Spin Relaxation Lifetime Research Articles

Ultrafast Spin Relaxation of Charge Carriers in Strongly Quantum Confined Methylammonium Lead Bromide Perovskite Magic-Sized Clusters.

Spin relaxation of charge carriers in strongly quantum confined perovskite magic-sized clusters has been probed, for the first time, by using polarization-controlled femtosecond transient absorption (fs-TA) spectroscopy. Fs-TA measurements with a circularly polarized pump and probe allowed for the determination of the exciton spin relaxation lifetime (∼1.5 ps) at room temperature based on the dynamics of a photoinduced absorption (PIA) feature peaked at 458 nm. This spin lifetime is shorter than that of perovskite quantum dots (PQDs) with a larger size, and the results suggest that exciton confinement and defects likely play a more important role in these strongly quantum confined magic-sized clusters with a larger surface-to-volume ratio.

Manipulation and Mechanistic Understanding of Exciton Spin Dynamics in a Chiral Inorganic Nanosystem via Facile pH‐Regulation

AbstractThis study demonstrates effective manipulation of exciton spin dynamics in a prototypical chiral inorganic nanosystem, i.e., cadmium selenide (CdSe) nanosheets capped with chiral cysteine ligands in aqueous solution, via facile pH‐regulation that can directly modify the electronic coupling between CdSe and cysteine's thiol group through Cd─S bonding. The comparative scrutiny by using transient circular dichroism spectroscopy enables to decipher the pertinent mechanisms behind the pH‐regulated spin‐flip dynamics. The hole‐trapping interaction between the valence‐band heavy‐hole spin state of CdSe and the cysteine‐induced “extrinsic” surface state is found to play a dominant role in prolonging the hole spin relaxation lifetime (by more than threefold). This study also demonstrates a relevant application in modulating the sensitivity of circularly polarized light detection. This work sets a paradigm for harnessing the elusive interactions in chiral inorganic nanosystems to achieve desired spin‐polarization regulation, refreshing the fundamental understanding about the mechanisms of spin dynamics involved therein.

Vanadium in silicon carbide: telecom-ready spin centres with long relaxation lifetimes and hyperfine-resolved optical transitions

Vanadium in silicon carbide (SiC) is emerging as an important candidate system for quantum technology due to its optical transitions in the telecom wavelength range. However, several key characteristics of this defect family including their spin relaxation lifetime (T1), charge state dynamics, and level structure are not fully understood. In this work, we determine the T1 of an ensemble of vanadium defects, demonstrating that it can be greatly enhanced at low temperature. We observe a large spin contrast exceeding 90% and long spin-relaxation times of up to 25 s at 100 mK, and of order 1 s at 1.3 K. These measurements are complemented by a characterization of the ensemble charge state dynamics. The stable electron spin furthermore enables high-resolution characterization of the systems’ hyperfine level structure via two-photon magneto-spectroscopy. The acquired insights point towards high-performance spin-photon interfaces based on vanadium in SiC.

Spin Preserved Exciton Funneling through a Polaronic Intermediate State in Layered Perovskite Single Crystals

AbstractThe spintronic device is another application of perovskite beyond solar cells, light emitting diodes, and photodetectors. Spin relaxation with a lifetime of several ps has been observed in both 3D and 2D perovskite. In this study, Ruddlesden–Popper (RP) perovskite single crystals with mixed n and n + 1 phase are synthesized. It is found that the spin‐polarized exciton can funnel from n phase to n + 1 phase with spin information retained effectively. Interestingly, the spin lifetime of the funneled exciton in n + 1 phase is much longer than that in the pristine n phase. This is attributed to the fact that the funneling is realized through a polaronic intermediate state, in which the spin relaxation is much slower. The polaronic intermediate states originate from the interaction between photogenerated excitons and the lattice distortion. The formation of the polaronic states can reduce overlap of electron–hole wave function and modified lattice symmetry; these two changes contribute to retain the exciton spin information in such polaronic states. The findings deepen the understanding of spin‐polarized exciton transportation and relaxation dynamics and provide a potentially novel approach to enhance the spin relaxation lifetime of perovskite single crystals.

Evaluating Lead Halide Perovskite Nanocrystals as a Spin Laser Gain Medium.

Spin-polarized charge endows conventional lasers with not only new functionalities but also reduced lasing thresholds thanks to the lifting of spin degeneracy. II-VI and III-V semiconductors have been extensively investigated as spin laser gain mediums; however, the degree of polarization is limited by the light hole and heavy hole degeneracy. Herein, we evaluate the potential of CsPbBr3 nanocrystals─ones that are featured with low band-edge degeneracy and therefore a high degree of polarization as a result of inverted band structure and large spin-orbit coupling─as a gain medium for spin lasers. Our experiment and numerical modeling results reveal that, within the spin relaxation lifetime, the optical gain threshold can be depressed by polarizing the charge using circularly polarized photoexcitation. However, prolonging the spin relaxation lifetime is required to realize a spin laser.

Efficient Optical Orientation and Slow Spin Relaxation in Lead-Free CsSnBr3 Perovskite Nanocrystals

Spin properties of lead halide perovskites have recently received considerable research attention, because the strong spin–orbit coupling (SOC) enables facile optical orientation of spin-polarized states, a property particularly useful for information science and spintronics. The strong SOC, however, also limits the spin relaxation lifetime. Herein, we study optical orientation and spin relaxation in lead-free CsSnBr3 perovskite nanocrystals. The SOC of CsSnBr3 (∼0.43 eV) is 3.5-fold weaker than that of lead halide perovskites but is still strong enough to ensure efficient optical orientation. Meanwhile, the room-temperature spin lifetime is prolonged to 18 ± 2 ps, longer than the reported lifetimes of various lead halide perovskites, and it is likely limited by electron–hole exchange rather than SOC. These results demonstrate the strong potential of nontoxic tin halide perovskites for spin-related applications.

Mott and generalized Wigner crystal states in WSe2/WS2 moiré superlattices.

Moiré superlattices can be used to engineer strongly correlated electronic states in two-dimensional van der Waals heterostructures, as recently demonstrated in the correlated insulating and superconducting states observed in magic-angle twisted-bilayer graphene and ABC trilayer graphene/boron nitride moiré superlattices1-4. Transition metal dichalcogenide moiré heterostructures provide another model system for the study of correlated quantum phenomena5 because of their strong light-matter interactions and large spin-orbit coupling. However, experimental observation of correlated insulating states in this system is challenging with traditional transport techniques. Here we report the optical detection of strongly correlated phases in semiconducting WSe2/WS2 moiré superlattices. We use a sensitive optical detection technique and reveal a Mott insulator state at one hole per superlattice site and surprising insulating phases at 1/3 and 2/3 filling of the superlattice, which we assign to generalized Wigner crystallization on the underlying lattice6-11. Furthermore, the spin-valley optical selection rules12-14 of transition metal dichalcogenide heterostructures allow us to optically create and investigate low-energy excited spin states in the Mott insulator. We measure a very long spin relaxation lifetime of many microseconds in the Mott insulating state, orders of magnitude longer than that of charge excitations. Our studies highlight the value of using moiré superlattices beyond graphene to explore correlated physics.

Excellent spin transport in spin valves based on the conjugated polymer with high carrier mobility.

Organic semiconductors (OSCs) are characteristic of long spin-relaxation lifetime due to weak spin-orbit interaction and hyperfine interaction. However, short spin diffusion length and weak magnetoresistance (MR) effect at room temperature (RT) was commonly found on spin valves (SVs) using an organic spacer, which should be correlated with low carrier mobility of the OSCs. Here, N-type semiconducting polymer P(NDI2OD-T2) with high carrier mobility is employed as the spacer in the SV devices. Exceedingly high MR ratio of 90.0% at 4.2 K and of 6.8% at RT are achieved, respectively, via improving the interface structure between the polymer interlayer and top cobalt electrode as well as optimal annealing of manganite bottom electrode. Furthermore, we observe spin dependent transport through the polymeric interlayer and a large spin diffusion length with a weak temperature dependence. The results indicate that this polymer material can be used as a good medium for spintronic devices.

Highly spin-polarized carrier dynamics and ultralarge photoinduced magnetization in CH3NH3PbI3 perovskite thin films.

Low-temperature solution-processed organic-inorganic halide perovskite CH3NH3PbI3 has demonstrated great potential for photovoltaics and light-emitting devices. Recent discoveries of long ambipolar carrier diffusion lengths and the prediction of the Rashba effect in CH3NH3PbI3, that possesses large spin-orbit coupling, also point to a novel semiconductor system with highly promising properties for spin-based applications. Through circular pump-probe measurements, we demonstrate that highly polarized electrons of total angular momentum (J) with an initial degree of polarization Pini ∼90% (i.e., -30% degree of electron spin polarization) can be photogenerated in perovskites. Time-resolved Faraday rotation measurements reveal photoinduced Faraday rotation as large as 10°/μm at 200 K (at wavelength λ = 750 nm) from an ultrathin 70 nm film. These spin polarized carrier populations generated within the polycrystalline perovskite films, relax via intraband carrier spin-flip through the Elliot-Yafet mechanism. Through a simple two-level model, we elucidate the electron spin relaxation lifetime to be ∼7 ps and that of the hole is ∼1 ps. Our work highlights the potential of CH3NH3PbI3 as a new candidate for ultrafast spin switches in spintronics applications.

Potassium spin polarization lifetime for a 30-carbon chain siloxane film

The siloxane film derived from the 30-carbon chain triacontyltrichlorosilane (TCTS) is studied as an anti-relaxation coating for atomic vapor cells. The longitudinal spin relaxation lifetime of optically pumped potassium atoms in the presence of TCTS is measured and the average number of non-relaxing atom-wall collisions, or bounces, enabled by the coated surface is determined. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) of TCTS were performed to investigate changes in chemical states and surface morphology of TCTS arising from K atom deposition on the film surface. TCTS was found to give approximately 530 bounces. Following lifetime measurements, K2p signals were clearly observed in XPS spectra. AFM images display non-preferential K deposition on the TCTS surface, however additional AFM studies with a TCTS surface exposed to Rb atoms show deposition occurs along surface defects. In agreement, Rb is found to preferentially deposit along the step edges of an 18-carbon chain monolayer film derived from 1-Octadecene. Finally, AFM indicates a much smoother surface for a tetracontane coating relative to TCTS. The importance of siloxane surface morphology versus film thickness with respect to coating performance is discussed.

Carrier spin relaxation in (Ga,Mn)As at room temperature

In this paper, the excitation energy density dependence of carrier spin relaxation is studied at room temperature for the as-grown and annealed (Ga,Mn)As samples using femtosecond time-resolved pump-probe Kerr spectroscopy. It is found that spin relaxation lifetime of electrons lengthens with increasing excitation energy density for both samples, and the annealed (Ga,Mn)As has shorter carrier recombination and electron spin relaxation lifetimes as well as larger Kerr rotation angle than the as-grown (Ga,Mn)As under the same excitation condition, which shows that DP mechanism is dominant in the spin relaxation process for (Ga,Mn)As at room temperature. The enhanced ultrafast Kerr effect in the annealed (Ga,Mn)As shows the potential application of the annealed (Ga,Mn)As in ultrafast all-optical spin switches, and also provides a further evidence for the p-d exchange mechanism of the ferromagnetic origin of (Ga,Mn)As.

Study of injection and relaxation of electron spins in InGaN film by time-resolved absorption spectroscopy

The injection and relaxation of electron spins in In0.1Ga0.9N film were studied by femtosecond time-resolved circularly polarized pump-probe spectroscopy. An initial degree of spin polarization of 02 was obtained, which agrees with 3∶1 ratio of heavy- to light-hole valence bands in transition strength, but not with the 1∶1 or 1∶0.94 ratios. A spin relaxation lifetime of 490±70ps was obtained at room temperature. The spin relaxation mechanism is discussed qualitatively, and is thought to be dominated by BAP mechanism here.

Cold-atom confinement in an all-optical dark ring trap

We demonstrate confinement of $^{85}\mathrm{Rb}$ atoms in a dark, toroidal optical trap. We use a spatial light modulator to convert a single blue-detuned Gaussian laser beam to a superposition of Laguerre-Gaussian modes that forms a ring-shaped intensity null bounded harmonically in all directions. We measure a $1∕e$ spin-relaxation lifetime of $\ensuremath{\approx}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ for a trap detuning of $4.0\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. For smaller detunings, a time-dependent relaxation rate is observed. We use these relaxation rate measurements and imaging diagnostics to optimize trap alignment in a programmable manner with the modulator. The results are compared with numerical simulations.

Effect of electron–hole spatial correlation on spin relaxation dynamics in InAs submonolayer

Using time-resolved photoluminescence and time-resolved Kerr rotation spectroscopy, we explore the unique electron spin behavior in an InAs submonolayer sandwiched in a GaAs matrix, which shows very different spin characteristics under resonant and non-resonant excitations. While a very long spin relaxation lifetime of a few nanoseconds at low temperature is observed under non-resonant excitation, it decreases dramatically under resonant excitation. These interesting results are attributed to the difference in electron–hole interactions caused by non-geminate or geminate capture of photo-generated electron–hole pairs in the two excitation cases, and provide a direct verification of the electron–hole spatial correlation effect on electron spin relaxation.

Spin lifetimes of electrons injected into GaAs and GaN

The spin relaxation times of electrons in GaAs and GaN are determined with a model that includes momentum scattering by phonons and ionized impurities, and spin scattering by the Elliot–Yafet, D’yakonov–Perel, and Bir–Aronov–Pikus mechanisms. Accurate bands generated using a long-range tight-binding Hamiltonian obtained from empirical pseudopotentials are used. The inferred temperature dependence of the spin relaxation lifetime agrees well with measured values in GaAs. We further show that the spin lifetimes decrease rapidly with injected electron energy and reach a local maximum at the longitudinal optical phonon energy. Our calculation predicts that electron spin lifetime in pure GaN is about three orders of magnitude longer than in GaAs at all temperatures, primarily as a result of the lower spin-orbit interaction and higher conduction band density of states.

Spatially resolved spin-injection probability for gallium arsenide.

We report a large spin-polarized current injection from a ferromagnetic metal into a nonferromagnetic semiconductor, at a temperature of 100 Kelvin. The modification of the spin-injection process by a nanoscale step edge was observed. On flat gallium arsenide [GaAs(110)] terraces, the injection efficiency was 92%, whereas in a 10-nanometer-wide region around a [111]-oriented step the injection efficiency is reduced by a factor of 6. Alternatively, the spin-relaxation lifetime was reduced by a factor of 12. This reduction is associated with the metallic nature of the step edge. This study advances the realization of using both the charge and spin of the electron in future semiconductor devices.