Hyperfine interaction of electrons and holes with nuclei probed by optical orientation in MAPbI$_3$ perovskite crystals

Optical orientation of electron and hole spins by circularly polarized light is investigated for MAPbI$_3$ single crystals. The Hanle and polarization recovery effects measured in transverse and longitudinal magnetic fields, respectively, evidence the hyperfine interaction with nuclear spins as the main factor determining the spin dynamics of charge carriers at cryogenic temperatures. The parameters of the nuclear spin fluctuations within the carrier localization volume are evaluated. Dynamic polarization of the nuclear spins is demonstrated by the Overhauser field reaching 5 mT for acting on the electrons and -30 mT for acting on the holes.

💡 Research Summary

This paper investigates the hyperfine interaction between charge carriers (electrons and holes) and the nuclear spin system in methylammonium lead iodide (MAPbI₃) single crystals by means of optical orientation experiments. Circularly polarized continuous‑wave Ti:Sapphire laser light (photon energy 1.722 eV, above the band gap of 1.652 eV) is used to generate spin‑polarized electrons and holes at cryogenic temperatures (1.6 K–15 K). The helicity of the excitation is either modulated at 400 kHz (σ⁺/σ⁻) or kept constant (σ⁺) to probe two complementary magnetic‑field effects: the Hanle effect in Voigt geometry (magnetic field perpendicular to the light propagation direction) and the polarization‑recovery (PR) effect in Faraday geometry (field parallel to the light propagation direction).

In the Hanle configuration the optically induced spin polarization precesses about the transverse magnetic field, leading to a Lorentzian decrease of the measured circular polarization degree P_oo as a function of field strength. In the PR configuration the longitudinal field suppresses spin dephasing caused by random hyperfine fields, resulting in a Lorentzian increase of P_oo with field. Both effects are fitted with a sum of three Lorentzian contributions, which the authors assign to (i) strongly localized holes, (ii) localized electrons, and (iii) weakly localized holes. The half‑width at half‑maximum (HWHM) values extracted from the fits are approximately 37 mT for the strongly localized holes, 9 mT for the electrons, and 0.5 mT for the weakly localized holes. These widths directly reflect the magnitude of the nuclear‑spin fluctuations experienced by each carrier type; a larger width corresponds to a smaller number of nuclei within the carrier’s localization volume.

The hyperfine coupling in MAPbI₃ is dominated by the ^207Pb nuclei (I = ½) for holes, while electrons couple to both ^207Pb and ^127I (I = 5/2). By combining the measured HWHM values with the known g‑factors (g_h ≈ ‑0.54 for holes, g_e ≈ ‑1.5 for electrons) the authors estimate the effective number of nuclei interacting with each carrier. Dynamic nuclear polarization (DNP) experiments reveal Overhauser fields of +5 mT acting on electrons and –30 mT acting on holes, confirming that holes generate a substantially larger nuclear polarization, consistent with their stronger hyperfine coupling to ^207Pb.

Temperature‑dependent measurements show that at the lowest temperature (1.6 K) the zero‑field optical orientation degree reaches about 50 % near the exciton resonance (1.638 eV) and about 6 % at 1.628 eV where excitonic contributions are negligible. As temperature increases to 10–15 K, the PR signal disappears entirely, indicating that carriers become delocalized and the hyperfine‑induced dephasing is no longer suppressed by a longitudinal field. The Hanle signal persists but with strongly reduced amplitude, reflecting a shorter spin lifetime and weaker hyperfine interaction due to carrier delocalization.

A key conclusion is that the carrier spin correlation time with nuclear spin fluctuations (τ_c) exceeds the carrier spin lifetime (T_s) in MAPbI₃ (T_s < τ_c). In this regime the ratio of PR to Hanle amplitudes is theoretically 3:1, but the experimental ratio deviates because the measured curves contain overlapping contributions from excitons and multiple carrier species, making precise amplitude extraction difficult. Nevertheless, the consistent HWHM values for both Hanle and PR curves across all carrier types support the τ_c > T_s scenario.

Overall, the work demonstrates that MAPbI₃ possesses a high degree of optical spin orientation, robust against excitation‑energy detuning, and that the Dyakonov‑Perel spin‑relaxation mechanism is suppressed by the material’s inversion symmetry. However, at low temperatures the dominant spin‑decoherence channel is the hyperfine interaction with the nuclear spin bath. The quantified Overhauser fields and nuclear‑spin fluctuation parameters provide essential benchmarks for future spin‑tronic and quantum‑information applications based on lead‑halide perovskites, where controlled nuclear spin polarization could be exploited for spin memory or coherent manipulation of carrier spins.