Why polar excitons stay sharp: parity protection of the center-of-mass recoil channel in exciton-phonon scattering
Abstract
In polar semiconductors the Fröhlich interaction is the dominant electron--phonon coupling, yet excitonic resonances in materials such as halide perovskites remain anomalously sharp. We show that standard frozen-center-of-mass treatments of the exciton--phonon problem miss the decisive kinematic degree of freedom: restoring the exact center-of-mass (COM) recoil reveals a universally open, parameter-free 1s→1s absorption channel at recoil momentum q*=√(2MexℏωLO)/ℏ, whose rate scales as NLO(T). We prove that this recoil channel is controlled by destructive electron--hole interference: the recoil linewidth vanishes with the mass asymmetry as γLOrecoil∝ℱ1s,1s(q*)2, and the elastic dressing obeys the exact suppression law SX/Sind=η2(6−η2)/5 within the hydrogenic Fröhlich model. The theory establishes a hierarchy of scattering regimes. In mass-asymmetric materials (GaAs, η=−0.74) the recoil channel is active (γLOrecoil=2.2~meV); in mass-symmetric materials (FAPbI3, η=0; MAPbI3, η=−0.11) it is killed by interference (0.00 and 0.12 meV), showing that the observed 27--40 meV perovskite linewidths cannot be accounted for by COM recoil and therefore require internal-state-changing and other inelastic channels, of which the constructive, η-robust 1s→ np resonance is the leading candidate within the present model. The Fröhlich constant α alone is therefore insufficient as a figure of merit: after projection onto the correlated exciton, the controlling parameters are η, q*aX, and the Rydberg detuning.
Related
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BibTeX
@misc{atambo2026polar,
title = {{Why polar excitons stay sharp: parity protection of the center-of-mass recoil channel in exciton-phonon scattering}},
author = {Michael O. Atambo},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.01600},
eprint = {2610.01600},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.01600},
}
RIS
TY - UNPB
TI - Why polar excitons stay sharp: parity protection of the center-of-mass recoil channel in exciton-phonon scattering
AU - Michael O. Atambo
PY - 2026
DA - 2026/10/01
PB - arXiv
DO - 10.48550/arxiv.2610.01600
UR - https://arxiv.org/abs/2610.01600
AB - In polar semiconductors the Fröhlich interaction is the dominant electron--phonon coupling, yet excitonic resonances in materials such as halide perovskites remain anomalously sharp. We show that standard frozen-center-of-mass treatments of the exciton--phonon problem miss the decisive kinematic degree of freedom: restoring the exact center-of-mass (COM) recoil reveals a universally open, parameter-free 1s→1s absorption channel at recoil momentum q_*=√(2M_exℏω_LO)/ℏ, whose rate scales as N_LO(T). We prove that this recoil channel is controlled by destructive electron--hole interference: the recoil linewidth vanishes with the mass asymmetry as γ_LO^recoil∝ℱ_1s,1s(q_*)^2, and the elastic dressing obeys the exact suppression law S_X/S_ind=η^2(6−η^2)/5 within the hydrogenic Fröhlich model. The theory establishes a hierarchy of scattering regimes. In mass-asymmetric materials (GaAs, η=−0.74) the recoil channel is active (γ_LO^recoil=2.2~meV); in mass-symmetric materials (FAPbI_3, η=0; MAPbI_3, η=−0.11) it is killed by interference (0.00 and 0.12 meV), showing that the observed 27--40 meV perovskite linewidths cannot be accounted for by COM recoil and therefore require internal-state-changing and other inelastic channels, of which the constructive, η-robust 1s→ np resonance is the leading candidate within the present model. The Fröhlich constant α alone is therefore insufficient as a figure of merit: after projection onto the correlated exciton, the controlling parameters are η, q_*a_X, and the Rydberg detuning.
ER -