An electron and its shadow
When a photon kicks an electron from the valence band into the conduction band, the empty spot it leaves behind acts like a positive particle (a hole). The two attract each other and form a bound pair called an exciton.
The exciton has its own discrete energy — slightly below the bare band gap — and it shows up in the optical spectrum as a sharp absorption peak. In CrSBr it sits around 1.375 eV.
Optically, it's a Lorentz oscillator
A textbook exciton is well described by a single Lorentzian contribution to the dielectric functionwith three numbers: its energy E_x, its width γ, and its oscillator strength f. The simulator literally uses these three knobs.
ε(E) = ε∞ + f / (E₀² − E² − iγE). The real part of n bumps up before the resonance and dips after it (anomalous dispersion); the absorption κ peaks right at E₀ — same shape as a Lorentzian.
Why magnetism matters here
The exciton energy in CrSBr depends on the local spin order: the AFM ground state has the lowest energy, the FM phase sits a bit higher, and the Mixed case lives in between. Magnetism is the lever, the exciton energy is what moves, and the reflectance spectrum is what we measure.
- An exciton is a Coulomb-bound electron–hole pair.
- It absorbs at one specific energy — a Lorentzian peak.
- Its position responds to magnetism in CrSBr — that's our handle.