Orbital Gravito-Magnetoelectric response and Orbital magnetic quadrupole moment correction

TL;DR

This paper develops a quantum formalism for the orbital gravito-magnetoelectric effect, highlighting the importance of orbital magnetic quadrupole moment corrections and analyzing effects in a PT-symmetric model with potential applications in detecting orbital magnetic moments.

Contribution

It introduces a comprehensive quantum framework for the orbital gravito-ME effect, including intrinsic and extrinsic parts, and emphasizes the role of orbital magnetic quadrupole moments for physical consistency.

Findings

Intrinsic effect requires orbital magnetic quadrupole correction.

Effect is enhanced near Dirac points in a PT-symmetric model.

Classification of effects based on magnetic point groups.

Abstract

The magnetoelectric effect has been actively studied in multiferroics since the first observation in an antiferromagnetic, $\mathrm{Cr_2O_3}$. This effect appears in systems without spatial inversion symmetry and time-reversal symmetry and is sensitive to detecting magnetic quadrupole moments. It is often discussed as inducing spin magnetizations; however, the orbital magnetoelectric effect in metals has recently attracted much attention since its observation in $\mathrm{MoS_2}$ and twisted bilayer graphene. In this work, we propose the full quantum formalism for the temperature gradient induced-orbital magnetoelectric effect (orbital gravito-ME effect). The effect consists of two parts, i.e., an extrinsic part and an intrinsic part. We demonstrate that the intrinsic part needs a correction from the orbital magnetic quadrupole moment besides the usual Kubo formula to avoid an unphysical divergence at zero temperature and to satisfy the Mott relation. Furthermore, we show the classification table with the magnetic point group for the intrinsic and extrinsic effects. Finally, we analyze the intrinsic part in a $\mathcal{PT}$-symmetric model exhibiting an orbital magnetization order, i.e., a loop current order, and demonstrate the enhancement near Dirac points. We believe that these results will contribute to the detection and usage of orbital magnetic moments beyond spin moments.