Researchers from the Technical University of Dortmund, including Vladimir L. Zhiliakov, Nataliia E. Kopteva, Irina A. Yugova, Dmitri R. Yakovlev, Ilya A. Akimov, and Manfred Bayer, have delved into the intricate world of exciton spin dynamics in lead halide perovskite semiconductors. Their work, published in the journal Physical Review B, offers a theoretical and experimental exploration of how these dynamics are influenced by crystal symmetry and magnetic fields.
Lead halide perovskites are a class of materials that have garnered significant attention in the energy sector, particularly for their potential in solar cells and light-emitting devices. These materials exhibit unique optical and electronic properties, which are largely governed by the behavior of excitons—bound pairs of electrons and holes that can transport energy through the material.
The researchers investigated the spin structure and dynamics of excitons in these materials, considering different crystal symmetries (cubic, tetragonal, and orthorhombic) and the effects of external magnetic fields. They found that the spin dynamics are significantly influenced by the crystal symmetry and the orientation of the magnetic field. For instance, in a longitudinal magnetic field, quantum beats between the bright exciton states were predicted under linearly polarized excitation and detection, while the dark exciton remained optically inactive. In a transverse magnetic field, all exciton spin states became optically active and could be excited by circularly polarized light.
The team also observed that reducing the crystal symmetry led to a zero-field offset of the exciton Larmor precession frequencies, modifying the Zeeman splitting energy dependence on the magnetic field. This theoretical framework allows for the extraction of the strength of the exchange interaction and the crystal symmetry.
Experimentally, the researchers measured the exciton spin coherence via time-resolved photoluminescence at a temperature of 1.6 K in longitudinal and transverse magnetic fields in orthorhombic MAPbI3 crystals. Polarization beats at the frequency of the bright exciton were observed in both configurations. Comparison with theory indicated that the excitons are in the strong exchange interaction regime, and the reduction of symmetry does not lead to a significant splitting of the exciton spin levels.
This research provides a deeper understanding of the fundamental properties of lead halide perovskites, which could pave the way for more efficient and stable energy devices. By manipulating the spin dynamics of excitons, it may be possible to enhance the performance of perovskite-based solar cells and light-emitting diodes, contributing to the ongoing efforts to transition to sustainable energy sources.
Source: Physical Review B
This article is based on research available at arXiv.