Spectral characterization of the Rashba spin-split band in a lead halide perovskite single crystal by photocurrent heterodyne interference spectroscopy

We characterized the temperature dependence of the complex dielectric susceptibility spectrum of a perovskite $\mathrm{C}{\mathrm{H}}₃\mathrm{N}{\mathrm{H}}₃\mathrm{Pb}{\mathrm{Br}}₃$ single crystal by using heterodyne interference spectroscopy. Owing to a time-domain measurement that is not influenced by incoherent lattice relaxations, we were able to detect a low-energy indirect-gap band. We observed two peaks in the energy region below the exciton transition at the $R$ point in the imaginary part of the spectrum. The temperature dependences of the energies of these peaks were different from the dependence of the energy of the direct-transition exciton. The trends of both the splitting width and peak intensities can be explained by assuming a Rashba spin-split band.