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Item type:Publication, Dispersion model for the optical absorption of two-dimensional materials(American Physical Society, 2026-01-07)The optical response of two-dimensional systems is strongly influenced by tightly bound excitons. Despite its relevance in helping predict device performance, the current derivation of the two-dimensional Elliott equation is rarely used to estimate exciton binding energy and band gap in these systems, primarily due to its lack of an analytical form and the complexity introduced by substrate interactions. In this work, we present a new approach based on optical absorption measurements via an extended Elliott band fluctuations model, which notably provides analytical expressions for isotropic systems. Our method accurately captures the optical absorption near the band edge, fully incorporating spin–orbit band splitting and substrate effects via the Keldysh effective potential. It also includes the influence of surface and interface contributions to the dielectric environment, which give rise to localized defect-related absorption features. We apply this approach to key transition metal dichalcogenides (MoS2, MoSe2, WS2, andWSe2) exhibiting small and large spin-orbit band splitting, on various substrates and over a broad temperature range. The results show good agreement with magnetoabsorption and photoluminescence measurements, allowing for an accurate description of excitonic properties using only optical measurements and is readily extendable to other 2D isotropic materials.2 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Description of excitonic absorption using the Sommerfeld enhancement factor and band-fluctuations(Institute of Physics, 2025)One of the challenges of excitonic materials is the accurate determination of the exciton binding energy and bandgap from optical measurements. The difficulty arises from the overlap of the discrete and continuous excitonic absorption at the band edge. Many researches have modeled the shape of the absorption edge of such materials on the seminal formulation proposed by Elliott in 1957 ( Phys. Rev. 108 1384–9) and its several modifications such as non-parabolic bands, magnetic potentials and electron–hole-polaron interactions. However, exciton binding energies obtained from optical absorption often vary strongly depending on the chosen ‘Elliott formula’. Here, we propose an alternative and rather simple approach, which has previously been successful in the determination of the optical bandgap of amorphous, direct and indirect semiconductors, based on the band-fluctuations (BFs) model. In this model, the fluctuations due to disorder, temperature or lattice vibrations give rise to the well known exponential shape of band tail states. The formulation results in an analytic equation for the fundamental absorption with 6 parameters only. To test it, the binding energy and optical bandgap of GaAs and the family of tri-halide perovskites ( MAPbX 3 ), X = Br , I , Cl , over a wide range of temperatures, are obtained by fitting the modified Elliott model. The results for the bandgap, linewidth and exciton binding energy are in good agreement with reports based on non-optical measurements. Moreover, due to the polar nature of perovskites, the retrieved binding energies can be compared with those computed with a model proposed by Kane (1978 Phys. Rev. B 18 6849). In the latter model, the exciton is surrounded by a cloud of virtual phonons interacting via the Frölich interaction. As a consequence, the upper bound for the binding energy of the exciton-polaron system can be estimated. These results are in good agreement with the optical parameters obtained with the proposed Elliott equation including BFs.1