Theory of Nonlinear Response for Charge and Spin Currents

TL;DR

This paper develops a comprehensive nonlinear theory for charge and spin currents driven by ac electric fields, revealing symmetry-dependent selection rules and proposing experimental methods to measure Berry curvature multipoles.

Contribution

It introduces a general nonlinear response framework considering charge/spin and longitudinal/transverse effects, with symmetry-based selection rules and experimental proposals for Berry curvature multipoles.

Findings

Only longitudinal odd harmonic charge currents exist when both TRS and IS are conserved.

All harmonic order charge and spin currents are nonzero when both TRS and IS are broken.

Berry curvature dipole and quadrupole can be extracted from second and fourth harmonic currents.

Abstract

The nonlinear Hall effect, which is the second-order harmonic charge Hall effect from the Berry curvature dipole in momentum space, has received much attention recently. As the responses to higher harmonics of the driving ac electric field are prominent and measurable, we develop a general nonlinear theory by taking the charge and spin currents as well as the longitudinal and transverse effects into account. We introduce the expansion order of the electric field and Berry curvature multipole moment, where the Berry curvature dipole is a particular one manifesting itself at the second harmonic order and the second expansion order of the electric field. There are four cases with conserving or breaking the time-reversal symmetry (TRS) and inversion symmetry (IS). We find a specific ``selection rule" that only longitudinal odd harmonic order charge currents exist for conserving both the TRS and IS, and with breaking both symmetries, all harmonic order charge and spin currents are nonzero. With conserving TRS and breaking IS, the charge Hall current exists at even harmonic order, and the longitudinal charge current occurs at odd harmonic order. Only the longitudinal spin current survives at even harmonic order. With breaking TRS and conserving IS, only odd harmonic order charge and spin currents can appear. Moreover, we observe that every harmonic order current contains a series of infinite-order expansion of the electric field. We further show that the Berry curvature dipole and quadrupole can be determined by measuring the second and fourth harmonic order currents in experiments. This may open a way to explore the higher responses of an ac driving system.