Direct detection of electron spins and doping effects in spin-polarized electron transport in gallium arsenide

The spin Hall effect allows the direct detection of spins in semiconductors. In this study, electron spins in n-doped gallium arsenide are detected and the effects of n-type doping in gallium arsenide under drift are studied. It is found that the effects in the spin-polarized electron transport increase with increasing doping density in the moderate range, indicating that the introduction of n-type dopants increases the electron-spin lifetimes in gallium arsenide. However, the transport drifting by a high field shows no effects because of destroying the electron-spin polarization in n-doped gallium arsenide. The results are discussed in details.     

Hole spin relaxation in Ge-Si core-shell nanowire qubits

Controlling decoherence is the biggest challenge in efforts to develop quantum information hardware. Single electron spins in gallium arsenide are a leading candidate among implementations of solid-state quantum bits, but their strong coupling to nuclear spins produces high decoherence rates. Group IV semiconductors, on the other hand, have relatively low nuclear spin densities, making them an attractive platform for spin quantum bits. However, device fabrication remains a challenge, particularly with respect to the control of materials and interfaces. Here, we demonstrate state preparation, pulsed gate control and charge-sensing spin readout of hole spins confined in a Ge-Si core-shell nanowire. With fast gating, we measure T(1) spin relaxation times of up to 0.6 ms in coupled quantum dots at zero magnetic field. Relaxation time increases as the magnetic field is reduced, which is consistent with a spin-orbit mechanism that is usually masked by hyperfine contributions.     

Electron spin coherence exceeding seconds in high-purity silicon

Silicon is one of the most promising semiconductor materials for spin-based information processing devices. Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a (28)Si form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in pure (28)Si material (residual (29)Si concentration <50 ppm) with donor densities of 10(14)-10(15) cm(-3). We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T(2) up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ~200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T(2) of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers, or quantum memories for systems such as superconducting qubits.     

Spectroscopic investigations on hybrid nanocomposites: CdS : Mn nanocrystals in a conjugated polymer

We have used electron paramagnetic resonance (EPR) spectroscopy for investigating the properties of spins, such as those carried by polarons which carry both spin and charge in poly (meta/para phenylene) PMPP: CdS doped Mn based nanocomposites. To identify the nature of paramagnetic species in PMPP matrix, we have studied the effect of different physical parameters. It was found that we are in presence of trapped polarons and localized spins which concentration has been estimated. Moreover, spin–spin and spin–lattice relaxation rates have been calculated. Then, we discussed the results of optical and EPR study on the hybrid nanocomposite (CdS nanostructures, doped with manganese (II) ions, incorporated in PMPP conjugated polymer matrix). The optical spectra of these nanocomposites were compared to the existing models of energy levels in quantum dots. Moreover, by the use of electronic paramagnetic resonance, conclusions about the location and the symmetry of Mn²⁺ ions have been drawn. The nanocomposite energy gap is in the 3.2–3.3 eV range. The size of the nanoparticle is about 3.3 nm and Mn²⁺ ions are located at or near the nanoparticle surface.     

Literature review on: Quantum readout of spin resonance in a silicon transistor

Phosphorus donor spins in silicon are promising quantum bit (qubit) candidates. They have a natural confinement potential, long spin lifetimes and decades of use in the semiconductor fabrication industry. Readout of a single qubit is a necessary step to build a quantum computer, which could potentially solve particular problems exponentially faster than conventional computers. Electrically detected magnetic resonance has previously been used to measure the spin state of an ensemble of spins. In this literature review, the concept of a quantum computer is introduced before the potential of using electrically detected magnetic resonance to measure the spin state of a single donor spin qubit in a silicon transistor is discussed.     

基于扫描隧道显微镜的单原子自旋共振技术

近年来,应用扫描隧道显微镜技术已经可以测量单个原子的电子自旋共振谱线,为实现原子尺度量子磁性的探测与操控迈出了重要一步。电子自旋共振扫描隧道显微镜具有原子分辨能力和几十个纳电子伏的超高能量分辨率,可以实现微弱信号的原子尺度探测,例如可以测量固体表面相距几纳米的两个原子之间的微弱磁偶极相互作用、单个原子的电子与核自旋之间的超精细相互作用,以及人工自旋阵列的量子涨落等。借助脉冲式电子自旋共振技术,可以进一步实现固体表面单个磁性原子以及耦合原子的量子相干操控,测量其拉比振荡、拉姆齐干涉条纹和自旋回波信号等。单原子脉冲式电子自旋共振的实现为应用单原子量子探针进行量子探测奠定了重要基础。另外,对具有原子级精度的人工自旋结构的量子相干操控,为多体系统的量子模拟提供了重要的固态实验平台。     

Quantum control in spintronics

Superposition and entanglement are uniquely quantum phenomena. Superposition incorporates a phase that contains information surpassing any classical mixture. Entanglement offers correlations between measurements in quantum systems that are stronger than any that would be possible classically. These give quantum computing its spectacular potential, but the implications extend far beyond quantum information processing. Early applications may be found in entanglement-enhanced sensing and metrology. Quantum spins in condensed matter offer promising candidates for investigating and exploiting superposition and entanglement, and enormous progress is being made in quantum control of such systems. In gallium arsenide (GaAs), individual electron spins can be manipulated and measured, and singlet-triplet states can be controlled in double-dot structures. In silicon, individual electron spins can be detected by ionization of phosphorus donors, and information can be transferred from electron spins to nuclear spins to provide long memory times. Electron and nuclear spins can be manipulated in nitrogen atoms incarcerated in fullerene molecules, which in turn can be assembled in ordered arrays. Spin states of charged nitrogen vacancy centres in diamond can be manipulated and read optically. Collective spin states in a range of materials systems offer scope for holographic storage of information. Conditions are now excellent for implementing superposition and entanglement in spintronic devices, thereby opening up a new era of quantum technologies.     

Ultrafast optical and magneto-optical studies of III–V ferromagnetic semiconductors

Abstract

We use ultrafast optical techniques to investigate the dynamics of charge and spin carriers and coherent phonons as well as magnetic order in III-V ferromagnetic semiconductors. We observe a rich array of dynamical phenomena that are absent in traditional nonmagnetic semiconductors or metallic ferromagnets. Very short charge and spin lifetimes of the photoinjected carriers (～2ps) and multi-level charge decay dynamics are observed, which are attributed to a large density of mid-bandgap states introduced during low temperature molecular beam epitaxy (LT-MBE) growth and highly p-type Mn doping. During the very short free carrier lifetime, the coercivity of the system is seen to be reduced. We attribute this photo-induced ‘softening’ to the transient modification of carrier-mediated ferromagnetic exchange coupling between Mn spins. After the photogenerated free electrons are trapped by defects, periodic oscillations appear in differential reflectivity due to the coherent generation of acoustic phonon wavepackets.     

Observation of Optically Induced Spin Dephasing and Dynamics Under Combined Electric and Magnetic Fields in Semiconductor Quantum Wells

Journal of Superconductivity and Novel Magnetism - Magnetic field dependence of the electronic spin polarization (P), polarization decay and spin oscillation in gallium arsenide quantum wells is...     

Atomic-scale magnetometry of distant nuclear spin clusters via nitrogen-vacancy spin in diamond

The detection of single nuclear spins is an important goal in magnetic resonance spectroscopy. Optically detected magnetic resonance can detect single nuclear spins that are strongly coupled to an electron spin, but the detection of distant nuclear spins that are only weakly coupled to the electron spin has not been considered feasible. Here, using the nitrogen-vacancy centre in diamond as a model system, we numerically demonstrate that it is possible to detect two or more distant nuclear spins that are weakly coupled to a centre electron spin if these nuclear spins are strongly bonded to each other in a cluster. This cluster will stand out from other nuclear spins by virtue of characteristic oscillations imprinted onto the electron spin decoherence profile, which become pronounced under dynamical decoupling control. Under many-pulse dynamical decoupling, the centre electron spin coherence can be used to measure nuclear magnetic resonances of single molecules. This atomic-scale magnetometry should improve the performance of magnetic resonance spectroscopy for applications in chemical, biological, medical and materials research, and could also have applications in solid-state quantum computing.     

Optical spin generation/detection and spin transport lifetimes

We generate electron spins in semiconductors by optical pumping. The detection of them is also performed by optical technique using time-resolved pump-probe photoluminescence polarization measurements in the presence of an external magnetic field perpendicular to the generated spin. The spin polarization in dependences of the pulse length, pump-probe delay and external magnetic field is studied. From the dependence of spin-polarization on the delay of the probe, the electronic spin transport lifetimes and the spin relaxation frequencies as a function of the strength of the magnetic field are estimated. The results are discussed based on hyperfine effects for interacting electrons.     

Transient Spectroscopy of Optical Spin Injection in ZnMnSe/ZnCdSe Quantum Structures

We show, by time-resolved magneto-photoluminescence (PL) spectroscopy in combination with selective laser excitation, that optical polarization of the ZnCdSe spin detector induced by spin injection from the ZnMnSe spin injector persists over a much longer time scale than the lifetime of the ZnMnSe excitons. This finding provides compelling experimental evidence that the dominant mechanism for the observed spin injection in the ZnMnSe/ZnCdSe structures should not be due to injection of the excitonic spins of the diluted magnetic semiconductor (DMS). It is rather due to e.g. a delayed spin injection arising from tunneling of individual carriers or/and trapped spins in ZnMnSe.     

Damage in gallium arsenide crystals produced by ion implantation,abrasion and ball-milling

Gallium arsenide single crystals implanted with tellurium and cadmium ions at 50 and 150 keV at room temperature were examined using RHEED. Damage depth profiles were measured. Annealing was carried out to investigate the effect of temperature on the implantation damage. These effects which proved to be very complicated, included decomposition of the gallium arsenide, formation of beta gallium oxide and gallium telluride, and preferred orientation of the gallium arsenide. Comparisons were made with the annealing behaviour of ball-milled gallium arsenide using X-ray diffraction line broadening. The effects of various types of mechanical damage associated with specimen polishing of the gallium arsenide single crystals were also investigated.     

Energy states, transport, and magnetotransport in diluted magnetic semiconductor (Ga, Mn)As with quantum well InGaAs

We investigate energy levels, thermodynamic, transport and magnetotransport properties of holes in GaAs structure with quantum well InGaAs delta-doped by C and Mn. We present self-consistent calculations for energy levels in the quantum well for different degrees of ionization of Mn impurity. The magnetoresistance of holes in the quantum well is calculated. We explain observed negative magnetoresistance by the reduction of spin-flip scattering on magnetic ions due to aligning of spins with magnetic field.     

Quantum-spin-liquid states in the two-dimensional kagome antiferromagnets ZnxCu4-x(OD)6Cl2

A three-dimensional system of interacting spins typically develops static long-range order when it is cooled. If the spins are quantum (S=1/2), however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating-valence-bond state, in which every pair of neighbouring quantum spins forms an entangled spin singlet (valence bonds) and these singlets are quantum mechanically resonating among themselves. Here we provide an experimental indication for such quantum paramagnetic states existing in frustrated antiferromagnets, Zn(x)Cu(4-x)(OD)(6)Cl(2), where the S=1/2 magnetic Cu2+ moments form layers of a two-dimensional kagome lattice. We find that in Cu(4)(OD)(6)Cl(2), where distorted kagome planes are weakly coupled, a dispersionless excitation mode appears in the magnetic excitation spectrum below approximately 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence-bond solid, that breaks translational symmetry. Doping with non-magnetic Zn2+ ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The valence-bond-solid state is suppressed, and for ZnCu(3)(OD)(6)Cl(2), where the kagome planes are undistorted and 90% occupied by the Cu2+ ions, the low-energy spin fluctuations become featureless.     

Mesoscopic systems: classical irreversibility and quantum coherence

Mesoscopic physics is a sub-discipline of condensed-matter physics that focuses on the properties of solids in a size range intermediate between bulk matter and individual atoms. In particular, it is characteristic of a domain where a certain number of interacting objects can easily be tuned between classical and quantum regimes, thus enabling studies at the border of the two. In magnetism, such a tuning was first realized with large-spin magnetic molecules called single-molecule magnets (SMMs) with archetype Mn(12)-ac. In general, the mesoscopic scale can be relatively large (e.g. micrometre-sized superconducting circuits), but, in magnetism, it is much smaller and can reach the atomic scale with rare earth (RE) ions. In all cases, it is shown how quantum relaxation can drastically reduce classical irreversibility. Taking the example of mesoscopic spin systems, the origin of irreversibility is discussed on the basis of the Landau-Zener model. A classical counterpart of this model is described enabling, in particular, intuitive understanding of most aspects of quantum spin dynamics. The spin dynamics of mesoscopic spin systems (SMM or RE systems) becomes coherent if they are well isolated. The study of the damping of their Rabi oscillations gives access to most relevant decoherence mechanisms by different environmental baths, including the electromagnetic bath of microwave excitation. This type of decoherence, clearly seen with spin systems, is easily recovered in quantum simulations. It is also observed with other types of qubits such as a single spin in a quantum dot or a superconducting loop, despite the presence of other competitive decoherence mechanisms. As in the molecular magnet V(15), the leading decoherence terms of superconducting qubits seem to be associated with a non-Markovian channel in which short-living entanglements with distributions of two-level systems (nuclear spins, impurity spins and/or charges) leading to 1/f noise induce τ(1)-like relaxation of S(z) with dissipation to the bath of two-level systems with which they interact most. Finally, let us mention that these experiments on quantum oscillations are, most of the time, performed in the classical regime of Rabi oscillations, suggesting that decoherence mechanisms might also be treated classically.     

介电/半导体功能集成薄膜的制备和特性调控

介电/半导体功能集成薄膜,主要是指将具有电、磁、声、光、热等功能特性的介电功能材料(主要是氧化物类介电功能材料)与硅、砷化镓或氮化镓等典型半导体类功能材料,以单层薄膜或多层薄膜的形式生长(甚至外延生长)在一起而形成的人工新材料,这类新材料有可能具有多功能一体化和功能特性之间的相互调制及耦合等特点,可望在新型电子和光电子器件中获得应用.介绍了介电/半导体功能集成薄膜产生的背景;从集成铁电薄膜与器件、HK/半导体集成薄膜与器件以及极性氧化物/GaN功能集成薄膜与器件等3个方面,分别介绍了介电/半导体功能集成薄膜的应用;概括介绍了介电/半导体功能集成薄膜的制备方法及特性调控.     

Resonant transmission through a double domain wall in magnetic nanowires

As experimentally verified, a large magnetoresistance arises due to domain walls creation (or destruction) in Ni nanowires and in some nanostructures based on GaMnAs magnetic semiconductors. Hence the presence and structuring of magnetic domain walls have important potential applications in magnetoelectronics devices. Here, we uncover a way of controlling the conductance via resonant transmission through a double domain wall structure. This phenomenon is due to quantum interference of charge carrier wave functions in spin quantum wells, which leads to the formation of quantized energy states in the potential well created by a double domain wall. When the energy of a state in the spin quantum well is resonant with the Fermi energy in the wire, the spin–flip transmission through the domain walls becomes most effective. This gives rise to a resonance-type dependence of the conductance on the distance between the domain walls or on the Fermi energy.     

Fast control of nuclear spin polarization in an optically pumped single quantum dot

Highly polarized nuclear spins within a semiconductor quantum dot induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin, or up to a few hundred mT for the hole spin. Recently this has been recognized as a resource for intrinsic control of quantum-dot-based spin quantum bits. However, only static long-lived Overhauser fields could be used. Here we demonstrate fast redirection on the microsecond timescale of Overhauser fields on the order of 0.5 T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using coherent control of an ensemble of 10(5) optically polarized nuclear spins by sequences of short radiofrequency pulses. These results open the way to a new class of experiments using radiofrequency techniques to achieve highly correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame leading to sub-μK nuclear spin temperatures, rapid adiabatic passage, and spin squeezing.     

Spin Transport in (110) GaAs-Based Cavity Structures

The anisotropic spin dephasing of optically generated electrons in an undoped (110) GaAs quantum well inside a microcavity structure is investigated by means of spatially resolved photoluminescence experiments at a temperature of T=80 K. The dynamic type-II potential modulation induced by a surface acoustic wave (SAW) is used to transport electrons spatially separated from holes. Thus, the D’yakonov–Perel’ (DP) and the Bir–Aronov–Pikus spin dephasing mechanisms are suppressed, and electron spins can be transported over long distances of about 24 μm, which correspond to spin lifetimes of at least 8 ns. The spin vector can be rotated by an external in-plane magnetic field or by the effective in-plane field resulting from the structural inversion anisotropy induced by an intense SAW. This rotation generates an in-plane spin component that is subject to the DP spin dephasing mechanism. By means of Hanle effect measurements the lifetime of this in-plane spin component is found to be of 0.7 ns.