Bimagnon studies in cuprates with resonant inelastic x-ray scattering at the O K edge. I. Assessment on La2CuO4 and comparison with the excitation at Cu L3 and Cu K edges

We assess the capabilities of magnetic resonant inelastic x-ray scattering (RIXS) at the O K edge in undoped cuprates by taking La2CuO4 as a benchmark case, based on a series of RIXS measurements that we present here. By combining the experimental results with basic theory we point out the fingerprints of bimagnon excitation in the O K edge RIXS spectra. These are a dominant peak around 450 meV, the almost complete absence of dispersion both with pi and sigma polarization, and the almost constant intensity vs the transferred momentum with sigma polarization. This behavior is quite different from Cu L-3 edge RIXS giving a strongly dispersing bimagnon tending to zero at the center of the Brillouin zone. This is clearly shown by RIXS measurements at the Cu L-3 edge that we present. The Cu L-3 bimagnon spectra and those at the Cu K edge-both from the literature and from our data-however, have the same shape. These similarities and differences are understood in terms of different sampling of the bimagnon continuum. This panorama points out the unique possibilities offered by O K RIXS in the study of magnetic excitations in cuprates near the center of the BZ.