Revealing hidden magneto-electric multipoles using Compton scattering

TL;DR

This paper demonstrates that anti-symmetric Compton scattering profiles can reveal hidden magneto-electric multipoles in materials, providing a new experimental signature for their detection.

Contribution

It introduces the anti-symmetric Compton profile as a novel signature for magneto-electric multipoles and validates this through symmetry analysis and first-principles calculations.

Findings

Anti-symmetric Compton profiles are signatures of magneto-electric multipoles.

The orientation of spin moments and spin-orbit coupling are crucial for the effect.

Validated with calculations on LiNiPO4 and Mn2Au.

Abstract

Magneto-electric multipoles, which are odd under both space-inversion $\cal I$ and time-reversal $\cal T$ symmetries, are fundamental in understanding and characterizing magneto-electric materials. However, the detection of these magneto-electric multipoles is often not straightforward as they remain "hidden" in conventional experiments in part since many magneto-electrics exhibit combined $\cal IT$ symmetry. In the present work, we show that the anti-symmetric Compton profile is a unique signature for all the magneto-electric multipoles, since the asymmetric magnetization density of the magneto-electric multipoles couples to space via spin-orbit coupling, resulting in an anti-symmetric Compton profile. We develop the key physics of the anti-symmetric Compton scattering using symmetry analysis and demonstrate it using explicit first-principles calculations for two well-known representative materials with magneto-electric multipoles, insulating LiNiPO$_4$ and metallic Mn$_2$Au. Our work emphasizes the crucial roles of the orientation of the spin moments, the spin-orbit coupling, and the band structure in generating the anti-symmetric Compton profile in magneto-electric materials.