SPICE Workshop on Ferrons and Magnons: friends or foes? July 7th - 9th, 2026
Kyung-Jin Lee
Orbital degrees of freedom in solids include both orbital angular momentum (OAM) and orbital quadrupoles, the latter being independent components of the orbital density matrix that are symmetry-distinct from OAM. Although orbital quadrupoles are essential for a complete description of orbital dynamics, previous studies have focused primarily on nonequilibrium OAM responses. We report optical detection of current-induced orbital quadrupole accumulation at the surface of titanium. The accumulation modifies the polarization rotation and ellipticity of reflected light in a symmetry channel distinct from the magnetic-dipole signals associated with spin and OAM. The quadrupolar optical response is twice as large as the dipolar one, establishing orbital quadrupoles as a major component of current-induced orbital responses. These results expand nonequilibrium physics beyond the conventional dipolar paradigm to encompass higher-order multipoles, and open a route to electrical manipulation of orbital-ordered phases.