Time-resolved Kerr Rotation Technique Research Articles

Coherent Spin Dynamics of a Two-Dimensional Electron Gas in the Mode of a Hall Ferromagnet

The spin-depolarized electron system in a GaAs quantum well at the filling factor ν = 1 is investigated using the time-resolved Kerr rotation technique. At temperatures above 5 K, a transition from the new spin-correlated state to a state with a low spin stiffness characteristic of a single-particle electron system is observed. A nonlinear contribution to the damping of Larmor oscillations that is manifested at low temperatures, when spin-spin correlations determine the ground state of the two-dimensional electron system, is identified. The parameters of the fluctuating magnetic field acting on individual electron spins are estimated.

Особенности когерентной спиновой динамики двумерного электронного газа в режиме холловского ферромагнетика

By means of the time-resolved Kerr rotation technique, a spin-depolarized electron system at a filling factor ν = 1 in a GaAs quantum well was studied. At temperatures above 5 K, a transition from a new spin-correlated state to a state with low spin stiffness characteristic of a single-particle electron system was found. A nonlinear contribution to the decay of Larmor oscillations arising at low temperatures, when spin-spin correlations determine the ground state of a two-dimensional electron system, is highlighted. The parameters of the fluctuating magnetic field acting on individual electron spins are estimated.

Spin dephasing of electrons and holes in isotopically purified ZnSe/(Zn,Mg)Se quantum wells

The coherent spin dynamics of resident electrons and holes in an isotopically purified ZnSe/(Zn,Mg)Se single quantum well is investigated in different regimes, requiring corresponding adaption of the applied time-resolved pump-probe Kerr rotation technique. The purification of the Zn and Se atom species in the crystal to the isotopes with zero nuclear spin is expected to lead to an extension of the spin dephasing times of resident carriers, due to the suppression of their interaction with the nuclear spins. Indeed, we find no indication of carrier-nuclear interaction in this sample and link the observed carrier spin relaxation to the spin-orbit interaction. Theoretical considerations support the experimental results.

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.

Anisotropic in-plane spin dynamics in (110)-oriented GaAs/AlGaAs multiple quantum well

We studied spin dephasing processes in GaAs/AlGaAs multiple quantum wells (MQWs) grown on a semi-insulating (110)-oriented GaAs substrate using the time-resolved Kerr rotation (TRKR) technique. The TRKR spectra gives the electron g-factor in MQWs, as well as the spin dephasing time (SDT) for electron spins within the sample plane. The electron g-factor shows a strong two-fold anisotropy, while the in-plane SDT remains almost isotropic. The anisotropy of the electron g-factor increases monotonically with the spread of more electron wave functions into the AlGaAs barrier. The two-fold symmetry of the electron g-factor is discussed with a phenomenological model based on spin splitting of energy bands caused by spin-orbit coupling.

Turnover of Exciton Spin States in CdTe/Cd0.88Mn0.12Te Quantum Wells

Photoinduced exciton spin states in diluted magnetic semiconductor CdTe/Cd0.88Mn0.12Te quantum wells (QWs) were investigated by the time-resolved Kerr rotation (TRKR) technique at various temperatures and applied magnetic fields. Two oscillation components were observed in the TRKR signals. One component is ascribed to the spin precession of a localized electron, which was not affected by the Mn ions in the CdMnTe barrier layers. Its frequency increased linearly with the applied magnetic field. The other component was strongly dependent on the temperature, and its frequency showed a minimum at around 20 K in the 8.4 nm QW. This result was well explained by the turnover of exciton spin states in the CdTe QW, which is determined by the competition between the exciton Zeeman effect and an exciton–Mn exchange interaction. The temperature dependence of the exciton spin relaxation is also discussed.

Negative initial phase shift of Kerr rotation generated from the building-up process of resident electron spin polarization in a CdTe single quantum well

Initial phase shift in a precessional motion of resident electron-spin polarization is studied in a ${\mathrm{CdTe}/\mathrm{Cd}}_{0.85}{\mathrm{Mg}}_{0.15}\mathrm{Te}$ single quantum well using a time-resolved Kerr rotation technique. The generation dynamics of resident electron-spin polarization involve the formation and transformation of the associated optically excited states and are complicated particularly in the early time region. A careful analysis of the phase shift gives a deep understanding of the generation processes. In the experiments, the negative phase shift of the resident electron-spin polarization is observed, and the mechanism associated especially with a quick hole spin flip in negative trions is studied through the dependences on excitation power and magnetic field strength.

Effective Nuclear Field Measurement in a Single Quantum Well via Time-Resolved Kerr Rotation Technique

Effective magnetic field of nuclear spin polarization (NSP) under circularly polarized pumps excitation and the dephasing time of resident electron spin polarization under pump and control excitation has been detected in a single CdTe quantum well (QW) by a time-resolved Kerr rotation (TRKR) technique. We deduced that the experimental method is verified with the effect, to a certain extent, through confirming the external magnetic field results. In addition, the nuclear field is revealed to be increased with the increasing electron spin component due to the enhanced electron-nuclear hyperfine interaction. Significant nuclear field is observed as 1.85 mT with the applied magnetic field tilted by about 15 degree. What is more, we found the spin dephasing time (SDT) has little relation with the created NSP field, but it decreases largely with the increasing spin polarization magnitude under pump excitation and the spin vector dispersions induced by control pulses.

Magnetic field effect on electron spin dynamics in (110) GaAs quantum wells

We study the electron spin relaxation in both symmetric and asymmetric GaAs/AlGaAs quantum wells (QWs) grown on (110) substrates in an external magnetic field B applied along the QW normal. The spin polarization is induced by circularly polarized light and is detected using the time-resolved Kerr rotation technique. In the asymmetric structure, where a δ-doped layer on one side of the QW produces the Rashba contribution to the conduction-band spin–orbit splitting, the lifetime of electron spins aligned along the growth axis exhibits an anomalous dependence on B in the range 0 < B < 0.5 T; this results from the interplay between the Dresselhaus and Rashba effective fields which are perpendicular to each other. For larger magnetic fields, the spin lifetime increases, which is a consequence of the cyclotron motion of the electrons and is also observed in (001)-grown quantum wells. The experimental results are in agreement with the calculation of the spin lifetimes in (110)-grown asymmetric quantum wells described by the point group Cs, where the growth direction is not the principal axis of the spin-relaxation-rate tensor.

Room temperature spin transport in undoped (110) GaAs/AlGaAs quantum wells

We are reporting on our first observation of a micrometer-order electron spin transport in a (110) GaAs/AlGaAs multiple quantum well (QW) at room temperature using a space- and time-resolved Kerr rotation technique. A 37-μm transport was observed within an electron spin lifetime of 1.2 ns at room temperature when using an in-plane electric field of 1.75 kV/cm. The spatio-temporal profiles of electron spins were well reproduced by the spin drift-diffusion equations coupled with the Poisson equation, supporting the validity of the measurement. The results suggest that (110) QWs are useful as a spin transport layer for semiconductor spintronic devices operating at room temperature.

Tuning of the Landé g-factor in AlxGa1−xAs/AlAs single and double quantum wells

We report on the spin dynamics of a high mobility two-dimensional electron gas in a AlxGa1−xAs/AlAs double quantum well structure. For high electron density samples, the g-factor was measured using time-resolved Kerr rotation technique. The g-factor tuning capability was observed by changing the aluminum content x independently in each well. Experiments demonstrated an unusual spin dephasing time robustness for high excitation power. The effect of the interaction between wells was analyzed in samples with different tunneling barriers. Results were compared with experiments on single well systems demonstrating higher spin polarization generation, longer spin dephasing time, and coupling for the double structures.

A strong anisotropy of spin dephasing time of quasi-one dimensional electron gas in modulation-doped GaAs/AlGaAs wires

We investigated the wire orientation dependence of ensemble electron spin dynamics by using a time-resolved Kerr rotation technique in wires made from a modulation-doped GaAs/AlGaAs quantum well. We observed a strong anisotropy of the electron spin dephasing time of as large as 40 depending on the wire orientation at 5 K by tuning the spin-orbit interaction. The wire width dependence of the spin dynamics is reproduced in a Monte Carlo simulation. In addition, two characteristic spin decay rates were observed in wires whose widths were close to the spin precession length. The dependence of the magnetic field direction on the spin dynamics of the wires was also studied to determine the difference from that of two dimensional electron gas.

Electron spin dynamics study of bulk p-GaAs: The screening effect

In this work, the electron spin dynamics of bulk p-GaAs doped with Be grown by molecular beam epitaxy is investigated by using the time-resolved magneto-optical Kerr rotation technique. The spin relaxation/dephasing times T1 and T2* are systematically investigated as a function of hole doping density and photo-excitation density as well as temperature. The complex hole doping density dependence of spin relaxation times T1 and T2* is observed experimentally, which agrees well with predictions of the kinetic spin Bloch equation theory published previously [J. Jiang and M. Wu, Phys. Rev. B 79, 125206 (2009)]. D'yakonov-Perel's mechanism [M. Dyakonov and V. Perel, Sov. Phys. Solid State 13, 3023 (1972)] is discussed to dominate the electron spin relaxation process in p-GaAs, with the effect of hole screening proven to play an important role.

Long-lived electron spins in a modulation doped (100) GaAs quantum well

We have measured T1 spin lifetimes of a 14 nm modulation-doped (100) GaAs quantum well using a time-resolved pump-probe Kerr rotation technique. The quantum well was selected by tuning the wavelength of the probe laser. T1 lifetimes in excess of 1 μs were measured at 1.5 K and 5.5 T, exceeding the typical T2* lifetimes that have been measured in GaAs and II-VI quantum wells by orders of magnitude. We observed effects from nuclear polarization, which were largely removable by simultaneous nuclear magnetic resonance, along with two distinct lifetimes under some conditions that likely result from probing two differently localized subsets of electrons.

Spin Dynamics in (111) GaAs/AlGaAs Undoped Asymmetric Quantum Wells

The electron spin dynamics is investigated by the time-resolved Kerr rotation technique in a pair of special GaAs/AlGaAs asymmetric quantum well samples grown on (111)-oriented substrates, whose structures are the same except for their opposite directions of potential asymmetry. A large difference of spin lifetimes between the two samples is observed at low temperature. This difference is interpreted in terms of a cancellation effect between the Dresselhaus spin-splitting term in the conduction band and another term induced by interface inversion asymmetry. The deviation decreases with the increasing temperature, and almost disappears when T > 100 K because the cubic Dresselhaus term becomes more important.

Suppression of electron spin relaxation in a wide GaAs/AlGaAs quantum well with a lateral confining potential

Coherent spin dynamics of electrons laterally confined in a wide GaAs quantum well using a special mosaic electrode deposited onto the sample plane has been investigated. Electron spin lifetime measurements have been carried out in structures with different sizes of orifices in the mosaic electrode using the time-resolved Kerr rotation technique. It has been found that the electron spin lifetime in the lateral traps increases significantly with decreasing orifice size in the mosaic electrode. The observed electron spin lifetime dependence can be interpreted in terms of D'yakonov–Perel' spin relaxation mechanism suppression.

Influence of electron localization on the spin dephasing anisotropy in bias GaAs/AlGaAs coupled quantum wells

Electron spin dephasing anisotropy is studied in GaAs/AlGaAs coupled quantum wells by means of a timeresolved Kerr rotation technique. It is found that the spin dephasing rate is strongly dependent on magnetic field and is significantly anisotropic in the quantum well plane. The presented theoretical model describes the experimental results by taking into account both the electron g-factor spreading and the irreversible electron spin relaxation which are caused by the electron localization. The suggested theoretical description is in a good agreement with experimental data.

Anisotropic in-plane spin splitting in an asymmetric (001) GaAs/AlGaAs quantum well

The in-plane spin splitting of conduction-band electron has been investigated in an asymmetric (001) GaAs/AlxGa1-xAs quantum well by time-resolved Kerr rotation technique under a transverse magnetic field. The distinctive anisotropy of the spin splitting was observed while the temperature is below approximately 200 K. This anisotropy emerges from the combined effect of Dresselhaus spin-orbit coupling plus asymmetric potential gradients. We also exploit the temperature dependence of spin-splitting energy. Both the anisotropy of spin splitting and the in-plane effective g-factor decrease with increasing temperature.PACS: 78.47.jm, 71.70.Ej, 75.75.+a, 72.25.Fe,

Temperature and electron density dependence of spin relaxation in GaAs/AlGaAs quantum well

Temperature and carrier density-dependent spin dynamics for GaAs/AlGaAs quantum wells (QWs) with different structural symmetries have been studied by using time-resolved Kerr rotation technique. The spin relaxation time is measured to be much longer for the symmetrically designed GaAs QW comparing with the asymmetrical one, indicating the strong influence of Rashba spin-orbit coupling on spin relaxation. D'yakonov-Perel' mechanism has been revealed to be the dominant contribution for spin relaxation in GaAs/AlGaAs QWs. The spin relaxation time exhibits non-monotonic-dependent behavior on both temperature and photo-excited carrier density, revealing the important role of non-monotonic temperature and density dependence of electron-electron Coulomb scattering. Our experimental observations demonstrate good agreement with recently developed spin relaxation theory based on microscopic kinetic spin Bloch equation approach.

Optical detection of zero-field spin precession of high mobility two-dimensional electron gas in a gated GaAs/AlGaAs quantum well

We investigated the effective magnetic field induced by spin–orbit interaction in a gated modulation-doped GaAs/AlGaAs quantum well (QW) structure. We measured the precession of the optically injected electron spins at zero magnetic field by a time-resolved Kerr rotation (TRKR) technique as a function of the gate voltage V g. The V g-dependence of the effective magnetic field extracted from the TRKR data was quantitatively analyzed by considering both Rashba and Dresselhaus spin–orbit interaction in a Monte Carlo simulation. With the Dresselhaus spin–orbit coupling parameter γ and the scattering time as fitting parameters, we reproduced the experimental TRKR data, from which we estimated γ∼13 eV Å 3.