The Rashba splitting state – a prototypical quantum phenomenon – causes a splitting of a doubly spin-degenerate band into two spin sub-bands shifted with respect to each other in k space by k0. The formation of the Rashba state exhibits a myriad of rich physical phenomenon and novel spin-dependent functionalities that is essential to spintronics, an enabler of future quantum information technology. Hybrid Metal Halides (HMHs) is a new class of synthetic semiconductors prepared by low-temperature solution processing with a large chemical and structure ‘universe’ given the synthetic versatility of their molecular cations. While the family of HMHs has shown remarkable performance in photovoltaic and optoelectronic applications, their rich spintronic functionalities have yet to be fully utilized [1-4]. Particularly, because of their large spin-orbit coupling induced by the heavy metal and halogen atoms, HMHs are prime candidates to explore the Rashba splitting state [5] for an efficient charge-to-spin interconversion [6-8].  However, the observation of the Rashba effect in cubic CH3NH3PbBr3 single crystals that possess bulk inversion symmetry is the subject of extensive debate due to the lack of conclusive experiments and theoretical explanations. Here we provide experimental evidence that Rashba state in cubic CH3NH3PbBr3 single crystal at room temperature occurs exclusively on the crystal surface and depends on specific surface termination that results in local symmetry breaking. We demonstrate this using a suite of spatially-resolved and depth-sensitive techniques, including circular photogalvanic effect, inverse spin Hall effect and multi-photon microscopy; and is supported by first-principle calculations. Our work resolves the existing controversy of Rashba states in three-dimensional HMHs, while offering a cautionary note in measuring and reporting this important phenomenon in HMHs.

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[8] Yang, S. et al. “Observation of Long Spin Lifetime in MAPbBr3 Single Crystals at Room Temperature” Journal of Physics: Materials 3, 15012 (2020).