Extended Data Fig. 4: Experiment versus theory.

a,b,d,e, Experimental background-subtracted (a,d) and calculated (b,e) ω–q EELS maps showing the dispersion of the magnon bands above 100 meV. c,f, Extracted spectra over a narrow momentum window (Δq = 0.22 Å⁻¹), to avoid spectral broadening through momentum averaging, at the value at which the intensity is maximum along Γ → M (q = 1.24 Å⁻¹) and Γ → X (q = 0.97 Å⁻¹), marked by the yellow arrows and dashed lines in a and d, respectively. Calculated spectra at the same wave vector and averaged over a similar momentum window highlight the shape of the peaks, with a rising edge from 80 meV reaching a maximum at about 100 meV, before a weaker feature extending up to 120 meV, with the peak observed along Γ → X being broader and more rounded.