Rapidly expanding spin-polarized exciton halo in a two-dimensional halide perovskite at room temperature

Tuning Spin-Polarized Lifetime at High Carrier Density through Deformation Potential in Dion-Jacobson-Phase Perovskites

The control of spin relaxation mechanisms is of great importance for spintronics applications as well as for fundamental studies. Layered metal-halide perovskites represent an emerging class of semiconductors with rich optical spin physics, showing potential for spintronic applications. However, a major hurdle arises in layered metal-halide perovskites with strong spin-orbit coupling, where the spin lifetime becomes extremely short due to D'yakonov-Perel' scattering and Bir-Aronov-Pikus at high carrier density. Using the circularly polarized pump-probe transient reflection technique, we experimentally reveal the important scattering for spin relaxation beyond the electron-hole exchange strength in the Dion-Jacobson (DJ)-type 2D perovskites (3AMP)(MA)n-1PbnI3n+1 [3AMP = 3-(aminomethyl)piperidinium, n = 1-4]. Despite a more than 10-fold increase in carrier concentration, the spin lifetimes for n = 3 and 4 are effectively maintained. We reveal neutral impurity and polar optical phonon scatterings as significant contributors to the momentum relaxation rate. Furthermore, we show that more octahedral distortions induce a larger deformation potential which is reflected on the acoustic phonon properties. Coherent acoustic phonon analysis indicates that the polaronic effect is crucial in achieving control over the scattering mechanism and ensuring spin lifetime protection, highlighting the potential of DJ-phase perovskites for spintronic applications.

Spontaneous Polarization Induced Optical Responses in a Two-Dimensional Ferroelectric Halide Perovskite

Two-dimensional (2D) halide perovskites exhibit unique structural and optical properties because large organic molecular cations distort the perovskite structure and the excitons confined in the 2D layers are stable. Here, we report the temperature dependences of the absorption spectra, second harmonic generation (SHG) intensity, and lattice constants of 2D perovskite (BA)2(EA)2Pb3I10 single crystals, where BA is n-butylammonium and EA is ethylammonium. We found that the Urbach tail of the absorption spectrum significantly changes at around 200 K and that the change is correlated with the SHG intensity and the in-plane lattice distortion. We concluded that a random distribution of spontaneous polarizations in the ferroelectric phase modifies the linewidth of the band-edge exciton transition and is the cause of the anomalous temperature dependence of the steepness parameter of the Urbach tail.

Biexciton dynamics in halide perovskite nanocrystals

Lead halide perovskite nanocrystals are attracting considerable interest as next-generation optoelectronic materials. Optical responses of nanocrystals are determined by excitons and exciton complexes such as trions and biexcitons. Understanding of their dynamics is indispensable for the optimal design of optoelectronic devices and the development of new functional properties. Here, we summarize the recent advances on the exciton and biexciton photophysics in lead halide perovskite nanocrystals revealed by femtosecond time-resolved spectroscopy and single-dot spectroscopy. We discuss the impact of the biexciton dynamics on controlling and improving the optical gain.