Observing dynamic and static Rashba effects in a thin layer of 3D hybrid perovskite nanocrystals using transient absorption spectroscopy

Abstract

The dynamic and static Rashba effects in hybrid methylammonium (MA) lead halide perovskites have recently been theoretically predicted. However, only the static effect was experimentally confirmed so far. Here, we report on the dynamic (sub-picosecond/picosecond timescale) and static (nanosecond/microsecond timescale) Rashba effects observed in a fully encapsulated layer with various thicknesses (ranging from ∼40 nm to ∼100 nm) of ∼20-nm-sized 3D MAPbBr3 nanocrystals (NCs) using transient absorption (TA) spectroscopy. The effect appears as a splitting of the corresponding peaks in TA spectra. We argue that the physical reason for the Rashba effect to be observed is fundamentally determined by configurational entropy loss in NCs possessing a strong spin asymmetry. Specifically, owing to an enhanced flexibility of the NC lattice, a built-in electric field initially induced by an ultrashort (100 fs) pumping pulse through the photo-Dember effect and subsequently developed due to dynamic charge separation throughout NCs is able to initiate the order–disorder transition associated with the MA cation reorientations, the process that efficiently breaks structural inversion symmetry and hence induces the Rashba spin–orbit interaction. The dynamic Rashba effect is found to be strongly dependent on photoexcited carrier density (pumping power), whereas it weakens sharply upon increasing the NC layer thickness up to ∼80 nm due to the NC stacking effect. The integrated intensities of the corresponding spin-split subbands demonstrate a photon-helicity-dependent asymmetry, thus proving the Rashba-type spin-splitting. The magnitudes of the Rashba and Fröhlich polaron effects and the methods of controlling the dynamic Rashba effect are discussed.