Aharonov-Casher-induced electric quadrupole of charge-neutral particles
arXiv:2607.13745
The paper demonstrates that charge‑neutral particles with magnetic moments acquire an electric quadrupole moment in solids via the Aharonov‑Casher effect, and calculates this effect for magnons in ferromagnetic pyrochlore and helical Fe langasite.
Abstract
For charged particles, their orbital angular momentum (OAM) in solid have a direct magnetic manifestation as an orbital magnetization. For charge-neutral particles, however, the physical manifestation of the OAM in solid remains unclear. Here, we show that a charge-neutral particle carrying a magnetic moment couples to electric-field gradient through the Aharonov-Casher (AC) effect, thereby exhibiting the electric quadrupole in crystalline solids. This AC-induced electric quadrupole (AC-EQ) contains the scalar, toroidal dipole, and reduced quadrupole components, which are conjugate to the divergence, circulation, and shear of the electric field, respectively. As representative examples, we calculate the AC-EQ of magnons in two magnetic systems. In ferromagnetic pyrochlore, Dzyaloshinskii-Moriya interaction (DMI) induces a sizable AC-EQ, whereas in helical Fe langasite the AC-EQ emerges from the helical spin configuration even in the absence of DMI. In both systems, the unit-cell-integrated AC-EQ components reach magnitudes comparable to typical nuclear electric quadrupole moments.
13 pages, 4 figures