Figure 2: Spin excitations of parent BFA.

(a,b) RIXS spectra of parent BFA along (0, 0)–(1, 0) and (0, 0)–(1, 1) directions in the first Brillouin zone obtained at 15 K with π polarized incoming light. The numbers displayed at the right side of each spectrum denote the absolute wave vector q_// projected along the sample a-b plane (as in e and Fig. 3a–c). (c) A representative spectrum at the BZ edge demonstrating the subtraction of the fluorescence background and the quasi-elastic peak in order to obtain the spin-excitation component. (d–e) Spin excitations along (0, 0)–(1, 0) (left) and (0, 0)–(1, 1) (right) directions after subtracting the background and quasi-elastic peak. The arrows mark the fitted peak centers. (f) Spin-excitation dispersions along (0, 0)–(1, 0) (left) and (0, 0)–(1, 1) (right) directions. q_// transfer is re-scaled to relative lattice units. The filled black diamonds are fitted energy positions from the data shown in d and e. Vertical bars represent errors from the fitting of the energy position of the spin excitations. The thick grey line is the calculated dispersion curve using the effective superexchange parameters from INS data³⁰ (see Methods).