Exciton spin structure in lead halide perovskite semiconductors explored via the spin dynamics in magnetic field

We theoretically investigate the spin structure and spin dynamics of excitons in bulk lead halide perovskite semiconductors with cubic, tetragonal, and orthorhombic crystal symmetry. The exciton spin structure and its modification by an external magnetic field are modeled for different regimes defined by the relative magnitude of the electron-hole exchange interaction (splitting between dark and bright states) and the Zeeman spin splitting. The effects of crystal symmetry and magnetic field orientation with respect to the crystal axes are considered for lead halide perovskite crystals with band gaps in the range 1.4 - 3.5 eV, having different ratios of electron and hole g-factors. For cubic symmetry, in a longitudinal magnetic field, our theory predicts quantum beats between the bright exciton states under linearly polarized excitation and detection, while the dark exciton remains optically inactive. In a transverse magnetic field, all exciton spin states become optically active and can be excited by circularly polarized light. Reduction of the crystal symmetry leads to a zero-field offset of the exciton Larmor precession frequencies, modifying the Zeeman splitting energy dependence on magnetic field. This theoretical framework allows for the extraction of the strength of the exchange interaction and the crystal symmetry. Experimentally, we measure 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 are observed in both configurations. Comparison with theory indicates 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.

💡 Research Summary

This paper presents a comprehensive theoretical and experimental study on the spin structure and dynamics of excitons in bulk lead halide perovskite semiconductors. The research aims to elucidate the fundamental spin physics of excitons in these materials, which is crucial for their applications in optoelectronics and spintronics.

The theoretical framework developed in the study models the exciton spin structure in perovskites with cubic, tetragonal, and orthorhombic crystal symmetries. The core of the model is the interplay between the electron-hole exchange interaction (which splits dark and bright exciton states) and the Zeeman splitting induced by an external magnetic field. The Hamiltonian is constructed in the basis of exciton spin states (the singlet |0,0⟩ and triplet |1,X⟩, |1,Y⟩, |1,Z⟩), and its form changes significantly depending on the magnetic field orientation relative to the light propagation direction (Faraday vs. Voigt geometry).

Key theoretical predictions are made: In a longitudinal magnetic field (Faraday), only the bright exciton states are optically active, leading to quantum beats under linearly polarized excitation, while the dark exciton remains inactive. In a transverse magnetic field (Voigt), all exciton spin states become optically active, enabling excitation by circularly polarized light and resulting in oscillations in the linear polarization degree of emission. The reduction of crystal symmetry from cubic to lower symmetries introduces anisotropy in the exchange constants, leading to a zero-field offset in the exciton Larmor precession frequencies and modifying the Zeeman energy dependence on the magnetic field. The theory is applied to five distinct cases of electron and hole g-factor combinations, which are shown to have universal dependencies on the bandgap energy in lead halide perovskites.

Experimentally, the predictions are tested using time-resolved photoluminescence at 1.6 K on an orthorhombic MAPbI3 crystal under both longitudinal and transverse magnetic fields. Polarization quantum beats at the frequency corresponding to the bright exciton are clearly observed in both configurations. A detailed comparison of the experimental data with the theoretical model allows the researchers to conclude that the excitons in MAPbI3 are in the strong exchange interaction regime (where the exchange splitting dominates over the Zeeman splitting). Furthermore, it is found that the symmetry reduction to orthorhombic does not lead to a significant splitting of the exciton spin levels in this specific case, meaning the spin dynamics are largely similar to those expected for cubic symmetry.

In summary, this work establishes a robust theoretical framework for understanding exciton spin dynamics in perovskites across different crystal symmetries and magnetic field configurations. The experimental validation in MAPbI3 confirms the theory’s predictive power and provides direct access to key parameters such as the bright exciton g-factor, its anisotropy, and the strength of the exchange interaction. This study significantly advances the fundamental understanding of spin properties in perovskite semiconductors, paving the way for their exploitation in spin-based quantum technologies.