Millisecond-long electron spin lifetime in CsPbI$_3$ perovskite nanocrystals revealed by optically detected magnetic resonance

Abstract

Perovskite nanocrystals are a convenient model system for optical spin orientation and manipulation. However, its real potential might be underestimated due to the incomplete knowledge on spin relaxation times, which are obscured by the limited sensitivity of measurement techniques as well as by the insufficient understanding of the spin relaxation mechanisms in perovskites. In this work, we study the spin relaxation of charge carriers in perovskite nanocrystals both experimentally and theoretically. We address the electron and hole spins in CsPbI$_3$ nanocrystals embedded in a glass matrix by the resonant spin inertia technique based on optically detected magnetic resonance. It allows us to determine the longitudinal spin relaxation time $T_1$ separately for electrons and holes, the $g$ factors, and the effective Overhauser field of the nuclear spin bath. At a temperature of 1.6 K, the $T_1$ time for electrons can be as long as 0.9 ms. We reveal the effect of the time-varying nuclear field fluctuations, which enhances the electron spin relaxation at low magnetic fields, and measure a rather long nuclear spin correlation time of about 60 $μ$s. We develop a model of the spin relaxation in nanocrystals based on a two-LO-phonon Raman process, which explains the observed temperature dependence of the time $T_1$.

Figures (3)

• Figure 1: Optically detected magnetic resonance in CsPbI$_3$ NCs. (a) ODMR spectra for different rf field frequencies. The modulation frequency is 0.6 kHz, the laser photon energy is 1.824 eV, the laser power is 0.5 mW. The inset shows the ODMR spectrum for $f_\text{rf} = 1.1$ GHz at an increased modulation frequency of 3 kHz and a laser power of 2 mW manifesting two resonances. (b) Magnetic field dependence of the magnetic resonance frequency allowing one to determine the electron $g$ factor of 1.68 from the linear fit shown by the red dashed line. (c) Magnetic field dependence of the width of the electron ODMR resonance (defined as the normal distribution standard deviation $\sigma$). The red dashed line shows a fit with Eq. \ref{['eq:dB']}. The temperature is 1.6 K.
• Figure 2: (a) Photoluminescence spectra of the two CsPbI$_3$ perovskite samples with different average NC sizes measured at $T = 6$ K. Dependence of electron $g$ factor on optical transition energy (b) and of electron ODMR peak width (c) measured at $f_\text{rf}$ = 1.13 GHz (magnetic field of about 45 mT). The red solid line shows the calculated nuclear field according to Ref. Meliakov2026Nucl, see text. The temperature in (b) and (c) is 1.6 K.
• Figure 3: (a) Dependence of the ODMR signal on the modulation frequency for the electron and hole resonances for a magnetic field of 100 mT (the corresponding rf frequencies are 2.2 and 0.6 GHz) and an increased laser power of 2 mW. The curves demonstrate the spin inertia effect making it possible to determine the times $T_1$ for the electrons and the holes separately. The dashed lines show fits with Eq. \ref{['eq:SI']}. The inset shows the dependence of $1 / T_1$ on the laser power for the electron with a linear fit shown by the dashed line. (b) Magnetic field dependence of the electron $T_1$ measured with a laser power of 0.125 mW. The dashed line shows a fit with Eq. \ref{['eq:T1']}. (c) Temperature dependence of the electron $T_1$ corresponding to a laser power of 0.125 mW. The dashed line shows a fit with an activation dependence \ref{['eq:act']}. The inset shows a sketch of virtual transitions to the NC exited state, accompanied by the emission and absorption of LO phonons resulting in a spin flip. The laser photon energy is 1.824 eV. The temperature in panels (a) and (b) is 1.6 K.