals".

Mira: The gist: The critical temperature for indirect exciton condensation can be strongly suppressed by intra-orbital Coulomb repulsion U of the d-orbitals,

Kai: First, who's behind it and why it matters.

Paper summary: Kai: So, we're looking at this paper "Spatially Indirect Exciton Condensation in Two-Dimensional Strongly Correlated Semimetals." Essentially, they're diving into two- and three-orbital Hubbard models on a triangular lattice to see if you can get an excitonic insulator state in these materials.

Mira: The main thesis here is that the strong on-site Coulomb repulsion, called U, can actually suppress the condensation temperature Tc for these excitons, especially when there's a decent electron-hole pair density.

Lev: That sounds like it could be tricky to simulate on real hardware if U is really high. How do they handle those strong interactions?

Kai: They use Cellular dynamical Mean-Field theory combined with determinant quantum Monte Carlo to capture the quantum fluctuations of these strongly interacting d-electrons, which is a non-perturbative way to look at it.

Mira: And beyond just the two orbitals, they build a three-orbital model where they find competition among different pairing channels, which further acts to reduce that critical temperature Tc.

Lev: Competition in pairing channels sounds complicated for error correction setups. What does that mean practically?

Kai: It means if you have multiple ways for the excitons to form, the system might choose a less condensed state when those channels fight each other out.

Conclusion: Mira: Looking at the whole picture of "Spatially Indirect Exciton Condensation in Two-Dimensional Strongly Correlated Semimetals," these authors are pointing toward how strong Coulomb forces affect the excitonic insulator physics we see in materials like Ta2Pd3Te5.

Kai: It seems like they're trying to explain that large gap between the exciton binding energy and the actual condensation temperature Tc, which is what people observe experimentally in those transition metal chalcogenides.

Lev: So, if this theory holds up, it means we have a better idea for predicting when these materials will actually show that excitonic insulator state under specific conditions.

Mira: The authors suggest that understanding the interplay between the on-site repulsion U and the inter-orbital interactions V is crucial because they find that U can fundamentally change how the condensation temperature depends on the carrier concentration n.

Kai: Basically, they're saying you can't just look at one interaction in isolation; you need to account for how everything fights for dominance in a multi-orbital system.

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