2, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, , Germany
4, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, , Germany
3, Bavarian Center for Applied Energy Research (ZAE Bayern), Erlangen, , Germany
Spin-orbit coupling and the resulting spin splittings in the band structure of lead halide perovskites were proposed to strongly influence the optical and transport properties of these novel materials. Calculations predict that Rashba-type spin splittings strongly enhance carrier lifetimes and electron-acoustic-phonon scattering. While spin-orbit coupling is a necessary consequence of the contribution of the Pb 6p orbitals to the conduction bands, few studies are available that determine the actual strength of the resulting Rashba splitting.
The most direct method to probe the electronic structure of a solid is angle-resolved photoelectron spectroscopy (ARPES). The technique is surface-sensitive with an information depth in the nm range and thus requires high-quality crystalline surfaces. ARPES measurements on (CH3NH3)PbBr3 single crystals cleaved in ultrahigh vacuum show circular valence-band maxima centered around the high-symmetry points, characteristic for a Rashba system. This interpretation is supported by the circular dichroism found in laser-based ARPES experiments. The extracted Rashba parameters are amongst the largest reported to date, with a Rashba energy of 160 meV (240 meV) in the low-temperature orthorhombic (room-temperature cubic) phase. These values are significantly larger than the ones found in calculations for the bulk material. The difference can be attributed to the breaking of the bulk crystal symmetry at the surface, resulting in additional fields which increase the Rashba splitting. The symmetry-breaking goes in hand with a preferential orientation of the long axis of the unit cell along the surface normal in the orthorhombic phase. The possibility of a Rashba splitting in the bulk material is investigated using photoluminescence and photocurrent spectroscopies. Both measurements find two optical transitions close to the band gap, consistent with a direct-indirect character of the band gap. The splitting between the two is larger in the low-temperature orthorhombic phase than in the room-temperature cubic one. For the lower-energy transitions, photocurrents induced in orthorhombic (CH3NH3)PbI3 by circularly polarized light depend on the helicity of the photons. This phenomenon is known as the circular photogalvanic effect. It implies a band structure with spin-splittings, in which spin currents are optically induced.
The strong Rashba splitting at the surface of (CH3NH3)PbBr3 and the possibility to optically induce spin currents in (CH3NH3)PbI3 make lead halide perovskites candidates for opto-spintronics applications. The findings also point out the importance of spin effects in understanding the exceptional photophysics in this class of materials.