SFB Extra Seminar

Combined structural and spin dynamics in lead halide perovskites

Lead halide perovskites (LHPs) form a class of semiconductors with a direct or close to direct band gap, whose size can be tuned across the near-infrared, visible, and near-ultraviolet spectral range by varying their chemical composition. Despite the ionic character of the materials, the effective masses of both electrons and holes are small. Therefore, possible applications of LHPs in optoelectronics and especially in thin-film solar cells have attracted significant research interest.

The unusual band structure of LHPs is the result of the distinct energetic ordering of bands of different orbital character, in combination with a strong spin-orbit coupling. These features distinguish the materials from traditional semiconductors like silicon or GaAs. The spin-orbit coupling does not only reduce carrier effective masses and affect the band ordering. In crystals without inversion symmetry, it also lifts the spin degeneracy of the electronic bands, an effect commonly referred to as the Rashba effect [1]. In LHPs, which have an inversion-symmetric cubic structure at sufficiently high temperature, the local symmetry can be broken by structural fluctuations due to soft optical phonon modes. For this situation, calculations predict a “dynamical” Rashba effect, even if the crystal maintains inversion-symmetry on average [2].

I will present electron [3] and optical spectroscopy [4] experiments which detect the fingerprints of the Rashba effect in lead halide perovskites. Experiments measuring optically excited spin currents [5] as a function of temperature indicate a dynamic origin of the effect. The dynamical Rashba effect should be general to a large class of materials with inversion symmetry containing heavy elements, possibly opening new perspectives for (opto-)spintronics devices.

[1] E. I. Rashba, V. I. Sheka, Fiz. Tverd. Tela: Collected Papers 2, 162 (1959)
[2] B. Monserrat, D. Vanderbilt; arXiv:1711.06274 (2017)
[3] D. Niesner et al.; Phys. Rev. Lett. 117, 126401 (2016)
[4] D. Niesner et al.; Phys. Rev. B 95, 075207 (2017)
[5] D. Niesner et al.; Proc. Natl. Acad. Sci. 115, 9509 (2018)