Signatures of hidden octupolar order from nonlinear Hall effects

Abstract: Detecting symmetry-breaking hidden orders with conventional probes has been a long-standing challenge in the field of magnetism. Higher-rank multipolar ordering $-$ anisotropic charge and magnetization distributions arising from a combination of spin-orbit coupling and crystalline environments $-$ is a quintessential example of such hidden orders, where new protocols of direct detection remain highly desirable. In this work, we propose non-linear Hall effects as a novel probe for multipolar ordering in metallic systems. Taking inspiration from the family of Pr-based heavy-fermion compounds, Pr(Ti,V)$2$Al${20}$, we formulate a minimal cubic-lattice model of conduction electrons coupled to a ferro-octupolar order parameter. The time-reversal-breaking order leads to a band structure that supports strong quadrupolar moments of the Berry curvature (BC). Using a semi-classical Boltzmann formalism in conjunction with a symmetry analysis, we demonstrate that the BC quadrupoles produce a third harmonic generation of the Hall voltage $[V_H(3 \omega)]$ measurable in an AC Hall experiment. Properties of the Hall response such as its anisotropy, its dissipationlessness, and its dependence on the order parameter are also examined. Our work encourages a new realm of investigation of multipolar ordering from non-linear transport experiments.