Optical tailoring of electron spin coherence in quantum dots

We address the precession of an ensemble of electron spins, each confined in a (In, Ga)As/GaAs self-assembled quantum dot. The quantum dot inhomogeneity is directly reflected in the precession of the optically oriented electron spins about an external magnetic field, which is subject to fast dephasing on a nanoseconds time scale. Proper periodic laser excitation allows synchronization of the electron spin precessions with the excitation cycle. The experimental conditions can be tailored such that eventually all (about a million) electron spins that are excited by the laser precess with a single frequency. In this regime the ensemble can be exploited during the single electron spin coherence times being in the microseconds range.     

Dynamics of spins in semiconductor quantum wells under drift

The dynamics of spins in semiconductor quantum wells under applied electric bias has been investigated by photoluminescence (PL) spectroscopy. The bias-dependent polarization of PL (P_PL) was measured at different temperatures. The P_PL was found to decay with an enhancement of increasing the strength of the negative bias, with an exception occurred for a low value of the negative bias. The P_PL was also found to depend on the temperature. The P_PL in the presence of a transverse magnetic field was also studied. The results showed that P_PL in the magnetic field oscillates under an applied bias, demonstrating that the dephasing of electron spin occurs during the drift transport in semiconductor quantum wells.     

Zero-field and Larmor spinor precessions in a neutron polarimeter experiment

We present a neutron polarimetric experiment where two kinds of spinor precessions are observed: one is induced by different total energy of neutrons (zero-field precession) and the other is induced by a stationary guide field (Larmor precession). A characteristic of the former is the dependence of the energy-difference, which is in practice tuned by the frequency of the interacting oscillating magnetic field ωR. In contrast the latter completely depends on the strength of the guide field, namely Larmor frequency ωL. Our neutron-polarimetric experiment exhibits individual tuning as well as specific properties of each spinor precession, which assures the use of both spin precessions for multi-entangled spinor manipulation.     

Effect of electric field and magnetic field on spin transport in bilayer graphene armchair nanoribbons: A Monte Carlo simulation study

In this article we study the effect of external magnetic field and electric field on spin transport in bilayer armchair graphene nanoribbons (GNR) by employing semiclassical Monte Carlo approach. We include D'yakonov-Perel' (DP) relaxation due to structural inversion asymmetry (Rashba spin-orbit coupling) and Elliott-Yafet (EY) relaxation to model spin dephasing. In the model we neglect the effect of local magnetic moments due to adatoms and vacancies. We have considered injection polarization along z-direction perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. To the best of our knowledge there has been no theoretical investigation of the effects of external magnetic field on spin transport in graphene nanoribbons. This theoretical investigation is important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene GNRs.     

Ultrafast optical study of magnons in the ferromagnetic semiconductor GaMnAs

Coherent oscillations of the magnetization were observed in magneto-optical Kerr measurements in thin films of the ferromagnetic semiconductor GaMnAs. For magnetic fields oriented in the film plane, two precession modes were observed. Their frequencies increase with the field when it is along the [100] axis, whereas they behave non-monotonically when the field is oriented along the in-plane hard axis [110]. Spectra are also presented for fields applied normal to the film plane. From the measured field-dependence of the magnon frequency, the spin stiffness and magnetic anisotropy constants were obtained.     

Spin echo of a single electron spin in a quantum dot

We report a measurement of the spin-echo decay of a single electron spin confined in a semiconductor quantum dot. When we tip the spin in the transverse plane via a magnetic field burst, it dephases in 37 ns due to the Larmor precession around a random effective field from the nuclear spins in the host material. We reverse this dephasing to a large extent via a spin-echo pulse, and find a spin-echo decay time of about 0.5 micros at 70 mT. These results are in the range of theoretical predictions of the electron spin coherence time governed by the electron-nuclear dynamics.     

Depolarization of the photoluminescence and spin relaxation in n-doped GaAs

Optically oriented electron spin lifetime in n-doped gallium arsenide was measured via depolarization of the photoluminescence (PL) in a transverse magnetic field (Hanle effect). In order to measure the PL polarization, a time-resolved pump-probe experiment, where a pump pulse generates spin-polarized electrons and a probe pulse monitors their polarization, was employed. The PL polarization in dependences of the pump-probe delay, external magnetic field as well as of the sample temperature was studied. The PL polarization was found to decay exponentially with the pump-probe delay, from which the spin lifetime of the electrons was measured. The measured value was found to depend on the strength of the magnetic field and sample temperature.     

Determining the sign of g factor via time-resolved Kerr rotation spectroscopy with a rotatable magnetic field

Time-resolved Kerr rotation spectroscopy is used to determine the sign of the g factor of carriers in a semiconductor material,with the help of a rotatable magnetic field in the plane of the sample.The spin precession signal of carriers at a fixed time delay is measured as a function of the orientation of the magnetic field with a fixed strength B.The signal has a sine-like form and its phase determines the sign of the g factor of carriers.As a natural extension of previous methods to measure the (time-resolved) photoluminescence or time-resolved Kerr rotation signal as a function of the magnetic field strength with a fixed orientation,such a method gives the correct sign of the g factor of electrons in GaAs.Furthermore,the sign of carriers in a (Ga,Mn)As magnetic semiconductor is also found to be negative.     

外磁场对(Ga，Mn)As有效g因子的影响

利用时间分辨Kerr旋光技术测量低温下稀磁半导体Ga_0.937Mn_0.063As中光注入极化载流子的自旋进动信号,并观察到自旋极化载流子的有效g因子值随外磁场的增强而增大的反常现象.这归结于磁场导致局域化空穴转化为非局域化空穴,从而使自发磁化强度增强,有效g因子值增大.基于此物理图像,进一步给出了(Ga,Mn)As的有效g因子与外磁场的关系式. 关键词： 时间分辨Kerr旋光测量 Zeeman效应 Ruderman-Kittel-Kasuya-Yosida模型     

Optical control of coherent interactions between electron spins in InGaAs quantum dots

Coherent interactions between spins in quantum dots are a key requirement for quantum gates. We have performed pump-probe experiments in which pulsed lasers emitting at different photon energies manipulate two distinct subsets of electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin dynamics are monitored through their precession about an external magnetic field. These measurements demonstrate spin precession phase shifts and modulations of the magnitude of one subset of oriented spins after optical orientation of the second subset. The observations are consistent with results from a model using a Heisenberg-like interaction with μeV strength.     

Generation of an external magnetic field with the spin orientation effect in a single layer Ising nanographene

In this work, the magnetic properties of the single layer Ising nanogaphene (SLING) are investigated by using Kaneyoshi approach (KA) within the effective field theory for different spin orientations of its magnetic atoms. We find that the magnetizations of the SLING has no phase transition, certain Curie temperature and distinct peak of susceptibility at T_c for the some spin orientations at the zero external magnetic field (H=0.0). Because these behaviors occur at H≠0.0, we suggest that the SLING generates an external magnetic field and behaves as an external magnetic field generator for these spin orientations. However, the SLING exhibits ferromagnetic behaviors for only one spin orientations. But, it exhibits antiferromagnetic behaviors for the others. For the AFM cases, diamagnetic susceptibility behaviors and type II superconductivity hysteresis behaviors are obtained. We hope that these results can open a door to obtain new class of single layer graphene and graphene-based magnetic field generator devices with the spin orientation effect.     

Gate control of quantum dot-based electron spin–orbit qubits

We investigate theoretically the coherent spin dynamics of gate control of quantum dot-based electron spin–orbit qubits subjected to a tilted magnetic field under electric-dipole spin resonance (EDSR). Our results reveal that Rabi oscillation of qubit states can be manipulated electrically based on rapid gate control of SOC strength. The Rabi frequency is strongly dependent on the gate-induced electric field, the strength and orientation of the applied magnetic field. There are two major EDSR mechanisms. One arises from electric field-induced spin–orbit hybridization, and the other arises from magnetic field-induced energy-level crossing. The SOC introduced by the gate-induced electric field allows AC electric fields to drive coherent Rabi oscillations between spin-up and -down states. After the crossing of the energy-levels with the magnetic field, the spin-transfer crossing results in Rabi oscillation irrespective of whether or not the external electric field is present. The spin–orbit qubit is transferred into the orbit qubit. Rabi oscillation is anisotropic and periodic with respect to the tilted and in-plane orientation of the magnetic field originating from the interplay of the SOC, orbital, and Zeeman effects. The strong electrically-controlled SOC strength suggests the possibility for scalable applications of gate-controllable spin–orbit qubits.     

Electron spin decoherence in quantum dots due to interaction with nuclei

We study the decoherence of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei. The decay is caused by the spatial variation of the electron wave function within the dot, leading to a nonuniform hyperfine coupling A. We evaluate the spin correlation function and find that the decay is not exponential but rather power (inverse logarithm) lawlike. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay time is given by (planck)N/A, where N is the number of nuclei inside the dot, and the amplitude of precession decays to a finite value. We show that there is a striking difference between the decoherence time for a single dot and the dephasing time for an ensemble of dots.     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.     

铁磁/重金属双层薄膜结构中磁性状态的稳定性分析

本文在理论上研究了铁磁/重金属双层薄膜结构中自旋霍尔效应自旋矩驱动的磁动力学. 通过线性稳定性分析, 获得了以电流和磁场为控制参数的磁性状态相图. 发现通过调节电流密度和外磁场, 可以获得不同的磁性状态, 例如: 平面内的进动态、平面内的稳定态以及双稳态. 当外磁场的方向在一定的范围时, 通过调节电流密度可以实现磁矩的翻转和进动. 同时, 通过数值求解微分方程, 给出了这些磁性状态磁矩的演化轨迹.     

Motional-narrowing-type dephasing of electron and hole spins of itinerant excitons in magnetically doped II-VI bulk semiconductors

Time-resolved optical spin-quantum-beat measurements performed on magnetically doped II-VI bulk semiconductors reveal an increase of the electron spin dephasing time with rising temperature typical for motional narrowing. With the dephasing being notably faster than in undoped II-VI semiconductors, the magnetic dopants must play a key role, modifying the known dephasing mechanisms and introducing new ones. Focusing on the latter, we theoretically explore the spin dephasing channel arising from magnetization fluctuations sampled by the itinerant excitons. This mechanism suffices to explain quantitatively the results of our time-resolved Faraday-rotation experiments on optically excited Cd(1-x)Mn(x)Te which we present here as a function of magnetic field, temperature and manganese dopant density. In addition to electron spin-quantum beats, some of our experiments reveal hole spin beats as well.     

Optical studies of spin precession in magnetic two-dimensional electron gases

Magnetic two-dimensional electron gases are studied using time-resolved Kerr and Faraday rotation spectroscopy in the Voigt geometry. The data directly reveal both electron and Mn spin precession in modest transverse fields. Scattering by Mn ions dominates the electron spin relaxation processes in these materials, and prevents the electron gas from acquiring a long-lived spin polarization as observed in non-magnetic structures. Nonetheless, a persistent Mn spin polarization occurs which creates a oscillating magnetic field within the electron gas for hundreds of picoseconds.     

High frequency tunable continuous wave ESR spectroscopy

We describe the ESR spectrometer we developed. Our aim was twofold: i) to reach the highest possible frequency and ii) to devise a frequency tunable spectrometer. The tunable source is an optically pumped far infrared laser which has been used from 160 GHz up to 525 GHz with magnetic fields of up to 19 T. We present measurements performed in semiconductor physics and in molecular chemical physics. These measurements allowed us to distinguish electric dipolar transitions from magnetic dipolar transitions. The increase ing-factor resolution was used to discriminate between entities withg-factors differing by a few 10^?5. This property together with the study of the line-width frequency dependence was used in geophysics. We studied the spin relaxation mechanisms of the model system Phosphorus doped Silicon. The variation of the spin relaxation time with temperature shows the importance of two-phonon mechanisms. High frequency tunable ESR makes possible the study of compounds with large zero field splitting which are ESR silent at standard frequencies.