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This review article summarizes recent developments related to the dynamical formation of persistent spin helices in GaAs- and CdTe-based heterostructures. We start with fundamental aspects of spin-orbit interaction in quantum wells, in particular the Dresselhaus and Rashba terms and their relation to the bulk and structural inversion asymmetries, respectively. In the regime of balanced interactions, their combined impact gives rise to the formation of the persistent spin helix, i.e., a regime where a unidirectional spin grating with enhanced coherence time is established. The experimental scheme relies on ultrafast Kerr microscopy and permits to excite the spin polarization and detect it with a simultaneous spatial and temporal resolution of micrometers and picoseconds, respectively. For a microscopic understanding and a description of the results, kinetic theory of spatio-temporal spin dynamics of two-dimensional electrons is presented. In addition, Monte Carlo simulations of the spin distribution function are performed. Based on these concepts we discuss three areas with recent advances in the field of spin helices. (i) Anisotropic spin transport and spin helix dynamics in a modulation-doped GaAs quantum well is analyzed. It is observed that application of an out-of-plane electric field changes spin-orbit interaction through the Rashba component and the cubic Dresselhaus term. Remarkably, a weak in-plane electric field substantially increases spin diffusion and also affects the spin helix wavelength. (ii) In-plane magnetic fields applied in two perpendicular orientations allow for the extraction of the individual spin–orbit coupling parameters. (iii) Finally, we explore the influence of optical doping on the spin helix in a CdTe quantum well. Most importantly, a non-uniform spatio-temporal precession pattern is observed. The kinetic theory of spin diffusion allows us to model this finding by incorporating a dependence on the photo-carrier density into the Rashba and the Dresselhaus parameters. [ABSTRACT FROM AUTHOR]

Interdot carrier and spin dynamics were studied in a two-dimensional array of high-density small quantum dots (SQDs) of InGaAs with an average diameter of 16 nm and a sheet density of 1.2 , in which 24 nm diametric large QDs (LQDs) were embedded as local potential minima. We observed a delayed photoluminescence (PL) rise from the lower-lying LQD states and a considerably faster PL decay from the higher-lying SQD states, indicating carrier transfer from the two-dimensionally coupled SQDs into the LQDs. In addition, inverse carrier tunneling from the LQDs into the SQDs was thermally induced, which is characterized by the thermal activation energy between the LQDs and SQDs. Moreover, circularly polarized transient PL behavior from the SQD states exhibits a suppression of the spin polarization decay in the initial time region, depending on the excited spin density. This tentatively suppressed spin relaxation can be quantitatively explained by selective interdot transfer of minority-spin electrons from the SQDs into LQDs, when the majority spin states in both QDs are sufficiently populated by excited spins. These findings indicate that the high-density SQDs behave as the main emitters with suppressed spin relaxation, while the scattered LQDs with lower potential behave as the receivers of minority-spin electrons. [ABSTRACT FROM AUTHOR]

Electronic and transport properties of stacked metal/semiconductor and metal/metal heterojunctions of transition metal dichalcogenides (TMDCs) are studied in this paper. MoS2, MoSe2, WS2, and WSe2 are considered for the semiconducting material, while NbS2, NbSe2, CoS2, and CoSe2 are considered for the metallic contact. Our density functional theory-based calculations reveal a strongly spin-polarized conductance for the studied systems. Furthermore, our results predict that the band gap of the semiconducting TMDC shifts above the Fermi energy of the connected system, leading to a diode-like semiconductor/metal contact. Moreover, the quantum conductance calculations for these junctions, carried out within the Landauer–Büttiker formalism for ballistic transport, shows that even the metal/metal heterojunctions have a diode-like character near the Fermi level. This interesting behaviour is discussed in terms of the misalignment between electronic states on each of the materials comprising the junction. [ABSTRACT FROM AUTHOR]

Closed-form expressions are derived for the relationship between the polarisation of the output and that of the pump for spin-polarised vertical-cavity surface-emitting lasers. These expressions are based on the spin-flip model (SFM) combined with the condition that the carrier recombination time is much greater than both the spin relaxation time and the photon lifetime. Allowance is also included for misalignment between the principal axes of birefringence and dichroism. These expressions yield results that are in excellent agreement both with previously published numerical calculations and with further tests for a wide range of parameters. Trends with key parameters of the SFM are easily deduced from these expressions. [ABSTRACT FROM AUTHOR]

Ferromagnetic Heusler compounds have vast and imminent applications for novel devices, smart materials thanks to density functional theory (DFT) based simulations, which have scored out a new approach to study these materials. We forecast the structural stability of Co2TaZ alloys on the basis of total energy calculations and mechanical stability criteria. The elastic constants, robust spin-polarized ferromagnetism and electron densities in these half-metallic alloys are also discussed. The observed structural aspects calculated to predict the stability and equilibrium lattice parameters agree well with the experimental results. The elastic parameters like elastic constants, bulk, Young’s and shear moduli, poison’s and Pugh ratios, melting temperatures, etc have been put together to establish their mechanical properties. The elaborated electronic band structures along with indirect band gaps and spin polarization favour the application of these materials in spintronics and memory device technology. [ABSTRACT FROM AUTHOR]

The Monte Carlo simulation method is applied to study the relaxation of excited electrons in monolayer graphene. The presence of spin polarized background electrons population, with density corresponding to highly degenerate conditions is assumed. Formulas of electron-electron scattering rates, which properly account for electrons presence in two energetically degenerate, inequivalent valleys in this material are presented. The electron relaxation process can be divided into two phases: thermalization and cooling, which can be clearly distinguished when examining the standard deviation of electron energy distribution. The influence of the exchange effect in interactions between electrons with parallel spins is shown to be important only in transient conditions, especially during the thermalization phase. [ABSTRACT FROM AUTHOR]

We performed first-principles calculations to investigate the formation energy, geometry structure, and electronic property of Fe-doped Bi2Ti2O7 systems with different Fe doping content. The calculated formation energies indicate that the substitutional configurations of Fe-doping Bi2Ti2O7 are easy to obtain under O-rich growth condition, but their thermodynamic stability decreases with the increase of Fe content. The calculated spin-resolved density of states and band structures indicate that the introduction of Fe into Bi2Ti2O7 brings high spin polarization. The spin-down impurity levels in FexBi2−xTi2O7 and spin-up impurity levels in FexBi2Ti2−xO7 systems locate in the bottom of conduction band and narrow the band gap significantly, thus leading to the absorption of visible light. Interestingly, the impurity states in FexBi2−xTi2O7 are the efficient separation center of photogenerated electron and hole, and less affected by Fe doping content, in comparison, the levels of impurity band in FexBi2Ti2−xO7 systems are largely effected by the Fe doping content, and high Fe doping content is the key factor to improve the separating rate of photogenerated electron and hole. [ABSTRACT FROM AUTHOR]

InAs/GaSb coupled quantum well heterostructures are important semiconductor systems with applications ranging from spintronics to photonics. Most recently, InAs/GaSb heterostructures have been identified as candidate two-dimensional topological insulators, predicted to exhibit helical edge conduction via fully spin-polarised carriers. We study an InAs/GaSb double quantum well heterostructure with an AlSb barrier to decouple partially the 2D electrons and holes, and find conduction consistent with a 2D hole gas, with an effective mass of 0.235 ± 0.005 m0, existing simultaneously with hybridised carriers with an effective mass of 0.070 ± 0.005 m0, where m0 is the bare electron mass. [ABSTRACT FROM AUTHOR]

We model the electron and hole spin dynamics in an undoped double quantum dot structure, considering the carrier tunneling between quantum dots. Taking the presence of an additional in-plane or tilted magnetic field into account, we enable the simulation of magneto-optical experiments, like the time-resolved Kerr rotation measurement, which are currently performed on such structures to probe the temporal spin dynamics. With our model, we reproduce the experimentally observed effect of the extension of the spin polarization lifetime caused by spatial charge separation, which may occur in structures of this type. Moreover, we provide a number of qualitative predictions concerning the necessary conditions for observation of this effect as well as about possible channels of its suppression, including the spin–orbit coupling, which leads to tunneling of carriers accompanied by a spin flip. We also consider the impact of the magnetic field tilting, which results in an interesting spin polarization dynamics. [ABSTRACT FROM AUTHOR]

A proposal of an electrically controlled spin transistor in a helical magnetic field is presented. In the proposed device, the transistor action is driven by the Landau–Zener transitions that lead to a backscattering of spin polarized electrons and switching the transistor into the high-resistance state (off state). The on/off state of the transistor can be controlled by the all-electric means using Rashba spin–orbit coupling that can be tuned by the voltages applied to the side electrodes. [ABSTRACT FROM AUTHOR]