Carrier Spin Dynamics Research Articles

Ultrafast magneto-optical dynamics in nickel (111) single crystal studied by the integration of ultrafast reflectivity and polarimetry probes

With the integration of ultrafast reflectivity and polarimetry probes, we observed carrier relaxation and spin dynamics induced by ultrafast laser excitation of Ni (111) single crystals. The carrier relaxation time within the linear excitation range reveals that electron–phonon coupling and dissipation of photon energy into the bulk of the crystal take tens of picoseconds. On the other hand, the observed spin dynamics indicate a longer time of about 120 ps. To further understand how the lattice degree of freedom is coupled with these dynamics may require the integration of an ultrafast diffraction probe.

Coherent Spin Dynamics of Electrons in CsPbBr3 Perovskite Nanocrystals at Room Temperature.

Coherent spin dynamics of charge carriers in CsPbBr3 perovskite nanocrystals are studied in a temperature range of 4-300 K and in magnetic fields of up to 500 mT using time-resolved pump-probe Faraday rotation and differential transmission techniques. We detect electron spin Larmor precession in the entire temperature range. At temperatures below 50 K, hole spin precession is also observed. The temperature dependences of spin-related parameters, such as Landè g-factor and spin dephasing time are measured and analyzed. The electron g-factor increases with growing temperature, which can not be described by the temperature-induced band gap renormalization. We find that photocharging of the nanocrystals with either electrons or holes depends on the sample cooling regime, namely the cooling rate and illumination conditions. The type of the charge carrier provided by the photocharging can be identified via the carrier spin Larmor precession.

Lead-Dominated Hyperfine Interaction Impacting the Carrier Spin Dynamics in Halide Perovskites.

The outstanding optical quality of lead halide perovskites inspires studies of their potential for the optical control of carrier spins as pursued in other materials. Entering largely uncharted territory, time-resolved pump-probe Kerr rotation is used to explore the coherent spin dynamics of electrons and holes in bulk formamidinium caesium lead iodine bromide (FA0.9 Cs0.1 PbI2.8 Br0.2 ) and to determine key parameters characterizing interactions of their spins, such as the g-factors and relaxation times. The demonstrated long spin dynamics and narrow g-factor distribution prove the perovskites as promising competitors for conventional semiconductors in spintronics. The dynamic nuclear polarization via spin-oriented holes is realized and the identification of the lead (207 Pb) isotope in optically detected nuclear magnetic resonance proves that the hole-nuclei interaction is dominated by the lead ions. A detailed theoretical analysis accounting for the specifics of the lead halide perovskite materials allows the evaluation of the underlying hyperfine interaction constants, both for electrons and holes. Recombination and spin dynamics evidence that at low temperatures, photogenerated electrons and holes are localized at different regions of the perovskite crystal, resulting in their long lifetimes up to 44μs. The findings form the base for the tailored development of spin-optoelectronic applications for the large family of lead halide perovskites and theirnanostructures.

Coherent Spin Dynamics of Electrons and Holes in CsPbBr3 Colloidal Nanocrystals.

The spin dynamics in CsPbBr3 lead halide perovskite nanocrystals are studied by picosecond pump-probe Faraday rotation in an external magnetic field. Coherent Larmor precession of electrons and holes with spin dephasing times of ∼600 ps is detected in a transversal magnetic field. The longitudinal spin relaxation time in weak magnetic fields reaches 80 ns at a temperature of 5 K. In this regime, the carrier spin dynamics is governed by nuclear spin fluctuations characterized by an effective hyperfine field strength of 25 mT. The Landé factors determining the carrier Zeeman splittings are ge = +1.73 for electrons and gh = +0.83 for holes. A comparison with a CsPbBr3 polycrystalline film and bulk single crystals evidences that the spatial confinement of electrons and holes in the nanocrystals only slightly affects their g factors and spin dynamics.

Impacts of Crystal Quality on Carrier Recombination and Spin Dynamics in (110)-Oriented GaAs/AlGaAs Multiple Quantum Wells at Room Temperature

We have systematically investigated the structural properties, carrier lifetimes, namely, photoluminescence (PL) lifetimes (τPL), and electron spin relaxation times (τs) in (110) GaAs/AlGaAs multiple quantum wells (MQWs) by using time-resolved PL measurements. The MQWs were grown by molecular beam epitaxy within a wide range of the growth temperature Tg (430–600 °C) and a high V/III flux ratio using As2. At 530 °C < Tg < 580 °C, we found that the quality of the heterointerfaces is significantly improved, resulting in τPL~40 ns at RT, one order of magnitude longer than those reported so far. Long τs (~6 ns) is also observed at RT.

Photogenerated carrier dynamics of TIPS-pentacene films as studied by photocurrent and electrically detected magnetic resonance.

The carrier generation process and spin dynamics through photoexcitation in the vacuum vapour deposition film of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were investigated by temperature dependence measurements of photocurrent and electrically detected magnetic resonance (EDMR). The EDMR signal was constructed from two components and showed a maximum at approximately 200 K. The temperature dependence was analysed using quantum mechanical simulation, assuming the carrier dynamics of the weakly coupled electron-hole pair (e-h pair). In addition, the analytical formula of photocurrent generation and EDMR signal intensity were also derived based on classical rate equations and used to understand the carrier dynamics. Through phase-shift analysis in quadrature detection of the EDMR signals, one of the two components was well analysed by using a narrow Lorentzian shape, and the other was by using a broad Gaussian.

Spin polarization recovery and Hanle effect for charge carriers interacting with nuclear spins in semiconductors

We report on theoretical and experimental study of the spin polarization recovery and Hanle effect for the charge carriers interacting with the fluctuating nuclear spins in the semiconductor structures. We start the theoretical description from the simplest model of static and isotropic nuclear spin fluctuations. Then we describe the modification of the polarization recovery and Hanle curves due to the anisotropy of the hyperfine interaction, finite nuclear spin correlation time, and the strong pulsed spin excitation. For the latter case, we describe the resonance spin amplification effect in the Faraday geometry and discuss the manifestations of the quantum Zeno effect. The set of the experimental results for various structures and experimental conditions is chosen to highlight the specific effects predicted theoretically. We show that the spin polarization recovery is a very valuable tool for addressing carrier spin dynamics in semiconductors and their nanostructures.

Origin of Rashba Spin-Orbit Coupling in 2D and 3D Lead Iodide Perovskites

We studied spin dynamics of charge carriers in the superlattice-like Ruddlesden-Popper hybrid lead iodide perovskite semiconductors, 2D (BA)2(MA)Pb2I7 (with MA = CH3NH3, and BA = CH3(CH2)3NH3), and 3D MAPbI3 using the magnetic field effect (MFE) on conductivity and electroluminescence in their light emitting diodes (LEDs) at cryogenic temperatures. The semiconductors with distinct structural/bulk inversion symmetry breaking, when combined with colossal intrinsic spin–orbit coupling (SOC), theoretically give rise to giant Rashba-type SOC. We found that the magneto-conductance (MC) magnitude increases monotonically with the emission intensity and saturates at ≈0.05% and 0.11% for the MAPbI3 and (BA)2(MA)Pb2I7, respectively. The magneto-electroluminescence (MEL) response with similar line shapes as the MC response has a significantly larger magnitude, and essentially stays constant at ≈0.22% and ≈0.20% for MAPbI3 and (BA)2(MA)Pb2I7, respectively. The sign and magnitude of the MC and MEL responses can be quantitatively explained in the framework of the Δg-based excitonic model using rate equations. Remarkably, the width of the MEL response in those materials linearly increases with increasing the applied electric field, where the Rashba coefficient in (BA)2(MA)Pb2I7 is estimated to be about 7 times larger than that in MAPbI3. Our studies might have significant impact on future development of electrically-controlled spin logic devices via Rashba-like effects.

Features of spin dynamics of magnetic ions and charge carriers in self-organized quantum dots CdSe/ZnMnSe

Self-organized disk-shaped quantum dots of CdSe embedded in diluted magnetic ZnMnSe barrier were studied by means of pump-probe time-resolved Kerr rotation (TRKR) technique at low temperature T = 7 K. In absence of the external magnetic field TRKR signal exhibits long-living spin dynamics with the decay time exceeding the period between laser pulses. Such spin dynamics is not typical for diluted magnetic semiconductors and nano-structures based on them and could be a trace of a bound magnetic polaron. Resonant spin amplification measured in transversal magnetic field up to 1 T shows the only one peak near B = 0. In B = 1 T the long-living non-precessing signal practically vanishes, while the precessing one appears with the Larmor frequency corresponding to the Mn2+ ions’ net spin precession around the magnetic field. It was found that the signal consists of three components with slightly different precession frequencies that could be due to the fine structure of the manganese spin sublevels occurring because of a stress in quantum dots.

Detection of Rashba spin splitting in 2D organic-inorganic perovskite via precessional carrier spin relaxation

The strong spin-orbit interaction in the organic-inorganic perovskites tied to the incorporation of heavy elements (e.g., Pb and I) makes these materials interesting for applications in spintronics. In conjunction with a lack of inversion symmetry associated with distortions of the metal-halide octahedra, surfaces and interfaces, or the application of a bias, the Rashba effect (used in spin field-effect transistors and spin filters) has been predicted to be much larger in these materials than in traditional III-V semiconductors such as GaAs. Evidence of strong Rashba coupling has been observed in both 3D (bulk) and 2D perovskites, with the relative role of bulk and surface Rashba contributions in the former case under active debate. The varying size of the reported spin splittings points to the need for more experimental studies of Rashba effects in the organic-inorganic perovskite family of materials. Here, we apply time-resolved circular dichroism techniques to the study of carrier spin dynamics in a 2D perovskite thin film [(BA)2MAPb2I7; BA = CH3(CH2)3NH3, MA = CH3NH3]. Our findings confirm the presence of a Rashba spin splitting via the dominance of precessional spin relaxation induced by the Rashba effective magnetic field (also known as D’yakonov Perel spin relaxation). The size of the Rashba spin splitting in our system was extracted from simulations of the measured spin dynamics incorporating LO-phonon and electron-electron scattering, yielding a value of 10 meV at an electron energy of 50 meV above the band gap, representing a 20 times larger value than in GaAs quantum wells.

Hyperfine interaction in atomically thin transition metal dichalcogenides.

The spin dynamics of localized charge carriers is mainly driven by hyperfine interaction with nuclear spins. Here we develop a theory of hyperfine interaction in transition metal dichalcogenide monolayers. Using group representation theory and the tight binding model we derive effective Hamiltonians of the intervalley hyperfine interaction in the conduction and valence bands. The spin–valley locking and pronounced spin–orbit splitting lead to a specific form of hyperfine interaction, which we call “helical”. We also demonstrate that the hyperfine interaction is noncollinear for chalcogen atoms in the general case. At the same time in the upper valence band the hyperfine interaction is purely of the Ising type, which suggests that the spin–valley polarization of localized holes in transition metal dichalcogenide monolayers can be conserved for a particularly long time.

RECENT ADVANCES IN ULTRAFAST TIME-RESOLVED SCANNING TUNNELING MICROSCOPY

Making smaller and faster functional devices has led to an increasing demand for a microscopic technique that allows the investigation of carrier and phonon dynamics with both high spatial and temporal resolutions. Traditional optical pump–probe methods can achieve femtosecond temporal resolution but fall short in the spatial resolution due to the diffraction limit. Scanning tunneling microscopy (STM), on the contrary, has realized atomic-scale spatial resolution relying on the high sensitivity of the tunneling current to the tip-sample distance. However, limited by the electronics bandwidth, STM can only push the temporal resolution to the microseconds scale, restricting its applications to probe various ultrafast dynamic processes. The combination of these two methods takes advantages of optical pump–probe techniques and highly localized tunneling currents of STM, providing one viable solution to track atomic-scale ultrafast dynamics in single molecules and low-dimensional materials. In this review, we will focus on several ultrafast time-resolved STM methods by coupling the tunneling junctions with pulsed electric waves, THz, near-infrared and visible laser. Their applications to probe the carrier dynamics, spin dynamics, and molecular motion will be highlighted. In the end, we will present an outlook on the challenges and new opportunities in this field.

Theory of spin inertia in singly charged quantum dots

The spin inertia measurement is a recently emerged tool to study slow spin dynamics, which is based on the excitation of the system by a train of circularly polarized pulses with alternating helicity. Motivated by the experimental results reported in E. A. Zhukov et al., arXiv:1806.11100 we develop the general theory of spin inertia of localized charge carriers. We demonstrate that the spin inertia measurement in longitudinal magnetic field allows one to determine the parameters of the spin dynamics of resident charge carriers and of photoexcited trions, such as the spin relaxation times, longitudinal g-factors, parameters of hyperfine interaction and nuclear spin correlation times.

Spin Dynamics of Charged and Neutral Excitons in Colloidal Nanocrystals

We review our studies on spin-dependent phenomena in various colloidal nanocrystals (quantum dots, dot-in-rod, and nanoplatelets). Several magneto-optical techniques are used in high magnetic fields: magnetic-field-induced polarization of neutral and charged excitons, spin-flip Raman scattering, and pump-probe Faraday rotation. We focus on spin properties and spin dynamics of neutral and charged excitons in order to identify them and measure g-factors and spin relaxation times. Pump-probe time-resolved Faraday rotation experiments with additional prepump allow us to measure coherent spin dynamics of carriers in CdS NCs at room temperature, and to study interplay between the negative and positive photocharging processes. We discuss the role of the dangling bond spins for recombination of the dark excitons and for formation of the dangling bond magnetic polaron.

Electron and Hole g-Factors and Spin Dynamics of Negatively Charged Excitons in CdSe/CdS Colloidal Nanoplatelets with Thick Shells.

We address spin properties and spin dynamics of carriers and charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies are performed by time-resolved and polarization-resolved photoluminescence, spin-flip Raman scattering and picosecond pump-probe Faraday rotation in magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets are negatively charged so that their photoluminescence is dominated by radiative recombination of negatively charged excitons (trions). Electron g-factor of 1.68 is measured, and heavy-hole g-factor varying with increasing magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for two-dimensional structures are calculated for various hole confining potentials for cubic- and wurtzite lattice in CdSe core. These calculations are extended for various quantum dots and nanoplatelets based on II-VI semiconductors. We developed a magneto-optical technique for the quantitative evaluation of the nanoplatelets orientation in ensemble.

Coherent spin dynamics of carriers in ferromagnetic semiconductor heterostructures with an Mn δ layer

The coherent spin dynamics of carriers in the heterostructures that contain an InGaAs/GaAs quantum well (QW) and an Mn δ layer, which are separated by a narrow GaAs spacer 2–10 nm thick, is comprehensively studied by the magnetooptical Kerr effect method at a picosecond time resolution. The exchange interaction of photoexcited electrons in QW with the ferromagnetic Mn δ layer manifests itself in magnetic-field and temperature dependences of the Larmor precession frequency of electron spins and is found to be very weak (several microelectron volts). Two nonoscillating components related to holes exist apart from an electron contribution to the Kerr signal of polarization plane rotation. At the initial stage, a fast relaxation process, which corresponds to the spin relaxation of free photoexcited holes, is detected in the structures with a wide spacer. The second component is caused by the further spin dephasing of energyrelaxed holes, which are localized at strong QW potential fluctuations in the structures under study. The decay of all contributions to the Kerr signal in time increases substantially when the spacer thickness decreases, which correlates with the enhancement of nonradiative recombination in QW.

Charged carrier spin dynamics in ZnO quantum wells and epilayers

Charged carrier spin dynamics in ZnO quantum wells and epilayers

Revealing giant internal magnetic fields due to spin fluctuations in magnetically doped colloidal nanocrystals.

Strong quantum confinement in semiconductors can compress the wavefunctions of band electrons and holes to nanometre-scale volumes, significantly enhancing interactions between themselves and individual dopants. In magnetically doped semiconductors, where paramagnetic dopants (such as Mn(2+), Co(2+) and so on) couple to band carriers via strong sp-d spin exchange, giant magneto-optical effects can therefore be realized in confined geometries using few or even single impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit. In nanoscale volumes, the statistical fluctuations of N spins are expected to generate giant effective magnetic fields Beff, which should dramatically impact carrier spin dynamics, even in the absence of any applied field. Here we directly and unambiguously reveal the large Beff that exist in Mn(2+)-doped CdSe colloidal nanocrystals using ultrafast optical spectroscopy. At zero applied magnetic field, extremely rapid (300-600 GHz) spin precession of photoinjected electrons is observed, indicating Beff ∼ 15 -30 T for electrons. Precession frequencies exceed 2 THz in applied magnetic fields. These signals arise from electron precession about the random fields due to statistically incomplete cancellation of the embedded Mn(2+) moments, thereby revealing the initial coherent dynamics of magnetic polaron formation, and highlighting the importance of magnetization fluctuations on carrier spin dynamics in nanomaterials.

Insensitivity of spin dynamics to the orbital angular momentum transferred from twisted light to extended semiconductors

We study the spin dynamics of carriers due to the Rashba interaction in semiconductor quantum disks and wells after excitation with light with orbital angular momentum. We find that although twisted light transfers orbital angular momentum to the excited carriers and the Rashba interaction conserves their total angular momentum, the resulting electronic spin dynamics is essentially the same for excitation with light with orbital angular momentum $l=+|l|$ and $l=-|l|$. The differences between cases with different values of $|l|$ are due to the excitation of states with slightly different energies and not to the different angular momenta per se, and vanish for samples with large radii where a $k$-space quasi-continuum limit can be established. These findings apply not only to the Rashba interaction but also to all other envelope-function approximation spin-orbit Hamiltonians like the Dresselhaus coupling.

Anomalous organic magnetoresistance from competing carrier-spin-dependent interactions with localized electronic and nuclear spins

We describe a new regime for low-field magnetoresistance in organic semiconductors, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere. The regime is studied by the controlled addition of localized electronic spins to a material that exhibits substantial room-temperature magnetoresistance ($\sim 20$\%). Although initially the magnetoresistance is suppressed by the doping, at intermediate doping there is a regime where the magnetoresistance is insensitive to the doping level. For much greater doping concentrations the magnetoresistance is fully suppressed. The behavior is described within a theoretical model describing the effect of carrier spin dynamics on the current.