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Electron spin dynamics of dilute nitride GaNAs quantum well (QW)-InAs quantum dots (QDs) tunnel-coupled structures having different QW thicknesses were studied via circularly polarized time-resolved photoluminescence. The rate equation fitting considering a capture of QD electron spins by the GaNAs localized states via tunnel transfer revealed that the spin amplification dynamics of the QDs depended on the QW thickness. For the QW thickness of 5 nm, although the temporal amplification of QD electron spin polarization was slow owing to the weak wavefunction coupling between the QW and QD, the long duration of high electron spin polarization was observed because of the suppressed capture of QD majority spins relative to the efficient removal of QD minority spins. When the QW thickness increased from 5 to 20 nm, the strong spin filtering in GaNAs and strong wavefunction coupling led to the fast amplification of QD electron spin polarization with high initial spin polarization. However, the spin polarization rapidly decays after the amplification owing to the removal of both QD majority and minority spins, originating from many effective localized states. These results indicate that the time-dependent QD electron spin polarization and the resultant spin amplification can be widely tuned by changing the thickness of the tunnel-coupled GaNAs QW. [ABSTRACT FROM AUTHOR]

The antiparallel current‐induced spin polarization (CISP) in an inversion‐symmetric double‐quantum‐well structure with the antiparallel Rashba effective magnetic field is calculated by solving the Boltzmann equation which is derived for the distribution operator in layer‐pseudospin space in the relaxation‐time approximation. The antiparallel CISP in an analytical form, obtained for constant momentum relaxation time in the limit of large Fermi energy, exhibits factor‐two increase with increasing the interlayer‐tunneling strength. This increase originates from the correction (to the semiclassical CISP) created by the subband mixing due to the momentum dependence of the Rashba effective magnetic field. The present interlayer‐coupling dependence of the antiparallel CISP is successfully explained as the layer‐pseudospin Hanle effect in which the spin in the original Hanle effect is replaced by the layer pseudospin. The present finding suggests that the correction by band mixing can be significant in the transport properties of various systems with the momentum‐dependent effective magnetic field. [ABSTRACT FROM AUTHOR]

The effective mass at the Fermi level is measured in the strongly interacting two-dimensional (2D) electron system in ultra-clean SiGe/Si/SiGe quantum wells in the low-temperature limit in tilted magnetic fields. At low electron densities, the effective mass is found to be strongly enhanced and independent of the degree of spin polarization, which indicates that the mass enhancement is not related to the electrons' spins. The observed effect turns out to be universal for silicon-based 2D electron systems, regardless of random potential, and cannot be explained by existing theories. [ABSTRACT FROM AUTHOR]

The effective mass at the Fermi level is measured in the strongly interacting two-dimensional (2D) electron system in ultra-clean SiGe/Si/SiGe quantum wells in the low-temperature limit in tilted magnetic fields. At low electron densities, the effective mass is found to be strongly enhanced and independent of the degree of spin polarization, which indicates that the mass enhancement is not related to the electrons' spins. The observed effect turns out to be universal for silicon-based 2D electron systems, regardless of random potential, and cannot be explained by existing theories. [ABSTRACT FROM AUTHOR]

A correlation between the spin-polarized carrier transfer and spin relaxation processes of a two-dimensional electron gas (2DEG) in an InGaN/GaN quantum well (QW) is investigated by time-resolved Kerr rotation spectroscopy at low temperature. Upon resonant excitation with the GaN barrier band edge energy, the spin polarization of the 2DEG in the QW is acquired from the transfer of spin-polarized photoexcited carriers. Significantly, the spin relaxation time of the 2DEG is enhanced to be as long as 1 ns along with the carrier transfer. It is demonstrated that by tailoring the Rashba and Dresselhaus spin–orbit couplings to approach a spin-degenerate surface, the screening effect of the polarization field leads to a longer spin relaxation time and effective manipulation of the spin relaxation. The polarization field screening induced enhancement of the spin relaxation time is significant in the way for the development of GaN-based spintronic devices. [ABSTRACT FROM AUTHOR]

Improving the efficiency of spin generation, injection, and detection remains a key challenge for semiconductor spintronics. Electrical injection and optical orientation are two methods of creating spin polarization in semiconductors, which traditionally require specially tailored p-n junctions, tunnel or Schottky barriers. Alternatively, we introduce here a novel concept for spin-polarized electron emission/injection combining the optocoupler principle based on vacuum spin-polarized light-emitting diode (spin VLED) making it possible to measure the free electron beam polarization injected into the III-V heterostructure with quantum wells (QWs) based on the detection of polarized cathodoluminescence (CL). To study the spin-dependent emission/injection, we developed spin VLEDs, which consist of a compact proximity-focused vacuum tube with a spin-polarized electron source (p-GaAs(Cs,O) or Na2KSb) and the spin detector (III-V heterostructure), both activated to a negative electron affinity (NEA) state. The coupling between the photon helicity and the spin angular momentum of the electrons in the photoemission and injection/detection processes is realized without using either magnetic material or a magnetic field. Spin-current detection efficiency in spin VLED is found to be 27% at room temperature. The created vacuum spin LED paves the way for optical generation and spin manipulation in the developing vacuum semiconductor spintronics. [ABSTRACT FROM AUTHOR]

We address the temperature influence on the precessional motion of electron spins under a transverse magnetic field, studied in GaAs/AlGaAs triple quantum wells, using pump-probe Kerr rotation. In the presence of an applied in-plane magnetic field, the TRKR measurements show the robustness of carrier's spin polarization against temperature, which can be easily traced in an extended range up to 250 K. By tuning the pump-probe wavelength to the exciton bound to a neutral donor transition, we observed a remarkably long-lasting spin coherence (with dephasing time T 2 * > 14 ns) limited by the spin hopping process and exchange interaction between the donor sites, as well as the ensemble spread of the g-factor. The temperature dependent spin dephasing time revealed a double linear dependence due to the different relaxation mechanisms active in respective temperature ranges. We observed that the increase in sample temperature from 5 K to 250 K leads to a strong T 2 * reduction by almost 98%/97% for the excitation wavelengths of 823/821 nm. Furthermore, we noticed that the temperature increase not only causes the reduction of spin lifetime, but can also lead to the variation of the electron g-factor. Additionally, the spin dynamics were studied through the dependencies on the applied magnetic field and optical pump power. [ABSTRACT FROM AUTHOR]

The impact of inter-valley scattering in modifying the density of optically injected spin polarized electrons in AlxGa1−xAs/GaAs heterostructure is investigated. Polarization resolved photoluminescence excitation measurements are performed for studying the spectral dependence of degree of circular polarization (DCP) of photoluminescence signal emanating from an adjacent quantum well layer. The experimental DCP spectra is found to deviate from the theoretically calculated curve at high excitation energy and the deviation is seen to widen with the Al-content of AlxGa1−xAs barrier layer. In the past, researchers had attributed similar deviations to the Dyakonov–Perel (DP) spin relaxation mechanism which dominates the electrons spin dynamics at high excitation energy. However, we find that the inter-valley scattering along with the DP mechanism decide the shape of optical spin orientation spectra at higher energy. In fact, at higher Al-content, the inter-valley scattering in AlxGa1–xAs layer becomes the most dominant mechanism to govern the steady state electron spin polarization at the band edge. A numerical model is also proposed for estimating the steady state electron spin polarization at zone centre by including the inter-valley scattering, energy relaxation and DP spin relaxation mechanisms. [ABSTRACT FROM AUTHOR]

Magnetic field dependence of the electronic spin polarization (P), polarization decay and spin oscillation in gallium arsenide quantum wells is investigated by spin-resolved photoluminescence measurements. In the presence of a magnetic field in the Voigt configuration, the spin-polarized electrons, generated by a circularly polarized light, are drifted by an electric field. The results show that P decays and oscillates with time as well as with the magnetic field. However, the electric field is found to play a vital role in spin dynamics and exchange coupling between the electron and hole spins in the wells. [ABSTRACT FROM AUTHOR]

A transmission-grating-sampled circular dichroism absorption spectroscopy (TGS-CDAS) and its theoretical model are developed sensitively to measure decay dynamics of a transient spin grating (TSG). A binary transmission grating with the same period as TSG is set behind TSG. It allows only a same small part of each period in TSG measured by circular dichroism absorption effect of a probe. In this way, the zero average of spin-dependent effects measured over a whole period in TSG is avoided so that TGS-CDAS has a high sensitivity to spin evolution in TSG. Spin transport experiments are performed on GaAs/AlGaAs quantum wells. Experimental results prove the feasibility and reliability of TGS-CDAS. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the effect of Be acceptors on the electroluminescence and the spin polarization in GaAs/AlAs p-i-n resonant tunneling diodes. The quantum well emission comprise two main lines separated by ~20meV attributed to excitonic and Be-related transitions, which intensities show remarkably abrupt variations at critical voltages, particularly at the electron resonant peak where it shows a high-frequency bistability. The circular-polarization degree of the quantum-well electroluminescence also shows strong and abrupt variations at the critical bias voltages and it attains relatively large values (of ~–75% at 15 T). These effects may be explored to design novel devices for spintronic applications such as a high-frequency spin-oscillators. [ABSTRACT FROM AUTHOR]

Bilinear magnetoresistance has been studied theoretically in 2D systems with isotropic cubic form of Rashba spin–orbit interaction. We have derived the effective spin–orbital field due to current-induced spin polarization and discussed its contribution to the unidirectional system response. The analyzed model can be applied to the semiconductor quantum wells as well as 2DEG at the surfaces and interfaces of perovskite oxides. • Magnetoresistance (MR) is studied in 2DEG with cubic Rashba spin-orbit interaction. • MR contains a term, that scales linearly with in-plane magnetic field and current. • Bilinear MR results from spin-orbital field due to current-induced spin polarization. [ABSTRACT FROM AUTHOR]

The effective mass of holes (m ∗) in GaAs quantum wells is investigated as a function of spin polarization, temperature, and carrier density using the Hartree–Fock approximation. The results reveal m ∗ initially decreases at low carrier densities before increasing with density. Additionally, as the interaction parameter increases, m ∗ decreases rapidly. Especially, as the temperature rises to 200 K, m ∗ asymptotically approaches a value of 0.45 m 0 , where m 0 is the electron mass. These findings provide insights into the influence of Coulomb interaction and exchange energy on the effective mass of semiconductors across a wide range of temperatures, spin polarizations, and densities. • Spin, temperature, and carrier density dependence of hole effective mass in quantum well is analyzed. • Majority mass increases with spin polarization while minority mass decreases. • Majority mass has a clear minimum at low every temperature an at low carrier density below 2 × 1010 cm−2. • The results are consistent with experimental results. [ABSTRACT FROM AUTHOR]

Spin transport was studied in a two-dimensional electron gas hosted in a wide GaAs quantum well occupying two subbands. Using space and time Kerr rotation microscopy to image drifting spin packets under an in-plane accelerating electric field, optical injection and detection of spin polarization were achieved in a pump–probe configuration. The experimental data exhibited high spin mobility and long spin lifetimes allowing us to obtain the spin–orbit fields as a function of the spin velocities. Surprisingly, above moderate electric fields of 0.4 V/cm with velocities higher than 2 µm/ns, we observed a dependence of both bulk and structure-related spin–orbit interactions on the velocity magnitude. A remarkable feature is the increase in the cubic Dresselhaus term to approximately half of the linear coupling when the velocity is raised to 10 µm/ns. In contrast, the Rashba coupling for both subbands decreases to about half of its value in the same range. These results yield new information on the application of drift models in spin–orbit fields and about limitations for the operation of spin transistors. [ABSTRACT FROM AUTHOR]

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]

Mechanisms of the suppression of the electron-hole exchange interaction in nonradiative excitons with a large in-plane wave vector in high-quality heterostructures with quantum wells are analyzed theoretically. It is shown that the dominant suppression mechanism is exciton-exciton scattering accompanied by the mutual spin flips of like carriers (either two electrons or two holes), comprising the excitons. As a result, the electron spin polarization in nonradiative excitons may be retained for a long time. The analysis of experimental data shows that this relaxation time can exceed one nanosecond. This long-term and optically controllable spin memory in an exciton reservoir may be of interest for future information technologies. [ABSTRACT FROM AUTHOR]

In this work we focus on the characterization methods allowing fast, cheap, and easy identification of photoanode TiO2 layer as well as quantum dots on the TiO2 surface for photovoltaic application. Application of the colloidal quantum dots material for the production of quantum dots solar cell is a key issue for mass production. At the same time there is a demand for a method to control the production steps for the deposition process for both photoanode and nanomaterial. The method has to be fast and cheap as well as efficient in order to meet the requirements imposed by production processes. For this purpose we propose to use a micro Raman spectroscopy as an efficient method to verify the presence of QDs. In this work we compare the results obtained by scanning electron microscopy with the Raman spectroscopy in theme of the pros and cons of these techniques. [ABSTRACT FROM AUTHOR]

Spin transport properties from the GaAs/AlGaAs superlattice (SL) to InGaAs quantum dots (QDs) are studied by circularly polarized time-resolved photoluminescence spectroscopy of QD excited states with the selective excitation of SL miniband states. For the SL with a thinner barrier, we observe an effective carrier transport in SL owing to the stronger overlap of wavefunctions of adjacent quantum wells and a simultaneous highly efficient carrier injection into QDs. Moreover, the SL with a thinner barrier demonstrates a quantum spin transport to QDs maintaining high spin polarization during the transport process. [ABSTRACT FROM AUTHOR]

The 1-D nanoscale structures are studied with a potential relief containing magnetic barriers separated by nonmagnetic quantum wells in the absence of exchange interaction between the magnetic moments of adjacent barriers. The considered structure is the sequence of N barriers divided into two “ferromagnetic domains” of the length n and N - n -- 1, both having parallel internal fields. In contrast, the internal field of the nth barrier is not coplanar with respect to the internal fields of the left and/or right “domains.” The degree of spin polarization of an electron wave transmitted through this system is calculated with the assumption that the incident wave is unpolarized with the degree of its polarization equals zero. It is shown that the degree of polarization of the transmitted wave as a function of the impulse exhibits a sequence of sufficiently wide plateaus, which are close to unity. In particular, there are significant deviations from the known spintronics phenomena found earlier in this system such as the valve, filter, and giant magnetoresistance effects. This circumstance provides opportunities for managing the transport properties of the system by varying the noncoplanar degrees of freedom of the system. [ABSTRACT FROM AUTHOR]

We propose a coaxial cylindrical quantum well wire which is composed of layers of a dilute magnetic semiconductor (DMS) and a nonmagnetic semiconductor (NMS). Using effective mass approximation, we present a numerical calculation of the eigenenergies and eigenstates of electrons in the heterostructure. The variation of the spin‐dependent energy levels is influenced by the giant Zeeman splitting and potential energies due to a vector potential for sufficiently low and high magnetic fields, respectively. It is found that crossing and anticrossing behaviors of energy levels can be controlled by the thickness of a NMS layer. The ballistic transport of the structure is also investigated. The ranges of the Fermi energy for obtaining full spin polarization are suggested in this work. [ABSTRACT FROM AUTHOR]

Carrier recombination and electron spin relaxation dynamics in asymmetric n-doped (110) GaAs/AlGaAs quantum wells are investigated with time-resolved pump-probe spectroscopy. The experiment results reveal that the measured carrier recombination time depends strongly on the polarization of pump pulse. With the same pump photon flux densities, the recombination time of spin-polarized carriers is always longer than that of the spin-balanced carriers except at low pump photon flux densities, this anomaly originates from the polarization-sensitive nonlinear absorption effect. Differing from the traditional views, in the low carrier density regime, the D'yakonov–Perel' (DP) mechanism can be more important than the Bir–Aronov–Pikus (BAP) mechanism, since the DP mechanism takes effect, the spin relaxation time in (110) GaAs QWs is shortened obviously via asymmetric doping. [ABSTRACT FROM AUTHOR]

Electron tunneling through a single magnetic barrier in a HgTe topological insulator has been theoretically investigated. We find that the perpendicular magnetic field would not lead to spin-flip of the edge states due to the conservation of the angular moment. By tuning the magnetic field and the Fermi energy, the edge channels can be transited from switch-on states to switch-off states and the current from unpolarized states can be filtered to fully spin polarized states. These features offer us an efficient way to control charge/spin transport in a HgTe/CdTe quantum well, and pave a way to construct the nanoelectronic devices utilizing the topological edge states. [ABSTRACT FROM AUTHOR]

The generation dynamics of a resident electron spin polarization involve the formation and transformation of the associated optically excited states depending on the excitation photon energy. Initial phase shift in the precession of a resident electron spin polarization gives the important clues to reveal the interplay between the associated excited states and resident electrons. In this work, the excitation energy dependence of the initial phase shift in Kerr rotation of a resident electron spin polarization is investigated in a CdTe/Cd [ABSTRACT FROM AUTHOR]

We propose a mechanism for the generation of spin polarization in semiconductor heterostructures with a quantum well and a magnetic impurity layer spatially separated from it. The spin polarization of carriers in a quantum well originates from spin-dependent tunneling recombination at impurity states in the magnetic layer, which is accompanied by a fast linear increase in the degree of circular polarization of photoluminescence from the quantum well. Two situations are theoretically considered. In the first case, resonant tunneling to the spin-split sublevels of the impurity center occurs and spin polarization is caused by different populations of resonance levels in the quantum well for opposite spin projections. In the second, nonresonant case, the spin-split impurity level lies above the occupied states of electrons in the quantum well and plays the role of an intermediate state in the two-stage coherent spin-dependent recombination of an electron from the quantum well and a hole in the impurity layer. The developed theory allows us to explain both qualitatively and quantitatively the kinetics of photoexcited electrons in experiments with photoluminescence with time resolution in Mn-doped InGaAs heterostructures. [ABSTRACT FROM AUTHOR]

Resorting to an external gate potential, we explore the spin–orbit coupling (SOC) in n-quantum-well structures with n subbands occupied, since n-well configuration is favorable to the flexible control of n-subband occupation for electrons in experiments. The charge transfer among the multiple local wells provides more possibilities of manipulation of the Rashba SOC. For all the systems we construct, from triple- to septuple-quantum-well structures, the SOC coefficient of the lowest subband is always much larger than those of the other subbands, which leads to greater spin splitting, conducive to desirable spin polarization. Furthermore, we demonstrate selective control of SOC via adjusting the gate, i.e., separately tuning the SOC coefficients of certain subbands while leaving those of the others unaffected by the gate. However, when more local wells are incorporated, the SOC coefficients of all subbands become weak gate-dependent in the case of n-subband occupation. All these findings are attributed to the charge redistribution among the subwells and the variation gradient of the potential energy along the growth direction of the quantum wells. Our results should provide new insight for experimental manipulation of the Rashba SOC in multiple-well structures. • Selective control of Rashba SOC of different subbands is realized in a wide gate potential range , facilitating the realization of persistent spin helix in quantum well semiconductors, which is crucial for spin-coherent transport. • Large spin splitting of the lowest subband is demonstrated, constructive to desirable spin polarization and detection. • When more local wells are incorporated, in the gate range where n subbands are kept occupied , Rashba SOC of all subbands become less gate-dependent. [ABSTRACT FROM AUTHOR]

We review experimental and theoretical study of dynamic electron spin injection in semiconductor nanostructures. The review is focused on spin-dependent recombination and spin-dependent tunneling phenomena leading to a photoluminescence polarization. These phenomena lead to fundamentally different dynamic spin polarization of the electrons in contrast to the polarization in a steady state under, stationary spin injection or in the case of s(p)–d exchange interaction in typical metal/ferromagnetic or semiconductor/ferromagnetic hybrid structures. Growing attention to the mechanisms leading to dynamic spin polarization is the result of (i) the possibility to precisely control the process by means of external gating and (ii) rich and still not well understood physics of the time dependent processes in semiconductor nanostructures. Moreover, non stationary processes allow to highlight the role of Coulomb correlations in the formation of carriers spin polarization. The described physics of dynamic polarization is expected in a variety of systems including semiconductor-QW based heterostructures, nanoplatelets and novel 2D materials. In the first part of the review the spin-dependent recombination via deep paramagnetic centers is discussed, which allows to obtain an unusually high spin polarization for carriers in nonmagnetic semiconductor alloys. The second part deals with the dynamic spin polarization in hybrid heterostructures semiconductor quantum well — magnetic dopant layer. Finally, possible implementation of the dynamic spin polarization in spin lasers is addressed. We also shortly discuss such perspective area as spin injection in microwave and terahertz frequency. We conclude with some open questions and other promising implementation of discussed effect which we consider very important for further development of spintronics. • The review is focused on spin-dependent recombination and spin-dependent tunneling. • A study of dynamic electron spin injection in nanostructures is discussed. • The described physics of dynamic polarization is expected in a variety of systems. • The considered phenomena are sensitive to electron correlations. • The obtained results open the possibility to effective and fast control of PL polarization. [ABSTRACT FROM AUTHOR]

We used spatially- and time-resolved Kerr rotation microscopy to show that in lateral wire-like structures, based on a modulation-doped GaAs-AlGaAs quantum well, an optically initialized spin polarization can be deterministically transferred to specific lateral positions, employing the persistent spin helix (PSH). To this end, we show that confinement in two directions leads to a strong enhancement of the effective decay time of spin polarization, which can be exploited to transfer spin polarization over relatively large lateral distances. This is demonstrated by the investigation of L-shaped wire-like lateral structures, where the legs are positioned in directions parallel and perpendicular to the wave vector of the PSH. [ABSTRACT FROM AUTHOR]

The recombination dynamics of photoexcited holes is studied in an In 0.53 Ga 0.47 As/InP quantum well in a quantizing magnetic field. The experimental data are analyzed in a framework of the four level system formed in the valence band by Landau levels. As a result, the characteristic times including the inter-Landau level transfer time, the spin relaxation time, and the interband recombination time are obtained. The inverse population of higher energy Landau level is found due to the slow transfer between Landau levels as compared to the fast interband recombination. It is demonstrated that optical spin injection led to about 50% spin polarization in the inversely populated Landau level, while the lowest energy Landau level is unpolarized. [ABSTRACT FROM AUTHOR]

The electronic and topological properties of heterovalent and heterocrystalline a-Sn/CdTe(001) quantum wells (QWs) are studied in dependence on the thickness of α-Sn by means of ab initio calculations. We calculate the topological Z2 invariants of the respective bulk crystals, which identify α-Sn as strong three-dimensional (3D) topological insulators (TIs), whereas CdTe is a trivial insulator. We predict the existence of two-dimensional (2D) topological interface states between both materials and show that a topological phase transition from a trivial insulating phase into the quantum spin Hall (QSH) phase in the QW structures occurs at much higher thicknesses than in the HgTe case. The QSH effect is characterized by the localization, dispersion, and spin polarization of the topological interface states. We address the distinction of the 3D and 2D TI characters of the studied QW structures, which is inevitable for an understanding of the underlying quantum state of matter. The 3D TI nature is characterized by two-dimensional topological interface states, while the 2D phase exhibits one-dimensional edge states. The two different state characteristics are often intermixed in the discussion of the topology of 2D QW structures, especially, the comparison of ab initio calculations and experimental transport studies. [ABSTRACT FROM AUTHOR]

We theoretically investigate the spin-dependent thermoelectric effects in a quantum dot that is connected with a normal metal lead and a ferromagnetic metal lead, by using the nonequilibrium Green's function method. The thermoelectric figure of merit ZT can be evaluated in several different ways, for example, by imposing a condition that the spin accumulation across the system is zero ( Δ μ s = 0 ) or that the spin current is zero ( I s = 0 ). We find that ZT in the case of Δ μ s = 0 is always greater, especially when the spin polarization degree of the ferromagnetic lead is large. Meanwhile, in each case the ZT value drops when the spin polarization degree is increased. Effects of electron interaction on thermoelectric properties are also discussed briefly. [ABSTRACT FROM AUTHOR]

The electric field-induced modifications of the spatial distribution of photoelectrons, photoholes, and electronic spins in optically pumped p+ GaAs are investigated using a polarized luminescence imaging microscopy. At low pump intensity, application of an electric field reveals the tail of charge and spin density of drifting electrons. These tails disappear when the pump intensity is increased since a slight differential drift of photoelectrons and photoholes causes the buildup of a strong internal electric field. Spatial separation of photoholes and photoelectrons is very weak so that photoholes drift in the same direction as photoelectrons, thus exhibiting a negative effective mobility. In contrast, for a zero electric field, no significant ambipolar diffusive effects are found in the same sample. [ABSTRACT FROM AUTHOR]

We present a theoretically investigation about the electron transport through a single-barrier structures in HgTe/CdTe quantum wells (QWs) with inverted band structure. For HgTe/CdTe QWs structure, the transmission probabilities show great dependence on the Rashba spin–orbit interaction (RSOI), the incident angle, the incident Fermi energy and the electric modulation potential. By tuning the modulation amplitude with gate voltage or the strength of RSOI modulation, the efficient spin-polarized P Z = 1 is achieved, which is formed due to the spin-split in band spectrum induced by the RSOI modulation. This electric mechanism provides us a way to manipulate the spin polarization electrically and convenient for experimental verification. [ABSTRACT FROM AUTHOR]

We demonstrate room-temperature spin-polarized ultrafast (∼10 ps) lasing in a highly optically excited GaAs microcavity. This microcavity is embedded with InGaAs multiple quantum wells in which the spin relaxation time is less than 10 ps. The laser radiation remains highly circularly polarized even when excited by nonresonant elliptically polarized light. The lasing energy is not locked to the bare cavity resonance, and shifts ∼10 meV as a function of the photoexcited density. Such spin-polarized lasing is attributed to a spin-dependent stimulated process of correlated electron-hole pairs. These pairs are formed near the Fermi edge in a high-density electron-hole plasma coupled to the cavity light field. [ABSTRACT FROM AUTHOR]

Low temperature scanning tunneling microscopy is used to image the doped topological insulator Bi2-xFexSe3. Interstitial Fe defects allow the detection of quasiparticle interference, and the reconstruction of the empty state band structure. Quantitative comparison between measured data and density functional theory calculations reveals the unexpected coexistence of quantum well states (QWSs) with topological surface states (TSSs) on the atomically clean surface of Bi2-xFexSe3. Spectroscopic measurements quantify the breakdown of linear dispersion due to hexagonal warping. Nonetheless, both QWSs and TSSs remain spin polarized and protected from backscattering to almost 1 eV above the Dirac point, suggesting their utility for spin-based applications. [ABSTRACT FROM AUTHOR]

We report ballistic transport commensurability minima in the magnetoresistance of ν=3/2 composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to ν=3/2, which we explain quantitatively by comparing three CF density models and concluding that the ν=3/2 CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe the shape and size of the CF Fermi contour. At a fixed electron density of ≃1.8×1011 cm−2, as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field (B∥) leads to a significant distortion of the CF Fermi contour as B∥ couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the ν=1/2 CF case at comparable B∥. Moreover, the applied B∥ further spin-polarizes the ν=3/2 CFs as deduced from the positions of the commensurability minima. [ABSTRACT FROM AUTHOR]

The spin polarization under low electric fields ( =300 V/cm) at low temperatures has been studied in undoped InGaAs/AlGaAs multiple quantum well. The spin polarization was created by optical spin orientation using circularly polarized light and the inverse spin-Hall effect was employed to measure the spin polarization current. We observed an obvious spin depolarization especially at lower temperatures (80-120K). We ascribed the spin depolarization of the photoinduced electrons to the heating effect from the low electric fields (the low field regime 50-300V/cm). This spin depolarization due to the heating effect is sensitive to temperature and electric field, suggesting a wide range of potential applications and devices. [ABSTRACT FROM AUTHOR]

In this paper, we study the spin-dependent electron transmission through a quantum well in semiconductor heterostructures with dipole-type oscillation field and homogeneous oscillation field. The numerical evaluations show that Dresselhaus spin-orbit coupling eliminates the spin degeneracy and leads to the splitting of asymmetric Fano-type resonance peaks in the transmissivity. The width of the splitting peaks is found to be highly sensitive to the direction of incident electron. The dipole modulation and the homogeneous modulation is approximately equivalent under certain conditions. The location and the line shape of the Fano-type resonance can be controlled by adjusting the energy and the direction of the incident electron, the oscillation frequency, and the amplitude of the external field. These interesting features may be used to devise tunable spin filters and realize pure spin transmission currents by the more practical dipole-type oscillation in the experimental setup. [ABSTRACT FROM AUTHOR]

We determine the lowest and higher order collective modes in both spin-conserving and spin-reversed sectors by calculating the energy differences of appropriate linear combinations of different levels of the excitons of the composite fermions and the fully spin-polarized ground states at filling factors v - 1/3, 2/5, 3/7, and 4/9. Apart from providing a detailed study of previously reported modes that have also been observed in the experiments, we predict additional higher energy modes at different filling factors. The lowest and the next higher spin-conserving modes have equal number of "magnetorotons" and the number is the same as the number of filled effective Landau-like levels of composite fermions. The higher energy modes at v = 1/3 merge with the lowest mode at long wavelengths. The spin-conserving modes do not merge at other filling factors. Apart from showing a zero-energy spin-wave mode at zero momentum, thanks to Larmor's theorem, the lowest spin-reversed modes at the ferromagnetic ground states of v = 2/5, 3/7, and 4/9 display one or more "spin rotons at negative energies, signaling unstable fully polarized ground states at sufficiently small Zeeman energies. The high-energy spin-reversed modes also have spin rotons but at positive energies. The energies of these excitations depend on the finite width of the quantum well as the Coulomb interaction gets screened. We perform a finite thickness correction to the Coulomb interaction by the standard method of local density approximation and use it to calculate the critical energies such as rotons and long- and short-wavelength modes, which are detectable in inelastic light scattering experiments. [ABSTRACT FROM AUTHOR]

We study the critical spin-polarization energy (αC) above which fractional quantum Hall states in two-dimensional electron systems confined to symmetric GaAs quantum wells become fully spin polarized. We find a significant decrease of a c as we increase the well width. In systems with comparable electron layer thickness, a c for fractional states near Landau level filling v = 3/2 is about twice larger than those near v = 1/2, suggesting a broken particle-hole symmetry. Theoretical calculations, which incorporate Landau level mixing through an effective three-body interaction, and finite layer thickness capture certain qualitative features of the experimental results. [ABSTRACT FROM AUTHOR]

Based on the dual-gated silicene, we have evaluated theoretically the spin-dependent transport in lateral resonant tunneling structure. By aligning the completely valley-polarized beam with spin-resolved well state in concerned structure, large spin polarization can be expected owing to spin-dependent resonant tunneling mechanism. Under the gate electric field modulation, the forming quantum well state can be externally manipulated, triggering further the emergence of externally-controllable spin polarization. Importantly, integrating the considered structure with a proper valley-filter, which might be constructed from valley-contrasting physics as that in graphene valleytronics, completely-polarized spin beam can also be attained without the assistance of ferromagnetic component, providing thus some profitable strategies to develop nonmagnetic spintronic devices residing on silicene. [ABSTRACT FROM AUTHOR]

The spin relaxation of a two-dimensional electron system (2DES) formed in a symmetric quantum well is studied theoretically when the quantum well is parallel to the (110) plane of the zinc-blende structure, the spin polarization is perpendicular to the well, and electrons occupy only the ground subband. The spin-relaxation rate is calculated as a function of the distribution of donor impurities which are placed in the well layer. Considered processes of the spin relaxation are (1) the intrasubband process by impurity-potential-induced spin-orbit interaction (SOI), which is the Elliott-Yafet mechanism in the 2DES, and (2) virtual intersubband processes consisting of a spin flip by (2a) well-potential-induced SOI or (2b) the Dresselhaus SOI, and a scattering from an impurity. It is shown that all of the above processes disappear when all impurities are located on the center plane of the well. Even if impurities are distributed over three (110) atomic layers, the spin-relaxation rate is two orders of magnitude lower than that for the uniform distribution over the well width of 7.5 nm. In GaAs/AlGaAs type-I quantum wells, the processes (1) and (2a) interfere constructively, being dominant over (2b) for the well width of ~10 nm, while in some type-II quantum wells, they can interfere destructively. [ABSTRACT FROM AUTHOR]

We propose a silicene-based lateral resonant tunneling device by placing silicene under the modulation of top nonmagnetic/ferromagnetic/nonmagnetic sandwich nanogates. Following the electric-tunable bandgap of silicene, lateral double-barrier structure is formed by imposing the flexible electrostatic modulation on top gates. By aligning the spin and valley-resolved confined states in magnetic well, remarkable spin/valley polarization can be accessed through spinor-relying resonant tunneling mechanism. Under the electrostatic, magnetic, and size manipulation, the confined well state can be efficiently engineered, and the observed spin and valley polarization can be further flexibly tuned, offering some helpful strategies to construct spinor-electronic logic atomically. [ABSTRACT FROM AUTHOR]

The typical bulk model describing 2D topological insulators (TI) consists of two types of spin-orbit terms, the so-called Dirac term, which induces out-of-plane spin polarization, and the Rashba term, which induces in-plane spin polarization. We show that for some parameters of the Fermi energy, the beam splitter device built on 2D TIs can achieve higher in-plane spin polarization than the one built on materials described by the Rashba model itself. Further, due to high tunability of the electron density and the asymmetry of the quantum well, spin polarization in different directions can be obtained. While in the normal (topologically trivial) regime the in-plane spin polarization would dominate, in the inverted regime, the out-of-plane polarization is more significant not only in the band gap but also for small Fermi energies above the gap. Further, we suggest a double beam splitter scheme to measure in-plane spin current ail-electrically. Although we consider here as an example HgTe/CdTe quantum wells, this scheme could be also promising for InAs/GaSb QWs where the in- and out-of-plane polarization could be achieved in a single device. [ABSTRACT FROM AUTHOR]

We investigate the electron-spin injection dynamics via tunneling from an In0.1Ga0.9As quantum well (QW) to In0.5Ga0.5As quantum dots (QDs) in coupled QW-QDs nanostructures. These coupled nanostructures demonstrate ultrafast (5 to 20 ps) spin injection into the QDs. The degree of spin polarization up to 45% is obtained in the QDs after the injection, essentially depending on the injection time. The spin injection and conservation are enhanced with thinner barriers due to the stronger electronic coupling between the QW and QDs. [ABSTRACT FROM AUTHOR]

In this work, we have investigated the spin polarization from two-dimensional hole gases (2DHG) formed in p–i–p GaAs/AlAs resonant tunnelling diodes (RTDs) under magnetic field parallel to the tunnel current. We have observed that the polarization degree from the quantum well (QW) and the 2DHG formed at the accumulation layer is highly voltage and light sensitive and exhibits a clear sign inversion. Our results indicate that the voltage dependence of the QW polarization degree is mainly due to an efficient hole-resonant tunnelling process through spin states of the QW. On the other hand, the voltage dependence of the 2DHG polarization degree seems to be dependent on the hole density which is controlled by the applied voltage across the RTDs. [ABSTRACT FROM AUTHOR]

Spin-split two-dimensional electronic states have been observed on ultrathin Sn(001) films grown on InSb(001) substrates. Angle-resolved photoelectron spectroscopy (ARPES) performed on these films revealed Dirac-cone-like linear dispersion around the ̄Γ point of the surface Brillouin zone, suggesting nearly massless electrons belonging to 2D surface states. The states disperse across a band gap between bulklike quantum well states in the films. Moreover, both circular dichroism of ARPES and spin-resolved ARPES studies show helical spin polarization of the Dirac-cone-like surface states, suggesting a topologically protected character as in a bulk topological insulator (TI). These results indicate that a quasi-3D TI phase can be realized in ultrathin films of zero-gap semiconductors. [ABSTRACT FROM AUTHOR]

We demonstrate that spin polarization and valley polarization have quantitatively similar effects on the resistivity of a two-dimensional electron gas in a silicon-on-insulator quantum well. In so doing, we also examine the dependence on disorder, leading to a coarse but global phenomenology of how the resistivity depends on its key parameters: spin and valley polarization, density, disorder, and temperature. [ABSTRACT FROM AUTHOR]

Four-quantum-well structure consisting of a ZnCdMnSe diluted magnetic semiconductor (DMS) well and three ZnCdSe non-magnetic semiconductor (NMS) wells separated by 15 nm-thick ZnSe barrier layers were prepared by using ALE and MBE technique. The sample was excited by CW and pulse lasers with photon energy larger than the barrier layer band gap, and circularly polarized PL spectra due to recombination of excitons localized in each well were measured as a function of external magnetic field in the Faraday geometry up to 8 T at 4 K. The PL from the NMS well next to the DMS well and from the NMS well next-but-one to the DMS well showed positive circular polarization due to LO phonon assisted transfer of positively spin polarized excitons from the DMS well to the NMS wells. The two NMS wells showed different magnetic field dependence of the degree of circular polarization of PL, which suggested that the exciton transfer from the DMS well to the NMS well next-but-one was governed by direct interaction between the two wells, not by cascade transfer process. The degree of circular polarization of PL from the NMS wells increased from zero to 20∼30% after pulse excitation. Raise time of the polarization degree was discussed on the basis of the difference of distance between the NMS well and the DMS well. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]