Dynamic Symmetry Breaking and Spin Splitting in Metal Halide Perovskites

TL;DR

This paper investigates how dynamic structural asymmetries in metal halide perovskites lead to spin splitting of electronic bands, influenced by thermal disorder and lone pair effects, with implications for their photophysical properties.

Contribution

It demonstrates that local asymmetry and thermal fluctuations cause variable spin splitting in lead iodide perovskites, revealing the underlying physics through advanced computational methods.

Findings

Spin splitting depends on local asymmetry around Pb and I sites.

Thermal disorder causes fluctuations in potential and band splitting.

The physics applies to both organic-inorganic and inorganic perovskites.

Abstract

Metal halide perovskites exhibit a materials physics that is distinct from traditional inorganic and organic semiconductors. While materials such as CH3NH3PbI3 are non-magnetic, the presence of heavy elements (Pb and I) in a non-centrosymmetric crystal environment result in a significant spin-splitting of the frontier electronic bands through the Rashba-Dresselhaus effect. We show, from a combination of \textit{ab initio} molecular dynamics, density-functional theory, and relativistic quasi-particle \textit{GW} theory, that the nature (magnitude and orientation) of the band splitting depends on the local asymmetry around the Pb and I sites in the perovskite structure. The potential fluctuations vary in time as a result of thermal disorder and a dynamic lone pair instability of the Pb(II) 6s$^{2}$6p$^{0}$ ion. We show that the same physics emerges both for the organic-inorganic CH3NH3PbI3 and the inorganic CsPbI3 compound. The results are relevant to the photophysics of these compounds and are expected to be general to other lead iodide containing perovskites.