Title: Charge Conservation Beyond Uniformity: Spatially Inhomogeneous Electromagnetic Response in Periodic Solids

Abstract: Nonlinear electromagnetic response functions have reemerged as a crucial tool for studying quantum materials. Most attention has been paid to responses to spatially uniform electric fields, relevant to optical experiments in conventional materials. However, magnetic and magnetoelectric phenomena are naturally connected by responses to spatially varying electric fields due to Maxwell's equations. Furthermore, in moir\'{e} materials, characteristic lattice scales are much longer, allowing spatial variation of optical electric fields to potentially have a measurable effect in experiments. To address these issues, we develop a formalism for computing linear and nonlinear responses to spatially inhomogeneous electromagnetic fields. Starting with the continuity equation, we derive an expression for the current operator that is manifestly conserved and model independent. Crucially, our formalism makes no assumptions on the form of the microscopic Hamiltonian and so is applicable to model Hamiltonians derived from tight-binding or ab initio calculations. We then develop a diagrammatic Kubo formalism for computing the wavevector dependence of linear and nonlinear conductivities, using Ward identities to fix the value of the diamagnetic current order-by-order in the vector potential. We apply our formula to compute the magnitude of the Kerr effect at oblique incidence for a model of a moir\'{e} Chern insulator and demonstrate the experimental relevance of spatially inhomogeneous fields in these systems. We further show how our formalism allows us to compute the magnetic multipole moments and magnetic susceptibility in insulators. We next use our formalism to compute the second-order transverse response to spatially varying transverse electric fields in our moir\'{e} Chern insulator model, with an eye towards the next generation of experiments in these systems.