Spin Filter Effect in a Parallel Double Quantum Dot

We present a theoretical study of the spectral and the spin-dependent transport properties of a few electron semiconductor parallel double quantum dot (DQD) in the presence of local induced Zeeman splittings at the quantum dots. Working in an extended Hubbard model and treating the coupled QD as a single coherent system, the linear response spin-dependent conductance is calculated at low temperatures. We analyze the conditions such that the device would operate as a bipolar spin filter by only varying the incident electron Fermi energy from non-magnetic leads.     

Spin-Dependent Electron Interferometers

We have studied theoretically the interplay between quantum interference and spin-orbit coupling in electron interferometers realized in semiconductor nanostructures. The correlation between spin quantization axis and propagation direction for electrons with fixed energy results in the appearance of spin-dependent interference fringes. We apply the spin-resolved scattering formalism to calculate transport in electronic versions of Youngs double-slit interferometer and the Mach-Zehnder interferometer. The possibility to tune spin-orbit coupling of the Rashba type using external gate voltages makes it possible to use such interferometer setups for magnet-less spintronics and quantum information processing.     

The interfacial roughness effect on spin-dependent transport in nonplanar junctions with double magnetic barriers

In this paper, we use the transfer matrix method to study the effect of roughness with nonplanar interfaces on spin-dependent transport properties in a magnetic tunnel junction, which consists of two ferromagnetic semiconductor barriers separated by a nonmagnetic (NM) layer. This trilayer is sandwiched between two NM electrodes. The roughness is modeled as a periodic corrugation interface. Based on the effective mass approximation, the spin-dependent transmission probability, and also the dependence of tunnel magnetoresistance (TMR) and electron spin polarization (SP) on the center nonmagnetic layer quantum well width are studied, theoretically, at several different temperatures. The numerical results show that the SP and TMR have an oscillatory behavior as a function of NM layer thickness and the interface roughness/islands degrade the transmission probability. Our results may be useful for the development of nanoelectronic devices.     

DNA Molecule as a Spintronic Device

In this work, we study the spin-dependent electron transport of a segment of DNA chain. We model the system by using the Fishbone model for DNA, which is characterized by a tight binding Hamiltonian. We predict that the spin-dependent transport can be observed in short DNA molecules coupled to metal contacts by applying an external magnetic field. A detailed analysis of spin-dependent current and spin polarization as a function of the bias voltages is carried out.     

Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons

We propose to use optical detection of magnetic resonance (ODMR) to measure the decoherence time T₂ of a single-electron spin in a semiconductor quantum dot. The electron is in one of the spin 1/2 states and a circularly polarized laser can only create an optical excitation for one of the electron spin states due to Pauli blocking. An applied electron spin resonance (ESR) field leads to Rabi spin flips and thus to a modulation of the photoluminescence or, alternatively, of the photocurrent. This allows one to measure the ESR linewidth and the coherent Rabi oscillations, from which the electron spin decoherence can be determined. We study different possible schemes for such an ODMR setup, including cw or pulsed laser excitation. An erratum to this article is available at .     

Electron Spin Relaxations in the Electric-Field Applied GaAs/InGaAs Heterostructure

We have investigated the electron spin relaxation rates in GaAs/InGaAs heterostructure in the presence of electric field by time-resolved photoluminescence (PL) measurements at 10 K. Spin-polarized electrons were optically generated in the bulk GaAs region, drifted driven by the electric field, and captured in two InGaAs quantum wells which work as spin detectors. The comparison of the degrees of PL polarizations from two wells, by adjusting excitation energy and electric field, enables us to investigate the electron spin relaxation rates in the different parts of the sample separately. We have found that the spin relaxation during the drift transport in the bulk region is accelerated in the high electric fields and that a significant spin relaxation takes place when electrons are captured into the quantum well.     

Fabrication and Characterization of Modulation-Doped ZnSe/(Zn,Cd)Se (110) Quantum Wells: A New System for Spin Coherence Studies

We describe the growth of modulation-doped ZnSe/(Zn,Cd)Se quantum wells on (110) GaAs substrates. Unlike the well-known protocol for the epitaxy of ZnSe-based quantum structures on (001) GaAs, we find that the fabrication of quantum well structures on (110) GaAs requires significantly different growth conditions and sample architecture. We use magnetotransport measurements to confirm the formation of a two-dimensional electron gas in these samples, and then measure transverse electron spin relaxation times using time-resolved Faraday rotation. In contrast to expectations based upon known spin relaxation mechanisms, we find surprisingly little difference between the spin lifetimes in these (110)-oriented samples in comparison with (100)-oriented control samples.     

Spin-Dependent Electron Interferometers

We have studied theoretically the interplay between quantum interference and spin-orbit coupling in electron interferometers realized in semiconductor nanostructures. The correlation between spin quantization axis and propagation direction for electrons with fixed energy results in the appearance of spin-dependent interference fringes. We apply the spin-resolved scattering formalism to calculate transport in electronic versions of Youngs double-slit interferometer and the Mach-Zehnder interferometer. The possibility to tune spin-orbit coupling of the Rashba type using external gate voltages makes it possible to use such interferometer setups for magnet-less spintronics and quantum information processing.     

Spin Relaxation in a Two-Dimensional Electron Gas in Si Quantum Wells

Electron spin resonance of two-dimensional (2D) electron gas in Si/SiGe quantum wells allows to evaluate both the longitudinal and the dephasing spin relaxation time. Diakonov–Perel (DP) relaxation, caused by Bychkov–Rashba (BR) spin orbit coupling, occurs to be the dominant mechanism in high mobility samples. For low mobility the Elliott–Yaffet mechanism dominates the longitudinal spin relaxation. When the BR effect is small, inhomogeneous broadening caused by potential fluctuations is seen. We compare spin relaxation of the 2D electron gas in Si and in GaAs quantum wells with respect to applications of these materials in spintronics.     

Anisotropic Spin Splitting and Spin Relaxation in AlGaAs/GaAs Quantum Structures

Spin relaxation due to the D'yakonov–Perel' mechanism is intimately related with the spin splitting of the electronic states. We calculate the spin relaxation rates from anisotropic spin splittings of electron subbands in n-(001)-AlGaAs/GaAs quantum structures obtained in a self-consistent multiband approach. The giant anisotropy of spin relaxation rates found for different spin components in the (001) plane can be ascribed to a mutual compensation of terms because of the asymmetry of the bulk crystal and the quantum well structure.     

Enhancement of spin coherence using Q-factor engineering in semiconductor microdisc lasers

Semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications and inspiring proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities, a new and promising research field, have revealed effects such as polarization beats, stimulated spin scattering and giant Faraday rotation. Here, we study the electron spin dynamics in optically pumped GaAs microdisc lasers with quantum wells and interface-fluctuation quantum dots in the active region. In particular, we examine how the electron spin dynamics are modified by the stimulated emission in the discs, and observe an enhancement of the spin-coherence time when the optical excitation is in resonance with a high-quality (Q approximately 5,000) lasing mode. This resonant enhancement, contrary to expectations from the observed trend in the carrier-recombination time, is then manipulated by altering the cavity design and dimensions. In analogy with devices based on excitonic coherence, this ability to engineer coherent interactions between electron spins and photons may provide new pathways towards spin-dependent quantum optoelectronics.     

Influence of Quantum Interference on the Tunnel Magnetoresistance in a Molecular Junction in the Presence of In-Plane Electric Field

Quantum interference effects on the spin-dependent transport in a molecular spin-valve, as FM/Benzene/FM model junction, in which a benzene molecule attached to ferromagnetic (FM) three-dimensional leads is numerically investigated. Using a generalized Green’s function method and in the framework of Landauer–Büttiker formalism, the variation of interference conditions is determined by rotation of the benzene molecule, and replacement of the connection configurations of the molecule to the FM leads from ortho to meta and then para configuration. We have found that transport characteristics, including the spin-dependent current and tunnel magnetoresistance (TMR) are strongly influenced by the quantum interference effects. Besides, the effect of an in-plane electric field on the spin-dependent transport characteristics is treated.     

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

自旋注入对有机半导体极化子电导的影响

在有机半导体自旋电子器件中,自旋从铁磁极注入到有机半导体后,自旋相上的极化子和自旋向下的极化子有不同的态密度,从而产生不同的电导.利用自旋漂移一扩散方程通过自洽计算得到了铁磁/有机半导体自旋注入结构中极化子自旋相关的电导和电流的自旋极化率.计算结果表明,极化子电导的自旋相关性是自旋注入引起的,和电流的自旋极化率密切相关;在自旋注入发生后,有机半导体内不同位置上极化子自旋态密度不同,由此产生的极化子电导也不相同,极化予电导是位置的函数.另外还发现,外电场会增强有机半导体电流的自旋极化率.     

Spin Injection Processes in ZnSe-Based Double Quantum Dots of Diluted Magnetic Semiconductors

Spin injection processes in the double quantum dots of ZnSe-based diluted magnetic semiconductors are discussed. Double quantum dots are fabricated from ZnSe-based double quantum wells by electron beam lithography and wet etching. In these samples, the photo-excited carriers in the magnetic dots are injected into the non-magnetic dots. The circular polarization degrees of photoluminescence from the non-magnetic dots are measured by micro-photoluminescence measurement system under the magnetic field up to 5 T. The maximum spin polarization degrees of injected carriers determined from our experiment are 10% for double quantum wells and 15% for double quantum dots. The spin injection efficiency was estimated both from the observed circular polarization degree and the diffusion length of carriers. We concluded that the spin injection efficiency is increased in the double quantum dots.     

D'yakonov–Perel' Spin Relaxation Controlled by Electron–Electron Scattering

The D'yakonov–Perel' mechanism of spin relaxation is connected with the spin splitting of the electron dispersion curve in crystals lacking a center of symmetry. In a two-dimensional noncentrosymmetric system, e.g. quantum well or heterojunction, the spin splitting is a linear function of k, at least for small values of k. We demonstrate that the spin relaxation time _s due to the spin splitting is controlled not only by momentum relaxation processes as widely accepted but also by electron–electron collisions which have no effect on the electron mobility. In order to calculate the time _s taking into account the electron–electron scattering, we have solved the two-dimensional kinetic equation for the electron spin density matrix. The result has been compared with that obtained assuming the momentum scattering to occur due to elastic scattering of electrons by ionized impurities. We have also extended the quasi-elastic approximation to describe the electron–electron collision integral for a spin-polarized three-dimensional electron gas.     

Influence of Barrier Material on Spin Splitting Due to Inversion Asymmetry in Heterostructures

Influence of barrier material on the spin splitting of conduction subbands in heterostructures because of structure inversion asymmetry (Bychkov–Rashba splitting) is studied. The spin splitting at a vanishing magnetic field is calculated for two heterostructures: InAs/SiO₂ and InAs/In_0.8Al_0.2As, having the same well material InAs but very different barrier materials. It is demonstrated that the barrier material strongly influences the spin splitting of the ground conduction subband in InAs. The spin splittings for both heterostructures are computed as functions of electron density, we obtain the splitting in InAs/SiO₂ almost twice larger than that in InAs/In_0.8Al_0.2As. The influence of spin-dependent part of the boundary conditions on the spin spin splitting is studied and it is shown that for considered heterostructures it changes the splitting up to 25% of its value. It is emphasized that the Bychkov–Rashba spin splitting is not proportional to the average electric field in heterostructure.     

Charge sensing of precisely positioned p donors in Si

Real-time sensing of (spin-dependent) single-electron tunneling is fundamental to electrical readout of qubit states in spin quantum computing. Here, we demonstrate the feasibility of detecting such single-electron tunneling events using an atomically planar charge sensing layout, which can be readily integrated in scalable quantum computing architectures with phosphorus-donor-based spin qubits in silicon (Si:P). Using scanning tunneling microscopy (STM) lithography on a Si(001) surface, we patterned a single-electron transistor (SET), both tunnel and electrostatically coupled to a coplanar ultrasmall quantum dot, the latter consisting of approximately four P donors. Charge transitions of the quantum dot could be detected both in time-averaged and single-shot current response of the SET. Single electron tunneling between the quantum dot and the SET island on a time-scale (τ ～ ms) two-orders-of-magnitude faster than the spin-lattice relaxation time of a P donor in Si makes this device geometry suitable for projective readout of Si:P spin qubits. Crucial to scalability is the ability to reproducibly achieve sufficient electron tunnel rates and charge sensitivity of the SET. The inherent atomic-scale control of STM lithography bodes extremely well to precisely optimize both of these parameters.     

Recombination echoes in disordered silicon

The influence of spin-selection rules on charge-carrier recombination in disordered semiconductors such as hydrogenated microcrystalline silicon (μc-Si : H) has been used for many years as a powerful tool for defect spectroscopy. Electrically detected magnetic resonance measures small conductivity changes that are induced by electron spin resonance, and is performed under quasi-continuous excitation of the ESR microwaves. Since the rate at which the spin population in μc-Si : H is altered is small compared to the spin-decoherence rate, no information on the dynamics of spin-dependent transitions can be obtained. In this paper, we will show that with strong microwave bursts the dynamics of spin-dependent recombination at dangling bonds is accessible. We will give experimental evidence that coherent spin phenomena, like spin-beat oscillations and recombination echoes, are observable in the photocurrent of μc-Si : H. A first qualitative model of recombination through dangling bonds in μc-Si : H is given.     

Detecting Spin Heat Accumulation by Sign Reversion of Thermopower in a Quantum Dot Side-Coupled to Majorana Bound States

We propose a scheme to detect the spin heat accumulation (SHA), an effective spin-dependent electron temperature, via sign reversion of thermopower induced by the Majorana bound states (MBSs) coupled to a quantum dot (QD). The SHA is generated in either a nonmagnetic material or a ferromagnet serving as an electrode connected to the QD and leads the spin-up and spin-down thermopowers to change signs at different temperatures with the help of QD-MBSs coupling. The existence of the SHA then can be detected by the variation of the spin-polarized or even charge thermopower with respect to the magnitude of the SHA. Our numerical results show that the transition temperature of the thermopower is sensitive to QD-MBSs coupling strength, hybridization between the MBSs, and the ferromagnetism on the leads. Around the transition temperature, either 100% spin-polarized or pure spin thermopower can be generated by the combined effects of SHA and MBSs. We also find that the intradot Coulomb interaction does not change the qualitative results of the present scheme.