Spin Dynamics In Semiconductors Research Articles

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.

Erratum: Origins of overshoots in the exciton spin dynamics in semiconductors [Phys. Rev. B 99, 165308 (2019)

Erratum: Origins of overshoots in the exciton spin dynamics in semiconductors [Phys. Rev. B 99, 165308 (2019)

Origins of overshoots in the exciton spin dynamics in semiconductors

We investigate the origin of overshoots in the exciton spin dynamics after resonant optical excitation. As a material system, we focus on diluted magnetic semiconductor quantum wells as they provide a strong spin-flip scattering for the carriers. Our study shows that overshoots can appear as a consequence of radiative decay even on the single-particle level in a theory without any memory. The magnitude of the overshoots in this case depends strongly on the temperature as well as the doping fraction of the material. If many-body effects beyond the single-particle level become important so that a quantum-kinetic description is required, a spin overshoot appears already without radiative decay and is much more robust against variations of system parameters. We show that the origin of the spin overshoot can be determined either via its temperature dependence or via its behavior for different doping fractions. The results can be expected to apply to a wide range of semiconductors as long as radiative decay and a spin-flip mechanism are present.

Effect of doping symmetry on electron spin relaxation dynamics in (110) GaAs/AlGaAs quantum wells

Considerable interest has been aroused in the study of the spin dynamics in semiconductors due to its potential applications in spintronics and quantum computation. In this paper, time-resolved circularly polarized pump-probe spectroscopy is used to study the carrier density dependences on the electron spin relaxation in approximately symmetrical and completely asymmetrical doping (110) GaAs/AlGaAs quantum wells. With the increase of the carrier density, the spin relaxation time first increases and then decrease obviously in both of the quantum wells, and the measured spin relaxation time of the approximately symmetrical doping quantum wells is always longer than that of the asymmetrical doping one. By analysis, we find that the spin relaxation is not dominated only by the Bir-Aronov-Pikus (BAP) mechanism in (110) GaAs quantum wells, that though the Dresselhaus spin-orbit coupling does not lead to any spin relaxation, the asymmetry of the doping position contributes to the asymmetry of potential energy structure, thus the built-in electric field which can induce the Rashba spin-orbit coupling to appear, and that the effective magnetic field induced by the Rashba spin-orbit coupling normal to the growth direction can lead to spin relaxation along the growth direction. Therefore, the Dyakonov-Perel (DP) mechanism plays an important role in asymmetrical doping (110) GaAs/AlGaAs quantum wells. In the approximately symmetrical and completely asymmetrical doping (110) GaAs/AlGaAs quantum wells, the DP mechanism dominates the spin relaxation at low carrier density, thus the spin relaxation time increases with carrier density increasing due to the strengthening of the electron-electron scattering and the decreasing of the momentum relaxation time. However, at high carrier density, BAP mechanism plays an important role, thus the spin relaxation time decreases obviously with carrier density increrasing, but the decay rates in both of the quantum wells are slower than that in the casethat only BAP mechanism dominates, because both the DP and BAP mechanism play an important role. The strength of the Rashba spin-orbit coupling depends on the symmetry of the quantum well. The DP mechanism in a completely asymmetrical doping quantum well is stronger than that in an approximately symmetrical doping quantum wells, thus the decay rate in a completely asymmetrical doping quantum wells is always slower than that in an approximately symmetrical doping quantum wells, and the spin relaxation time in a completely asymmetrical doping quantum wells is shorter than that in an approximately symmetrical doping quantum wells.

Transient analysis of oblique Hanle signals observed in GaAs

A transient response of nuclear spins in GaAs to a change in a magnetic field was analyzed based on the time evolution of nuclear spin temperature. Simulation results well reproduced our experimental results on transient oblique Hanle signals observed in an all-electrical spin injection device, enabling a quantitative understanding of nuclear spin dynamics in the presence of a hyperfine interaction between nuclei and polarized electrons. Analysis regarding the time evolution of nuclear spin temperature revealed that the hysteretic nature of a nuclear field with respect to the sweep direction of an external field was caused by the delay of time response of nuclear spin temperature to a change in the magnetic field. This analysis provides a deep understanding of nuclear spin dynamics in semiconductors.

Analysis of the transient response of nuclear spins in GaAs with/without nuclear magnetic resonance

As an alternative to studying the steady-state responses of nuclear spins in solid state systems, working within a transient-state framework can reveal interesting phenomena. The response of nuclear spins in GaAs to a changing magnetic field was analyzed based on the time evolution of nuclear spin temperature. Simulation results well reproduced our experimental results for the transient oblique Hanle signals observed in an all-electrical spin injection device. The analysis showed that the so called dynamic nuclear polarization can be treated as a cooling tool for the nuclear spins: It works as a provider to exchange spin angular momentum between polarized electron spins and nuclear spins through the hyperfine interaction, leading to an increase in the nuclear polarization. In addition, a time-delay of the nuclear spin temperature with a fast sweep of the external magnetic field produces a possible transient state for the nuclear spin polarization. On the other hand, the nuclear magnetic resonance acts as a heating tool for a nuclear spin system. This causes the nuclear spin temperature to jump to infinity: i.e., the average nuclear spins along with the nuclear field vanish at resonant fields of 75As, 69Ga and 71Ga, showing an interesting step-dip structure in the oblique Hanle signals. These analyses provide a quantitative understanding of nuclear spin dynamics in semiconductors for application in future computation processing.

Phase effects due to previous pulses in time-resolved Faraday rotation measurements

Time-resolved Faraday rotation measurements have proved transformative in the investigation of spin dynamics in semiconductors. In materials with spin lifetimes which are on the order of, or greater than, the laser repetition time, the collective effect of spin polarization due to the whole pump pulse train becomes important. Here, we discuss a relative phase shift which results from these spins. We derive and experimentally validate a closed-form expression which describes this phase shift and characterize it throughout parameter space. A spin lifetime measurement based on this phase shift is described, and we discuss situations in which the model used must be augmented to be applicable.

Ultrafast supercontinuum fiber-laser based pump-probe scanning magneto-optical Kerr effect microscope for the investigation of electron spin dynamics in semiconductors at cryogenic temperatures with picosecond time and micrometer spatial resolution

We describe a two-color pump-probe scanning magneto-optical Kerr effect microscope which we have developed to investigate electron spin phenomena in semiconductors at cryogenic temperatures with picosecond time and micrometer spatial resolution. The key innovation of our microscope is the usage of an ultrafast "white light" supercontinuum fiber-laser source which provides access to the whole visible and near-infrared spectral range. Our Kerr microscope allows for the independent selection of the excitation and detection energy while avoiding the necessity to synchronize the pulse trains of two separate picosecond laser systems. The ability to independently tune the pump and probe wavelength enables the investigation of the influence of excitation energy on the optically induced electron spin dynamics in semiconductors. We demonstrate picosecond real-space imaging of the diffusive expansion of optically excited electron spin packets in a (110) GaAs quantum well sample to illustrate the capabilities of the instrument.

Frequency-domain optical probing of coherent spins in nanocrystal quantum dots

Spin-photon interactions such as the Faraday effect provide techniques for measuring coherent spin dynamics in semiconductors. In contrast to typical ultrafast pulsed laser techniques, which measure spin dynamics in the time domain with an intense, spectrally broad probe pulse, we demonstrate a frequency-domain spin-photon resonance effect using modulated continuous-wave lasers which enables measurement of GHz-scale coherent spin dynamics in semiconductors with minimal spectral linewidth. This technique permits high-resolution spectroscopic measurements not possible with ultrafast methods. We have employed this effect to observe coherent spin dynamics in CdSe nanocrystals using standard diode lasers. By fitting the results to the expected model, we extract electron g-factors, and spin coherence and dephasing times in agreement with time-domain measurements.

Efficient data averaging for spin noise spectroscopy in semiconductors

Spin noise spectroscopy (SNS) is the perfect tool to investigate electron spin dynamics in semiconductors at thermal equilibrium. We simulate SNS measurements which utilize real-time fast Fourier transformation instead of an ordinary spectrum analyzer and show that ultrafast digitizers with low resolution enable surprisingly sensitive, high bandwidth SNS in the presence of strong optical background shot noise. The simulations reveal that optimized input load at the digitizer is crucial for efficient spin noise detection while the resolution of the digitizer, i.e., the bit depth, influences the sensitivity rather weakly.

Note: A time-resolved Kerr rotation system with a rotatable in-plane magnetic field

A time-resolved Kerr rotation system with a rotatable in-plane magnetic field has been constructed to study anisotropic spin relaxation of electrons in semiconductors. A permanent magnet magic ring is placed on top of a motor-driven rotation stage (RS) to create the rotatable in-plane magnetic field. The RS is placed on a second translation stage to vary the local magnetic field around a sample. The in-plane magnetic field in such a system varies from 0.05 to 0.95 T, with full-round 360° rotatablity, thus offering a convenient and low-cost way to study the anisotropy of spin dynamics in semiconductors. Its performance was demonstrated via measurement of the anisotropy of the spin dephasing time (SDT) of electrons in a two-dimensional electron system embedded in a GaAs/Al(0.35)Ga(0.65)As heterostructure. The SDT with B∥[110] was observed to be 10% larger than that with B∥[110], consistent with the results of others, which was measured via rotating sample.

Gigahertz spin noise spectroscopy inn-doped bulk GaAs

We advance spin noise spectroscopy to an ultrafast tool to resolve high-frequency spin dynamics in semiconductors. The optical nondemolition experiment reveals the genuine origin of the inhomogeneous spin dephasing in $n$-doped GaAs wafers at densities at the metal-to-insulator transition. The measurements prove in conjunction with depth-resolved spin noise measurements that the broadening of the spin dephasing rate does not result from thermal fluctuations or spin-phonon interaction, as suggested previously, but from spatial $g$-factor variations.

Spin Noise Spectroscopy in GaAs

We observe the noise spectrum of electron spins in bulk GaAs by Faraday-rotation noise spectroscopy. The experimental technique enables the undisturbed measurement of the electron-spin dynamics in semiconductors. We measure exemplarily the electron-spin relaxation time and the electron Landé g factor in -doped GaAs at low temperatures and find good agreement of the measured noise spectrum with a theory based on Poisson distribution probability.

Ultrafast geometric manipulation of electron spin and detection of the geometric phase via Faraday rotation spectroscopy

Time-resolved Faraday rotation spectroscopy is currently exploited as a powerful technique to probe spin dynamics in semiconductors. We propose here an all-optical approach to geometrically manipulate electron spin and to detect the geometric phase by this type of extremely sensitive experiment. The global nature of the geometric phase can make the quantum manipulation more stable, which may find interesting application in quantum devices.

Spin coherence in semiconductors: storage, transport and reduced dimensionality

We review the use of femtosecond-resolved optical techniques for the study of electron spin dynamics in semiconductors and show how these methods are able to address many of the issues that are centrally important to spin polarized semiconductor-based electronics. Pump–probe studies can be used to optimize spintronic material parameters, and have led to the observation of 100 ns spin lifetimes in n-doped bulk GaAs and spin transport across macroscopic length scales (100 μm) . We have found that spin coherence is preserved across a semiconductor heterojunction over a broad range of temperatures, and can be protected by transport to regions of intrinsically lower decoherence. Studies of CdSe quantum dots ranging from 22 to 80 Å in diameter indicate that spin coherence in these structures is thermally robust, with nanosecond-scale spin lifetimes persisting to room temperature.