Patrick Grenzer | Searcharxiv

arXiv2026

A Visible-Frequency Excitonic Reststrahlen Band in (PEA)$_2$PbI$_4$ Slabs

Layered halide perovskites host exceptionally strong excitons, whose optical signatures are usually interpreted as absorptive resonances on a smooth dielectric background. Strong excitons, however, can also reshape the dielectric response itself and drive the real permittivity negative, opening a reflective band: the visible, excitonic analogue of an infrared Reststrahlen band. Whether bare (PEA)$_2$PbI$_4$ slabs reach this regime has remained unclear. Here we show that low-temperature transmission of (PEA)$_2$PbI$_4$ slabs, driven by the intralayer-exciton manifold, evolves with increasing thickness from an excitonic dip into a broad near-zero-transmission interval with compressed Fabry-P\'erot-like fringes. Transfer-matrix analysis with an effective Lorentz-oscillator dielectric response reproduces this crossover, reconstructs a finite negative-Re($\varepsilon$) window, and implies near-ultrastrong exciton-photon coupling. Calculated field maps show suppressed in-plane field penetration within this interval and a driven longitudinal response near the high-energy ($\varepsilon=0$) edge. These results identify (PEA)$_2$PbI$_4$ slabs as a cavity-free visible-frequency excitonic Reststrahlen material.

cond-mat.mtrl-sci↗

arXiv2026

Intra- and Interlayer Excitonic Fine Structure of the Two-Dimensional Perovskite (PEA)$_2$PbI$_4$

Two-dimensional halide perovskites host strongly bound excitons whose fine structure controls polarization selection rules and radiative recombination, yet several spectral features in (PEA)$_2$PbI$_4$ remain controversially assigned. Here, polarization-resolved low-temperature photoluminescence combined with first-principles G$_0$W$_0$+BSE calculations resolves both the intralayer and interlayer excitonic fine structure of this prototypical n=1 Ruddlesden-Popper perovskite. The low-energy multiplet is consistently described as a purely excitonic intralayer fine structure governed by crystal symmetry, octahedral distortions, and the two-layer unit cell, without invoking Rashba or exciton-polaron mechanisms as the primary origin. A weaker doublet ~45 meV above the bright intralayer states is identified as interlayer excitons from its agreement with the calculated interlayer manifold in energy and splitting. Although the static calculations underestimate their oscillator strength and do not reproduce the observed orthogonal polarizations, distortion-induced mixing with bright intralayer excitons strongly enhances interlayer optical activity and provides a plausible explanation for their visibility. Our results establish interlayer excitons in (PEA)$_2$PbI$_4$ and refine the excitonic description of fine structure in two-dimensional perovskites.

cond-mat.mes-hall↗