Degree Of Electron Spin Polarization Research Articles

Spin polarization of electrons by ultraintense lasers

In a strong magnetic field, ultra-relativistic electrons or positrons undergo spin flip transitions as they radiate, preferentially spin polarizing in one direction -- the Sokolov-Ternov effect. Here we show that this effect could occur very rapidly (in less than 10 fs) in high intensity ($I\gtrsim10^{23}$ W/cm$^{2}$) laser-matter interactions, resulting in a high degree of electron spin polarization (70%-90%).

Spin-polarized electrons upon nondipole photodetachment of fullerene anions

SynopsisSpin-polarization of electrons ejected from fullerene anions in the geometry where only nondipole photodetachment effects matter is studied at photon energies of only few tens of eV. The degree of electron spin-polarization is found to be on the order of 5 to 10%, at certain energies. This is remarkable, given that this happens at low photon energies.

Effect of Rashba and Dresselhaus Spin–Orbit Couplings on Electron Spin Polarization in a Hybrid Magnetic–Electric Barrier Nanostructure

A theoretical study has been carried out on the spin-dependent electron transport in a hybrid magnetic–electric barrier nanostructure with both Rashba and Dresselhaus spin–orbit couplings, which can be experimentally realized by depositing a ferromagnetic strip and a Schottky metal strip on top of a semiconductor heterostructure. The spin–orbit coupling-dependent transmission coefficient, conductance, and spin polarization are calculated by solving the Schrodinger equation exactly with the help of the transfer-matrix method. We find that both the magnitude and sign of the electron spin polarization vary strongly with the spin–orbit coupling strength. Thus, the degree of electron spin polarization can be manipulated by properly adjusting the spin–orbit coupling strength, and such a nanosystem can be employed as a controllable spin filter for spintronics applications.

Effect of Quasi-Fermi Level on the Degree of Electron Spin Polarization in GaAs**Supported by the National Natural Science Foundation of China under Grant Nos 11504194 and 11274189, the Project of Shandong-Provincial Higher Educational Science and Technology Program under Grant No J14LJ06, and the Application Foundation Research Program of Qingdao under Grant No 14-2-4-101-jch.

With spin-polarized-dependent band gap renormalization effect taken into account, the energy-dependent evolution of electron spin polarization in GaAs is calculated at room temperature and at a low temperature of 10 K. We consider the exciting light with right-handed circular polarization, and the calculation results show that the degree of electron spin polarization is dependent strongly on the quasi-Fermi levels of |1/2〉 and | − 1/2〉 spin conduction bands. At room temperature, the degree of electron spin polarization decreases sharply from 1 near the bottom of the conduction band, and then increases to a stable value above the quasi-Fermi level of the | − 1/2〉 band. The greater the quasi-Fermi level is, the higher the degree of electron spin polarization with excessive energy above the quasi-Fermi level of the | − 1/2〉 band can be achieved. At low temperature, the degree of electron spin polarization decreases from 1 sharply near the bottom of the conduction band, and then increases with the excessive energy, and in particular, up to a maximum of 1 above the quasi-Fermi level of the |1/2〉 band.

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.

Effects of spin diffusion on electron spin relaxation time measured with a time-resolved microscopic photoluminescence technique

We performed measurements at room temperature for a GaAs/AlGaAs multiple quantum well grown on GaAs(110) using a time-resolved microscopic photoluminescence (micro-PL) technique to find what effects spin diffusion had on the measured electron spin relaxation time, τs, and developed a method of estimating the spin diffusion coefficient, Ds, using the measured data and the coupled drift-diffusion equations for spin polarized electrons. The spatial nonuniformities of τs and the initial degree of electron spin polarization caused by the pump intensity distribution inside the focal spot were taken into account to explain the dependence of τs on the measured spot size, i.e., a longer τs for a smaller spot size. We estimated Ds as ∼100 cm2/s, which is similar to a value reported in the literature. We also provided a qualitative understanding on how spin diffusion lengthens τs in micro-PL measurements.

Ultrafast electron spin dynamics in bulk CdTe investigated by femtosecond pump-probe reflection spectroscopy

The transient spin polarization dynamics in bulk cadmium telluride (CdTe) at 70 K is investigated by the circularly polarized pump-probe reflection technique. A general expression is derived from the rate equations of a two-level system with small signal approximation to describe the light-helicity-dependent reflection spectrum. The initial degree of electron spin polarization in the excited state and the electron spin relaxation time in bulk CdTe at low temperature with different carrier density are analyzed according to this model. Our experimental results reveal that the Dʼyakonov–Perelʼ mechanism based on a fully microscopic kinetic spin Bloch equation (microscopic KSBE) approach dominates in the electron spin relaxation process in bulk CdTe crystal. • The transient spin polarization dynamics in bulk CdTe at 70 K is investigated. • A general expression is derived from the rate equations of a two-level system. • P s 0 and τ s in bulk CdTe are analyzed according to this model. • The DP mechanism based on KSBE dominates the electron spin relaxation process.

Search for spin-polarized photoemission from GaAs using light with orbital angular momentum

Laser light with photon energy near the band gap of GaAs and in Laguerre-Gaussian modes with different amounts of orbital angular momentum was used to produce photoemission from unstrained GaAs. The degree of electron spin polarization was measured using a micro-Mott polarimeter and found to be consistent with zero with an upper limit of \ensuremath{\sim}3$%$ for light with up to $\ifmmode\pm\else\textpm\fi{}5\ensuremath{\hbar}$ of orbital angular momentum. In contrast, the degree of spin polarization of 32.3 \ifmmode\pm\else\textpm\fi{} 1.4$%$ using circularly polarized laser light at the as the same wavelength, which is typical for bulk GaAs photocathodes.

Electron spin orientation under in-plane optical excitation in GaAs quantum wells

We study the optical orientation of electron spins in GaAs/AlGaAs quantum wells for excitation in the growth direction and for in-plane excitation. Time- and polarization-resolved photoluminescence excitation measurements show, for resonant excitation of the heavy-hole conduction band transition, a negligible degree of electron spin polarization for in-plane excitation and nearly 100% for excitation in the growth direction. For resonant excitation of the light-hole conduction band transition, the excited electron spin polarization has the same (opposite) direction for in-plane excitation (in the growth direction) as for excitation into the continuum. The experimental results are well explained by an accurate multiband theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling.

Comparison of measurement techniques for electron spin relaxation time in a GaAs/AlGaAs multiple quantum well

We compared the performance of three measurement techniques for electron spin relaxation time, i.e., polarization- and time-resolved photoluminescence (PTRPL) measurement, time-resolved reflectance (TRR) measurement, and time-resolved Kerr rotation measurement. With all three techniques similar electron spin relaxation times (0.83–1.2ns) were obtained in a (110) GaAs/AlGaAs multiple quantum well with excitation wavelengths near the heavy-hole exciton resonance at 77K. We extracted the spin-independent contribution of the Coulomb interaction which obscures TRR signals as an offset without limiting the excitation wavelength to heavy-hole exciton resonance, using the degree of electron spin polarization simultaneously obtained by the PTRPL measurement. It was found that the Coulomb interaction contribution is dominant in TRR signals over both heavy- and light-hole exciton resonances.

Cyclotron Spin-Flip Excitations in aν=1/3Quantum Hall Ferromagnet

Inelastic light scattering spectroscopy discloses a novel type of cyclotron spin-flip excitation in a quantum Hall system around the nu = 1/3 filling. The excitation energy follows qualitatively the degree of electron spin polarization, reaching a maximum value at nu = 1/3. This characterizes the new excitation as a nu = 1/3 ferromagnet eigenmode. The mode energy exceeds drastically the theoretical prediction obtained within the renowned single-mode approximation. We develop a new theoretical approach where the basis set is extended by adding a double-exciton component representing the cyclotron magnetoplasmon and spin wave coupled together. This double-mode approximation, inferred to be responsible for substantially reducing the gap between theoretical and experimental results, shows that the cyclotron spin-flip excitation is effectively a four-particle state.

ELECTRON SPIN IMAGING IN QUANTUM HALL DEVICES BY KERR ROTATION MEASUREMENT

The electron spin polarization images in a single layer GaAs / AlGaAs quantum well in the quantum Hall effect (QHE) regime have been observed by using an optical fiber-based high sensitive Kerr rotation microscope. The observed Kerr signal image at the Landau level filling factor ν = 1 reveals the small electron density fluctuation less than 1% through the degree of electron spin polarization. We have also observed the Kerr rotation images in the current flowing QHE devices. At ν = 1 QHE plateau, the winding strip of just ν = 1 incompressible state, which contributes the dissipationless electron transport, has been clearly observed in the bulk region of the devices. The highly current dependent Kerr images have valuable information for studying current density distribution and the distribution of spin polarization in the QHE regime. The experimental results and techniques of the Kerr imaging in the QHE regime have been reported in detail.

Optical orientation in quantum wells

In this paper, we review the optical selection rules in semiconductor quantum wells and discuss the degree of electron spin polarization in dependence of the excitation energy and the optical excitation direction. We describe the very fast initial spin relaxation at large excess energies in very thin quantum wells and quantify the dependence on excitation density. As examples for optical studies and the rich spin dynamics in quantum wells, we present the electron spin dynamics in (110)-oriented GaAs quantum wells unveiling the intersubband spin relaxation mechanism and experiments on the complex spin dynamics of coupled electron–hole spins.

Energy-dependent evolution of electron spin polarization in bulk intrinsic GaAs

Time-resolved circularly polarized pump-probe spectroscopy is used to study the electron spin coherence dynamics in intrinsic GaAs at 9.6K. It is found that the oscillation amplitude of absorption quantum beats reflecting electron spin coherence varies nonmonotonically with photon energy increaing. A circularly dichromatic pump-probe model is developed with both spin-polarized-dependent band filling and band-gap renormalization effects taken into account. The model shows that the oscillation amplitude of quantum beats is dependent on the initial degree of electron spin polarization, spin-detectable sensitivity and band-filling factor whose product results in the non-monotonic variation of the quantum-beat amplitude and agrees very well with our experimental results. The degree of electron-spin polarization involved in energy-split two-level system is defined for the first time. It is found that a degree of electron spin polarization of up to 100% can be photocreated at higher excess-energy levels.

Electron-spin polarization in anti-parallel double [formula omitted]-magnetic-barrier nanostructures

We present a theoretical study on spin-dependent transport of electrons in any anti-parallel double δ -magnetic-barrier nanostructure, which can be experimentally realized by depositing a ferromagnetic stripe on the surface of a semiconductor heterostructure. A general fomula of tranmission probability for electrons tunneling through this kind of nanostructures, is obtained. It is shown that large spin-polarized current can be achieved in such a device. It also is shown that the degree of electron-spin polarization is strongly dependent upon magnetic-strength difference of two δ -barriers. These interesting properties may provide an alternative scheme to spin-polarize electrons into semiconductors, and this device may be used as a tunable spin-filter.

Optical orientation of electron spins in GaAs quantum wells

We present a detailed experimental and theoretical analysis of the optical orientation of electron spins in GaAs/AlAs quantum wells. Using time and polarization resolved photoluminescence excitation spectroscopy, the initial degree of electron spin polarization is measured as a function of excitation energy for a sequence of quantum wells with well widths between 63 Ang and 198 Ang. The experimental results are compared with an accurate theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling. We find in wide quantum wells that the measured initial degree of polarization of the luminescence follows closely the spin polarization of the optically excited electrons calculated as a function of energy. This implies that the orientation of the electron spins is essentially preserved when the electrons relax from the optically excited high-energy states to quasi-thermal equilibrium of their momenta. Due to initial spin relaxation, the measured polarization in narrow quantum wells is reduced by a constant factor that does not depend on the excitation energy.

Spin polarization of two-dimensional electron gas in a hybrid magnetic–electric barrier

We report on a theoretical study of spin-dependent electron transport in a two-dimensional electron gas (2DEG) modulated by a stripe of ferromagnetic metal under an applied voltage. A general formula of transmission probability for electronic tunneling through this system is obtained. Based on this formula, it is shown that large spin-polarized current can be achieved in such a device. It is also shown that the degree of electron-spin polarization is strongly dependent upon the applied voltage to the stripe in the device. These interesting properties may provide an alternative scheme to spin-polarize electrons into semiconductors, and this device may be used as a voltage-tunable spin-filter.

Electron-spin polarization in both magnetically and electrically modulated nanostructures

We investigate theoretically the spin-dependent transport properties of electrons in realistic magnetic-electric-barrier (MEB) nanostructures produced by the deposition, onto a heterostructure, of a metallic ferromagnetic stripe. We find the degree of electron-spin polarization to be closely tied to the voltage applied to the stripe, despite the fact that this voltage in itself induces no spin-polarization effect. As a positive (negative) voltage is applied, the electron-spin polarization shifts in the low- (high-) energy direction and increases (decreases). Our results imply that the degree of electron-spin polarization can be tuned through the applied voltage. This implication might prove useful in the design and application of spintronic devices based on magnetic-barrier nanostructures. PACS Nos.: 73.40.Gk, 73.23.-b, 75.70.Cn

ELECTRON-SPIN POLARIZATION IN BOTH MAGNETICALLY AND ELECTRICALLY-MODULATED NANOSTRUCTURES

We theoretically investigate the spin-dependent transport properties of electrons in realistic magnetic-electric barrier nanostructures, which are produced by the deposition, on top of a heterostructure, of a metallic ferromagnetic stripe with an applied voltage. The degree of the electron-spin polarization is found to be closely associated with this voltage, although the use of applied voltage itself induces no spin polarization effect. As a positive voltage is applied to the stripe the electron-spin polarization shifts towards the low-energy region and increases; it shifts towards the high-energy direction and reduces for a negative applied voltage. These results shown in this work imply that the degree of electron-spin polarization can be tuned by means of an applied voltage on the stripes of system, which may result in a practical voltage-controlled spin filter.

Electron-spin polarization of photoions produced through photoionization from the laser-excited triplet state of Sr.

We report the detailed experimental study on the production of electron-spin-polarized Sr+ ions through one-photon resonant two-photon ionization via laser-excited 5s5p 3P1 (MJ = +1) of Sr atoms produced by laser-ablation. We have experimentally confirmed that the use of laser-ablation for the production of Sr atoms prior to photoionization does not affect the electron-spin polarization. We have found that the degree of electron-spin polarization is 64+/-9%, which is in good agreement with our recent theoretical prediction. As we discuss in detail, we infer, from a simple analysis, that photoelectrons, being the counterpart of electron-spin-polarized Sr+ ions, have approximately the same degree of electron-spin polarization. Our experimental results demonstrate that the combined use of laser-ablation technique and pulsed lasers for photoionization would be a compact and effective way to realize a pulsed source for spin-polarized ions and electrons for the studies of various spin-dependent dynamics in chemical physics.