Numerical investigations on exciton spin relaxation mechanisms in colloidal CsPbBr3 perovskite nano-crystals.

Lead halide perovskites are a promising candidate for opto-electronic functional devices owing to a unique fine structure splitting (FSS) of their exciton states, which results from an inherent spin-orbit coupling and shape anisotropy instead of strong quantum confinement. However, a deeper understanding of spin decoherence mechanisms in these materials is still lacking. Although empirical suggestions on spin relaxation by Elliot- Yafet (EY), D’yakonov-Perel’ (DP) or Maialle-Silva-Sham (MSS) mechanisms exist, importantly, an exact disentanglement of the regimes where each of these mechanisms dominate is missing. In this work we present results of our recent Monte Carlo simulations to unravel the experimentally observed, robust, coherent dynamics of exciton FSS states in colloidal caesium lead bromide perovskite nano-crystals. Using a careful fit of the experimentally measured evolution of the spin polarisation to the one obtained from the Monte Carlo simulations we conclude that the average spin precession frequency, which is proportional to the FSS energy splitting and hence the oscillatory spin dynamics, is dependent on the nano-crystal size but independent of the temperature. We also decode the experimentally observed anomalous temperature dependence of the spin decoherence and identify different temperature regimes where the individual spin relaxation mechanisms dominate. Our work provides a comprehensive understanding of the exciton spin decoherence in these materials and helps to build an unambiguous picture of the complex interplay between the underlying spin relaxation mechanisms. This would pave way for engineering robust high temperature opto-spintronics and quantum information devices.