Unified first-principles theory of exciton polarons across coupling regimes

Develop a unified ab initio framework for exciton polarons that seamlessly bridges weak- and strong-coupling regimes by formulating and solving self-consistent exciton–phonon many-body equations (for example, a generalization of the Hedin–Baym equations to the two-particle electron–hole Green’s function) that yield exciton-polaron energies, wavefunctions, and dynamics in real materials.

Background

The review surveys recent progress on exciton polarons and self-trapped excitons, noting that current methods treat weak and strong coupling separately. While exciton-polaron equations and supercell approaches exist, they do not yet constitute a single all-coupling, self-consistent theory comparable to charged-polaron frameworks.

A unified theory would clarify how exciton–phonon interactions produce localization and spectral features across materials, and would connect exciton-polaron equations to Fock-space exciton–phonon Hamiltonians or generalized Green’s-function approaches.

References

Here, we limit ourselves to drawing the reader's attention to six open questions that in our view are the most urgent. First, the first-principles theory of exciton polarons is still in its infancy. Recent developments in ab initio methods have succeeded in addressing the weak- and strong-coupling limits of this problem, yet a unifying conceptual framework that seamlessly bridges these regimes is still lacking.

— Polarons from first principles  (2512.06176 - Dai et al., 5 Dec 2025) in Section 7: Conclusions and Outlook

The open questions follow directly from this hierarchy. The spacer dependence of anharmonic dynamics remains insufficiently mapped; the crossover between large-polaron and self-trapped exciton regimes requires a unified treatment; and exciton fine-structure ordering across composition, size and structural symmetry remains unresolved.

— Band-like Carriers in a Soft, Anharmonic Lattice: Lead-Halide Perovskites  (2608.28104 - Lee et al., 28 Aug 2026) in Section 10.4, Status

Whether dominant exciton fields induce the lattice distortion or whether the complexes form based on pre-existing local distortions remains to be tested in future experiments.

— Extraordinary Lifetime Enhancement of Coherent Phonon-Amplitude Modes in the Excitonic Insulator Phase of Ta2Pd3Te5  (2609.26485 - M et al., 22 Sep 2026) in Conclusions

The lattice COM-gauge coefficient $\tfrac32\eta$ is obtained from a signed-minimum-image vertex which, for $q=2\pi n/L$, $n\neq0$, is not the periodic lattice density operator; its replacement by the periodic vertex, and the resulting intraband share, are under audit in the revision of Ref. (cluster ledger: OPEN).

— Why polar excitons stay sharp: parity protection of the center-of-mass recoil channel in exciton-phonon scattering  (2610.01600 - Atambo, 1 Oct 2026) in Section II, subsection “On-shell linewidths: COM recoil and inelastic resonance”