Exciton effects on electric circular dichroism in halide perovskites: first-principles calculations in the length and velocity gauge

Circular dichroism (CD) spectroscopy is a fundamental technique for investigating the symmetry and optical properties of chiral molecules and solids. For nonmagnetic systems, the electric circular dichroism (ECD) requires consideration of the magnetic dipole response. First principles calculations of ECD for molecules are well-established, but in solids, additional challenges arise from the definition of the position operator and the inclusion of many-body effects. Here, we develop a first-principles formalism, based on the GW plus Bethe Salpeter equation (GW-BSE) approach, for calculating ECD in both the velocity and length gauges. We explore contributions to ECD from the magnetic dipole, spin magnetic dipole, and electric quadrupole and determine the role of exciton effects by comparing the GW-BSE framework with an independent particle picture based on Density Functional Theory (DFT). We apply our method to study ECD spectra of two layered chiral hybrid organic-inorganic perovskites, (S-NEA)₂PbBr₄ and (S-MBA)₂PbI₄, and show the significance of both quasiparticle and excitonic effects in the CD. We show that compared to the more conventionally used velocity gauge, our implementation in the length gauge, achieved through the enforcement of a locally smooth gauge for both the electronic and exciton wavefunctions, dramatically improves convergence with respect to the number of bands, naturally accounts for the nonlocal potential in pseudopotential calculations, and is more numerically stable in the presence of band crossings and degeneracies.