Sharp transitions in the spectra of small Frenkel-like excitons for multi-orbital lattice systems

Man-Yat Chu; Mona Berciu

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arXiv · 2509.05927

Sharp transitions in the spectra of small Frenkel-like excitons for multi-orbital lattice systems

Man-Yat Chu·Mona Berciu

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Abstract

We propose a method for calculating exciton spectra and wavefunctions for model lattice Hamiltonians, based on real-space electron-hole propagators. We verify that our results agree with those of the continuum approximation in the limit of large Wannier excitons, and propose a simple criterion to estimate the exciton size above which the continuum approximation is quantitatively accurate. We then investigate simple one- and two-dimensional multi-orbital lattice models and show that small, Frenkel-like excitons, whose size approaches the lattice constant, can display physics that disagrees with the simplest continuum descriptions (a single-valley quadratic expansion around the minimum gap) not just quantitatively, but qualitatively. Specifically, we identify sharp transitions in the character and momentum of the lowest-energy exciton, enabled by the multi-orbital nature of the lattice models.

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Man-Yat Chu, Mona Berciu. 2025-09-07. Sharp transitions in the spectra of small Frenkel-like excitons for multi-orbital lattice systems. https://arxiv.org/abs/2509.05927

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