Space-and time-resolved measurements of spin drift and diffusion are performed on a GaAs-hosted two-dimensional electron gas. For spins where forward drift is compensated by backward diffusion, we find a precession frequency in absence of an external magnetic field. The frequency depends linearly on the drift velocity and is explained by the cubic Dresselhaus spin-orbit interaction, for which drift leads to a spin precession angle twice that of spins that diffuse the same distance.Drift and diffusion of charge carriers in semiconductor nanostructures are the foundation of information technology. The spin of the electron is being investigated as an additional or complementary degree of freedom that can enhance the functionality of electronic devices and circuits [1][2][3]. In the presence of spin-orbit interaction (SOI), the spins of moving electrons precess about effective magnetic fields that depend on the electron momentum vector, k [4]. In a two-dimensional electron gas (2DEG), this precession has been proposed as a gatetunable switching mechanism [5,6]. Spin diffusion and spin drift have been studied using optical [7][8][9][10][11] and electrical techniques [12,13]. A local spin polarization expands diffusively into a spin mode with a spatial polarization pattern that is characteristic of the strength and symmetry of the SOI [14]. An additional drift induced by an electric field does not modify the spatial precession period in the case of linear SOI [15][16][17][18]. This is because spins that travel a certain distance and direction precess on average by the same angle, irrespective of how the travel is distributed between diffusion and drift. Therefore, no spin precession occurs for quasi-stationary electrons, i.e. for electrons where drift is compensated by diffusion.In this letter, we experimentally observe such unexpected drift-induced spin precession of stationary electron spins in the absence of an external magnetic field. Using an optical pump-probe technique, we investigate the spatiotemporal dynamics of locally excited spin polarization in an n-doped GaAs quantum well. Spin polarization probed at a fixed position is found to precess with a finite frequency, ω. This is identified as a consequence of cubic SOI, which affects spin drift and spin diffusion differently. A simple model predicts that drifting spins precess twice as much as spins that diffuse the same distance. This difference leads to a dependence ω ∝ β 3 v dr , where β 3 is the cubic SOI coefficient and v dr the drift velocity. We demonstrate quantitative agreement between model and experiment, and extract a β 3 in agreement with literature values. Monte-Carlo simulations confirm the validity of the model and pinpoint deviations that occur when the drift-induced SOI field is small compared arXiv:1602.05095v3 [cond-mat.mes-hall]

Using the recently reported mode locking effect [1] we demonstrate a highly robust control of electron spin coherence in an ensemble of (In,Ga)As quantum dots during the single spin coherence time. The spin precession in a transverse magnetic field can be fully controlled up to 25 K by the parameters of the exciting pulsed laser protocol such as the pulse train sequence, leading to adjustable quantum beat bursts in Faraday rotation. Flipping of the electron spin precession phase was demonstrated by inverting the polarization within a pulse doublet sequence. PACS numbers: 72.25.Dc, 72.25.Rb, 78.47.+p, 78.55.Cr The spin of an electron in a quantum dot (QD) is an attractive quantum bit candidate [2,3,4,5] due to its favorable coherence properties [1,6,7,8]. As the interaction strength is rather small for direct spin manipulation, the idea to swap spin into charge has been furbished [6,9,10]. For example, the electron may be converted into a charged exciton by optical injection of an electronhole pair [10], depending on the residual electron's spin orientation, leading to distinctive polarization selection rules.The fundamental quantity regarding spin coherence is the transverse relaxation time T 2 . In a QD ensemble, this time is masked by dephasing, mostly caused by dotto-dot variations of the spin dynamics. The dephasing time does not exceed 10 ns, much shorter than T 2 . This leads to the general believe that manipulations ought to -1 0 1 2 3 4 5 -1 0 1 2 3 4 5 T D = 1.86 ns 3.26 ns 3.66 ns 3.76 ns 3.86 ns T D = 4.26 ns FR amplitude (arb. units) B = 6 T, T = 6 K 4.92 ns 5.22 ns 5.42 ns 5.62 ns 5.92 ns Time (ns) FIG. 1: Faraday rotation traces measured as function of delay between probe and first pump pulse at time zero. A second pump pulse was applied, delayed relative to the first one by TD , indicated at each trace. The top left trace gives the FR without second pump.be performed on a single spin. Measurement of a single electron spin polarization, however, also results in dephasing due to temporal sampling of varying nuclear spin configurations [11,12], as statistically significant measurements on a single QD may require multiple repetition of the experiment. The dephasing can be overcome by spin-echo techniques, which give a single electron spin coherence time on the scale of micro-seconds [8]. This long coherence time derived by spin-echo is result of a refocusing of the electron spin and possibly the nuclear spin configuration [11], and it is viewed as an upper bound on the free-induction decay of spin coherence [11,13].Recently, however, we have shown that mode locking of electron spin coherence allows one to overcome the ensemble dephasing [14] and to measure the single electron spin relaxation time T 2 without applying spin-echo refocusing [1]. For monitoring the coherence, pump-probe Faraday rotation (FR) measurements [15] on a QD ensemble were used: after optical alignment of the spins normal to an external magnetic field the electron spins precess about this field. Due to precession frequency varia...

The temperature dependence of electron-spin coherence in singly negatively charged ͑In,Ga͒As/GaAs quantum dots is studied by time-resolved Faraday rotation. The decoherence time T 2 is constant on the microsecond scale for temperatures below 15 K; for higher temperatures it shows a surprisingly sharp drop into the nanosecond range. The decrease cannot be explained through inelastic scattering with phonons, and it may be related to elastic scattering due to phonon-mediated fluctuations of the hyperfine interaction.Solid-state systems are interesting for implementation of quantum information processing because they may provide controllable qubits sufficiently protected from environmentinduced classicality. 1,2 Specifically, in semiconductor quantum dots ͑QDs͒, a qubit can be defined by the two-level system of a confined electron spin, 3 which currently attracts great attention because of its long relaxation times. The spin relaxation can be characterized by two time scales, the longitudinal relaxation time T 1 , limited by inelastic scattering, and the transverse relaxation time T 2 ͑also called decoherence time͒, for which limitations may arise also from elastic scattering. The relation between these times is nontrivial and is often summarized by the simple relationFor the T 1 time of a QD electron spin, a number of investigations exist, from both experiment and theory. Compared to higher-dimensional systems, the T 1 times are very much enhanced because the QD confinement protects the spin from the main inelastic-scattering mechanism: the electron-spin coupling with its orbital motion. In high magnetic fields, T 1 has been shown to persist over tens of milliseconds or even longer at cryogenic temperatures, 4,5 in accord with theoretical calculations. 6 Further, its dependence on external parameters such as temperature and magnetic field for neutral and charged quantum dots has been studied. [7][8][9][10] On the other hand, the information about the T 2 time is still limited. Considering that inelastic scattering would be the only channel for decoherence, T 2 may be as large as 2T 1 . However, studies at cryogenic temperatures show T 2 times in the microsecond range, showing that the elastic relaxation channel due to hyperfine interaction plays the dominant role under these conditions. 11,12 Recently, several calculations for T 2 times have been reported. [13][14][15][16][17][18][19][20][21] An important figure of merit of electron-spin qubits is stability under temperature changes. A temperature increase enhances the lattice phonon occupation, so that decoherence mechanisms involving phonons gain importance. Here we study the QD electron-spin coherence as a function of temperature. We show that coherence can be initiated by short laser pulses for temperatures up to ϳ100 K. The coherence time, however, is temperature independent only up to 15 K; above it shows a sharp drop. From model calculations we conclude that this sharp drop is not related to spin-orbit coupling but arises from hyperfine interaction fluctuatio...

The experimental demonstration of the spin Hall effect in a high mobility two-dimensional electron system is reported. The spatial dependence is studied by Kerr rotation as a function of the external magnetic field using an applied electric field amplitude and direction as control parameters. We observe that the effect is robust in a bilayer structure with a nonzero Rashba coefficient displayed by an electrically controllable internal magnetic field, a large spin Hall conductivity in the range of the universal intrinsic value, and a mobility-enhanced spin diffusion constant. With the application of an unidirectional electric field, the role of the spin drift was also studied. The data was analyzed following both phenomenological and microscopic approaches and compared with experimental references in a single-layer configuration.

Understanding the electronic structure of semiconductor nanostructures is not complete without a detailed description of their corresponding spin-related properties. Here we explore the response of the shell structure of InAs self-assembled quantum dots to magnetic fields oriented in several directions, allowing mapping of the g-tensor modulus for the s and p shells. We find that the g tensors for the s and p shells exhibit a very different behavior. The s state, being more localized, probes the confinement potential details by sweeping the magnetic-field orientation from the growth direction towards the in-plane direction. For the p state, the g-tensor modulus is closer to that of the surrounding GaAs, consistent with a larger delocalization. In addition to the assessment of the g tensor, these results reveal further details of the confining potentials of self-assembled quantum dots that have not yet been probed.