Searching for hidden order with photons

Among the other possible probes, a recently designed technique [1] to detect these time-odd, parity-odd magneto-electric multipoles is based on their interactions with photons in a Compton scattering measurement, which measures the electron density as a function of momentum. Guided by the non-trivial duality between the real space and momentum space, the asymmetric electron spin density of the magneto-electric multipoles in real space transforms into asymmetric electron density in the momentum space. The projection of this electron momentum density along the direction of the photon momentum transfer is captured by the Compton scattering profile. As a result, the asymmetric electron momentum density manifests as an anti-symmetric component in the Compton profile of an inversion asymmetric magnetic material, hosting magneto-electric multipoles.

Our recent works [2, 3] provide some physical understanding of the effect, taking the well-known magneto-electric material LiNiPO4 as an example. The weak anti-symmetric signal in the material, however, prevents the unambiguous detection of the signal in present experimental setups. Armed with first principles methods based on density functional theory and symmetry analysis, our challenge is, therefore, to search for suitable materials with strong enough antisymmetric signal so that it can be detected unambiguously in experiments beyond the level of the measurement noise.