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Integrating conventional superconductors with common III-V semiconductors provides a versatile platform to implement tunable Josephson junctions (JJs) and their applications. We propose that with gate-controlled time-dependent spin-orbit coupling, it is possible to strongly modify the current-phase relations and Josephson energy and provide a mechanism to drive the JJ dynamics, even in the absence of any bias current. We show that the transition between stable phases is realized with a simple linear change in the strength of the spin-orbit coupling, while the transition rate can exceed the gate-induced electric field GHz changes by an order of magnitude. The resulting interplay between the constant effective magnetic field and changing spin-orbit coupling has direct implications for superconducting spintronics, controlling Majorana bound states, and emerging qubits. We argue that topological superconductivity, sought for fault-tolerant quantum computing, offers simpler applications in superconducting electronics and spintronics., Comment: 7 pages, 5 figures, published in Phys. Rev. Applied as a Letter

With non-Abelian statistics and nonlocal degrees of freedom, Majorana bound states (MBS) are suitable for implementing fault-tolerant topological quantum computing. While the main efforts to realize MBS have focused on one-dimensional systems, it requires delicate parameter tuning and its geometric constraints pose significant challenges for the demonstration of non-Abelian statistics. Building on recent experimental advances in planar Josephson junctions (JJs), we propose how to overcome this obstacle in topological JJs and demonstrate non-Abelian statistics with phase or mini-gate control, detected by charge sensing using quantum point contacts. Our proposals, supported by the experiments, would constitute an important milestone towards topological quantum computing.

Neutral and charged excitons (trions) in atomically-thin materials offer important capabilities for photonics, from ultrafast photodetectors to highly-efficient light-emitting diodes and lasers. Recent studies of van der Waals (vdW) heterostructures comprised of dissimilar monolayer materials have uncovered a wealth of optical phenomena that are predominantly governed by interlayer interactions. Here, we examine the optical properties in NbSe$_2$ - MoSe$_2$ vdW heterostructures, which provide an important model system to study metal-semiconductor interfaces, a common element in optoelectronics. Through low-temperature photoluminescence (PL) microscopy we discover a sharp emission feature, L1, that is localized at the NbSe$_2$-capped regions of MoSe$_2$. L1 is observed at energies below the commonly-studied MoSe$_2$ excitons and trions, and exhibits temperature- and power-dependent PL consistent with exciton localization in a confining potential. Remarkably, L1 is very robust not just in different samples, but also under a variety of fabrication processes. Using first-principles calculations we reveal that the confinement potential required for exciton localization naturally arises from the in-plane band bending due to the changes in the electron affinity between pristine MoSe$_2$ and NbSe$_2$ - MoSe$_2$ heterostructure. We discuss the implications of our studies for atomically-thin optoelectronics devices with atomically-sharp interfaces and tunable electronic structures., Comment: 30 pages, 5 figures, 2 tables

Pristine materials seldom appear as we want them. Instead, their appeal typically comes from suitable modifications. Proximity effects are a versatile method to transform a given material by acquiring the properties of its neighbors and becoming, superconducting, magnetic, valley-polarized, or topologically nontrivial [1-3]. This approach is particularly suitable for 2D materials in which the length of the proximity effects exceeds their thickness [1,2]. Advances in (quantum) spin and anomalous Hall effect, as well as (anomalous) valley Hall effect suggest the electronic degrees of freedom (spin, charge and valley) can be used as different information carriers [3]. The realization of multiple Hall effects in a single material provides a fascinating opportunity to manipulate the implementation of such information. Here we predict the realization and manipulation of multiple Hall effects in the proximitized 2D materials based on first-principles calculations and tight binding models. Harnessing such Hall effects associated with multiple degrees of freedom of electrons could enable novel applications in electronics, spintronics, and valleytronics. [1]. P. Lazic, K. D. Belashchenko, I. Žutic, Phys. Rev. B 2016, 93, 241401. [2]. B. Scharf, A. Matos-Abiague, J. E. Han, E. M. Hankiewicz, I. Žutic, Phys. Rev. Lett. 2016, 117, 166806. [3]. T. Zhou, J. Zhang, Y. Xue, B. Zhao, H. Zhang, H. Jiang, Z. Yang, Phys. Rev. B 2016, 94, 235449.

Valleytronic materials, characterized by local extrema (valley) in their bands, and topological insulators have separately attracted great interest recently. However, the interplay between valleytronic and topological properties in one single system, likely to enable important unexplored phenomena and applications, has been largely overlooked so far. Here, by combining a tight-binding model with first-principles calculations, we find the large-band-gap quantum spin Hall effects (QSHEs) and valley Hall effects (VHEs) appear simultaneously in the Bi monolayers decorated with halogen elements, denoted as Bi2XY (X, Y = H, F, Cl, Br, or I). A staggered exchange field is introduced into the Bi2XY monolayers by transition metal atom (Cr, Mo, or W) doping or LaFeO3 magnetic substrates, which together with the strong SOC of Bi atoms generates a time-reversal-symmetry-broken QSHE and a huge valley splitting (up to 513 meV) in the system. With gate control, QSHE and anomalous charge, spin, valley Hall effects can be observed in the single system. These predicted multiple and exotic Hall effects, associated with various degrees of freedom of electrons, could enable applications of the functionalized Bi monolayers in electronics, spintronics, and valleytronics., accepted in npj Quantum Materials

Inversion symmetry breaking combined with strong spin-orbit coupling in transition metal dichalcogenides such as MoS2 offers important opportunities for spintronics. We investigate excitons in MoS2 monolayers in an applied in-plane electric field. Tight-binding and Bethe-Salpeter equation calculations predict a large quadratic Stark shift. The scaling of the Stark shifts with the exciton binding energy and the dielectric environment provides a path to engineering the MoS2 electro-optical response. Our results suggest that the excitonic Stark effect can be observed experimentally in a MoS2 monolayer and we explain its implications for spintronic devices.

We theoretically show how the interplay between spin-orbit coupling (SOC) and magnetism can result in a finite tunneling Hall conductance, transverse to the applied bias. For two-dimensional tunnel junctions with a ferromagnetic lead and magnetization perpendicular to the current flow, the detected anomalous Hall voltage can be used to extract information not only about the spin polarization but also about the strength of the interfacial SOC. In contrast, a tunneling current across a ferromagnetic barrier on the surface of a three-dimensional topological insulator (TI) can induce a planar Hall response even when the magnetization is oriented along the current flow[1]. The tunneling nature of the states contributing to the planar Hall conductance can be switched from the ordinary to the Klein regimes by the electrostatic control of the barrier strength. This allows for an enhancement of the transverse response and a giant Hall angle, with the tunneling planar Hall conductance exceeding the longitudinal component. Despite the simplicity of a single ferromagnetic region, the TI/ferromagnet system exhibits a variety of functionalities. In addition to a spin-valve operation for magnetic sensing and storing information, positive, negative, and negative differential conductances can be tuned by properly adjusting the barrier potential and/or varying the magnetization direction. Such different resistive behaviors in the same system are attractive for potential applications in reconfigurable spintronic devices. [1] B. Scharf, A. Matos-Abiague, J. E. Han, E. M. Hankiewicz, and I. Zutic, arXiv:1601.01009 (2016).

In condensed-matter systems Majorana bound states (MBSs) are emergent quasiparticles with non-Abelian statistics and particle-antiparticle symmetry. While realizing the non-Abelian braiding statistics under exchange would provide both an ultimate proof for MBS existence and the key element for fault-tolerant topological quantum computing, even theoretical schemes imply a significant complexity to implement such braiding. Frequently examined 1D superconductor/semiconductor wires provide a prototypical example of how to produce MBSs, however braiding statistics are ill-defined in 1D and complex wire networks must be used. By placing an array of magnetic tunnel junctions (MTJs) above a 2D electron gas formed in a semiconductor quantum well grown on the surface of an s-wave superconductor, we have predicted the existence of highly tunable zero-energy MBSs and have proposed a novel scheme by which MBSs could be exchanged [1]. This scheme may then be used to demonstrate the states’ non-Abelian statistics through braiding. The underlying magnetic textures produced by MTJ array provides a pseudo-helical texture which allows for highly-controllable topological phase transitions. By defining a local condition for topological nontriviality which takes into account the local rotation of magnetic texture, effective wire geometries support MBS formation and permit their controlled movement in 2D by altering the shape and orientation of such wires. This scheme then overcomes the requirement for a network of physical wires in order to exchange MBSs, allowing easier manipulation of such states. [1] G. L. Fatin, A. Matos-Abiague, B. Scharf, and I. Zutic, arXiv:1510.08182, preprint.

Andreev reflection spectroscopy of ferromagnet/superconductor (F/S) junctions is a sensitive probe of the junction interface as well as the spin polarization. We theoretically investigate spin-polarized transport in F/S junctions in the presence of Rashba and Dresselhaus interfacial spin-orbit fields and show that Andreev reflection can be controlled by changing the magnetization orientation. This suggests a similar control of the superconducting proximity effect and Majorana states. We predict a giant in- and out-of-plane magnetoanisotropy of the junction conductance. If the ferromagnet is highly spin polarized - in the half-metal limit - the magnetoanisotropic Andreev reflection depends universally on the spin-orbit fields only. Our results show that Andreev reflection spectroscopy can be used for sensitive probing of interfacial spin-orbit fields in F/S junction. This work has been supported by DFG SFB 689, the International Doctorate Program Topological Insulators of the Elite Network of Bavaria, DOE-BES Grant DE-SC0004890, and ONR N000141310754. P. H\"{o}gl, A. Matos-Abiague, Igor \v{Z}uti\'c, J. Fabian, Phys. Rev. Lett. 115, 116601 (2015)

Introduction Historical Overview: From Electron Transport in Magnetic Materials to Spintronics Albert Fert Spin Transport and Magnetism in Magnetic Metallic Multilayers Basics of Nano-Thin Film Magnetism Bretislav Heinrich Micromagnetism as a Prototype for Complexity. Anthony S. Arrott Giant Magnetoresistance: Experiment Jack Bass Giant Magnetoresistance: Theory Evgeny Y. Tsymbal, D.G. Pettifor, and Sadamichi Maekawa Spin Injection, Accumulation, and Relaxation in Metals Mark Johnson Spin Torque Effects: Experiment Maxim Tsoi Spin Torque in Magnetic Systems: Theory A. Manchon and Shufeng Zhang Hot Carrier Spin Transport Ron Jansen Spin Transport and Magnetism in Magnetic Tunnel Junctions Tunneling Magnetoresistance: Experiment (Non-MgO) Patrick R. LeClair and Jagadeesh S. Moodera Tunnel Magnetoresistance in MgO-Based Magnetic Tunnel JunctionsExperiment Shinji Yuasa Tunneling Magnetoresistance: Theory Kirill D. Belashchenko and Evgeny Y. Tsymbal Spin-Filter Tunneling Tiffany S. Santos and Jagadeesh S. Moodera Spin Torques in Magnetic Tunnel Junctions. Yoshishige Suzuki and Hitoshi Kubota Multiferroic Tunnel Junctions Manuel Bibes and Agnes Barthelemy Spin Transport and Magnetism in Semiconductors Spin Relaxation and Spin Dynamics in Semiconductors Jaroslav Fabian and M.W. Wu Electrical Spin Injection and Transport in Semiconductors Berend T. Jonker Spin-Polarized Ballistic Hot-Electron Injection and Detection in Hybrid Metal Semiconductor Devices Ian Appelbaum Magnetic Semiconductors: IIIV Semiconductors Carsten Timm Magnetism of Dilute Oxides J.M.D. Coey Tunneling Magnetoresistance and Spin Transfer with (Ga,Mn)As H. Jaffres and Jean Marie George Spin Transport in Organic Semiconductors Valentin Dediu, Luis E. Hueso, and Ilaria Bergenti Spin Transport in Ferromagnet/IIIV Semiconductor Heterostructures Paul A. Crowell and Scott A. Crooker Spin Polarization by Current Sergey D. Ganichev, Maxim Trushin, and John Schliemann Anomalous and Spin-Injection Hall Effects Jairo Sinova, Jorg Wunderlich, and Tomas Jungwirth Spin Transport and Magnetism at the Nanoscale Spin-Polarized Scanning Tunneling Microscopy Matthias Bode Point Contact Andreev Ref lection Spectroscopy Boris E. Nadgorny Ballistic Spin Transport. Bernard Doudin and N.T. Kemp Graphene Spintronics Csaba Jozsa and Bart J. van Wees Magnetism and Transport in Diluted Magnetic Semiconductor Quantum Dots Joaquin Fernandez Rossier and R. Aguado Spin Transport in Hybrid Nanostructures Saburo Takahashi and Sadamichi Maekawa Nonlocal Spin Valves in Metallic Nanostructures Yoshichika Otani and Takashi Kimura Molecular Spintronics Stefano Sanvito Applications Magnetoresistive Sensors Based on Magnetic Tunneling Junctions Gang Xiao Magnetoresistive Random Access Memory. Johan Akerman Emerging Spintronics Memories. Stuart Parkin, Masamitsu Hayashi, Luc Thomas, Xin Jiang, Rai Moriya, and William Gallagher GMR Spin-Valve Biosensors Drew A. Hall, Richard S. Gaster, and Shan X. Wang Semiconductor Spin-Lasers Rafal Oszwaldowski, Christian Gothgen, Jeongsu Lee, and Igor Zutic Spin Logic Devices Hanan Dery

We present a seamless integration of spin-based memory and logic circuits. The building blocks are magnetologic gates based on a hybrid graphene/ferromagnet material system. We use network search engines as a technology demonstration vehicle and simulate a high-speed, small-area, and low-power spin-based circuit.

If magnetic semiconductors are ever to find wide application in real spintronic devices, their magnetic and electronic properties will require tailoring in much the same way that band gaps are engineered in conventional semiconductors. Unfortunately, no systematic understanding yet exists of how, or even whether, properties such as Curie temperatures and band gaps are related in magnetic semiconductors. Here we explore theoretically these and other relationships within 64 members of a single materials class, the Mn-doped II-IV-V2 chalcopyrites, three of which are already known experimentally to be ferromagnetic semiconductors. Our first-principles results reveal a variation of magnetic properties across different materials that cannot be explained by either of the two dominant models of ferromagnetism in semiconductors. Based on our results for structural, electronic, and magnetic properties, we identify a small number of new stable chalcopyrites with excellent prospects for ferromagnetism., Comment: 6 pages with 4 figures, plus 3 supplementary figures; to appear in Nature Materials

Injecting spin-polarized carriers into semiconductor lasers provides important opportunities to extend what is known about spintronic devices, as well as to overcome many limitations of conventional (spin-unpolarized) lasers. By developing a microscopic model of spin-dependent optical gain derived from an accurate electronic structure in a quantum well-based laser, we study how its operation properties can be modified by spin-polarized carriers, carrier density, and resonant cavity design. We reveal that by applying a uniaxial strain, it is possible to attain a large birefringence. While such birefringence is viewed as detrimental in conventional lasers, it could enable fast polarization oscillations of the emitted light in spin lasers which can be exploited for optical communication and high-performance interconnects. The resulting oscillation frequency ($>200$ GHz) would significantly exceed the frequency range possible in conventional lasers., 15 pages, 14 figures

We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photoexcited holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding non-magnetic ZnSe matrix. In sample B (ZnTe/ZnMnSe) on the other hand, the holes are confined in the non-magnetic ZnTe QDs and the electrons move in the magnetic ZnMnSe matrix. The magnetic polaron formation energies, E MP , in these samples were measured from the temporal red-shift of the excitonic emission peak. The magnetic polarons in the two samples exhibit distinct characteristics. In sample A, the magnetic polaron is strongly bound with E MP =35 meV. Furthermore, E MP has unconventionally weak dependence of on both temperature T and magnetic field B appl . In contrast, magnetic polarons in sample B show conventional characteristics with E MP decreasing with increasing temperature and increasing external magnetic field. We attribute the difference in magnetic polaron properties between the two types of QDs to the difference in the location of the Mn ions in the respective structures.

We review several proposed spintronic devices that can provide new functionality or improve available functions of electronic devices. In particular, we discuss a high mobility field effect spin transistor, an all-metal spin transistor, and our recent proposal of an all-semiconductor spin transistor and a spin battery. We also address some key issues in spin-polarized transport, which are relevant to the feasibility and operation of hybrid semiconductor devices. Finally, we discuss a more radical aspect of spintronic research--the spin-based quantum computation and quantum information processing., Comment: 17 pages, 3 figures

The angular dependence of the nonlinear transverse magnetic moment of untwinned high-quality single crystals of YBa{sub 2}Cu {sub 3}O{sub 6.95} has been studied at a temperature of 2.5thinspthinspK using a low frequency ac technique. The absence of any signature at angular period 2{pi}/4 is analyzed in light of the numerical predictions of such a signal for a pure d{sub x{sup 2}{minus}y{sup 2}} order parameter with line nodes. Implications of this null result for the existence of a nonzero gap at all angles on the Fermi surface are discussed. {copyright} {ital 1999} {ital The American Physical Society}

Measurements of the nonlinear transverse magnetic moment have been carried out on disk-shaped samples of YBCO, using an ac technique. Numerical calculations for a dx2-y2 pairing state predict a fourfold modulation of this moment as a function of the angle between the applied magnetic and the crystalline axes for a magnetic field applied in the a–b plane of the sample. Our results imply a minimum nonzero gap at all points on the Fermi surface.

We present a novel design concept for spintronic nanoelectronics that emphasizes a seamless integration of spin-based memory and logic circuits. The building blocks are magneto-logic gates [1,2] based on a hybrid graphene/ferromagnet material system. We use network search engines as a technology demonstration vehicle and present a spin-based circuit design with smaller area and lower energy consumption than the state-of-the-art CMOS counterparts. This design can also be applied in applications such as data compression, coding and image recognition. In the proposed scheme, over 100 spin-based logic operations are carried out before any need for a spin-charge conversion. Consequently, supporting CMOS electronics requires little power consumption. The spintronic-CMOS integrated system can be implemented on a single 3-D chip. These nonvolatile logic circuits hold potential for a paradigm shift in computing applications.

We present a magneto-optical study of magnetic polarons in type-II Zn,Mn Te quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to 150 K, with a formation energy of 40 meV. The emission from these dots exhibits an unusually small Zeeman shift with applied magnetic field 2 meV at 8 T and at the same time a very large circular polarization. We attribute this apparently contradictory behavior by a low and weakly temperaturedependent magnetic susceptibility due to antiferromagnetic coupling of the Mn spins.

We study the influence of deformations on magnetic ordering in quantum dots doped with magnetic impurities. The reduction of symmetry and the associated deformation from circular to elliptical quantum confinement lead to the formation of piezomagnetic quantum dots. The strength of elliptical deformation can be controlled by the gate voltage to change the magnitude of magnetization, at a fixed number of carriers and in the absence of applied magnetic field. We reveal a reentrant magnetic ordering with the increase of elliptical deformation and suggest that the piezomagnetic quantum dots can be used as nanoscale magnetic switches., Comment: 4 pages, 3 figures

We formulate an analytical model for vertical-cavity surface-emitting lasers (VCSELs) with injection (pump) of spin-polarized electrons. Our results for two different modes of carrier recombination allow for a systematic analysis of the operational regimes of the spin-VCSELs. We demonstrate that threshold reduction by electrically-pumped spin-polarized carriers can be larger than previously assumed possible. Near the threshold, such VCSELs can act as effective non-linear filters of circularly-polarized light, owing to their spin-dependent gain., Comment: 4 pages, 3 figures

Optical orientation is a highly efficient tool for the generation of nonequilibrium spin polarization in semiconductors. Combined with spin-polarized transport it offers new functionalities for conventional electronic devices, such as pn junction bipolar diodes or transistors. In nominally nonmagnetic junctions optical orientation can provide a source for spin capacitance--the bias-dependent nonequilibrium spin accumulation--or for spin-polarized current in bipolar spin-polarized solar cells. In magnetic junctions, the nonequilibrium spin polarization generated by spin orientation in a proximity of an equilibrium magnetization gives rise to the spin-voltaic effect (a realization of the Silsbee-Johnson coupling), enabling efficient control of electrical properties such as the I-V characteristics of the junctions by magnetic and optical fields. This article reviews the main results of investigations of spin-polarized and magnetic pn junctions, from spin capacitance to the spin-voltaic effect., Comment: 9 pages, 10 figures; appeared in the special issue of Semicond. Sci. Technol. on Optical Orientation, in honor of B. P. Zakharchenya

Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field., tutorial review; 342 pages, 132 figures

We propose a model of carrier-mediated ferromagnetism in semiconductors that accounts for the temperature dependence of the carriers. The model permits analysis of the thermodynamic stability of competing magnetic states, opening the door to the construction of magnetic phase diagrams. As an example we analyze the stability of a possible reentrant ferromagnetic semiconductor, in which increasing temperature leads to an increased carrier density, such that the enhanced exchange coupling between magnetic impurities results in the onset of ferromagnetism as temperature is raised., 4 pages, 3 figures

Many problems in computational magnetics involve computation of fields which decay within a skin depth $\delta$, much smaller than the sample size $d$. We discuss here a novel perturbation method which exploits the smallness of $\epsilon \equiv \delta / d$ and the asymptotic behavior of the solution in the exterior and interior of a sample. To illustrate this procedure we consider the computation of the magnetic dipole and quadrupole moments of an anisotropic, unconventional, three dimensional superconductor. The method significantly reduces the required numerical work and can be implemented in different numerical algorithms., Comment: Three pages. To appear in Journal of Applied Physics (MMM-Intermag issue)

Exotic superconducting properties of Sr$_{2}$RuO$_{4}$ have provided strong support for an unconventional pairing symmetry. However, the extensive efforts over the past decade have not yet unambiguously resolved the controversy about the pairing symmetry in this material. While recent phase-sensitive experiments using flux modulation in Josephson junctions consisting of Sr$_{2}$RuO$_{4}$ and a conventional superconductor have been interpreted as conclusive evidence for a chiral spin-triplet pairing, we propose here an alternative interpretation. We show that an overlooked chiral spin-singlet pairing is also compatible with the observed phase shifts in Josephson junctions and propose further experiments which would distinguish it from its spin-triplet counterpart., Comment: 4 pages, 1 figure

A magnetic bipolar transistor is a bipolar junction transistor with one or more magnetic regions, and/or with an externally injected nonequilibrium (source) spin. It is shown that electrical spin injection through the transistor is possible in the forward active regime. It is predicted that the current amplification of the transistor can be tuned by spin., 4 pages, 2 figures

In an inhomogeneously doped magnetic semiconductor spin relaxation time T_1 can be determined by all-electrical measurements. Nonequilibrium spin injected in a magnetic p-n junction gives rise to the spin-voltaic effect where the nonequilibrium spin-induced charge current is very sensitive to T_1 and can flow even at no applied bias. It is proposed that T_1 can be determined by measuring the I-V characteristics in such a geometry. For a magnetic p-n junction where the results can be calculated analytically, it is in addition possible to extract the g-factor and the degree of injected carrier spin polarization., 4 pages, 2 figures

A theory of spin-polarized transport in inhomogeneous magnetic semiconductors is developed and applied to magnetic/nonmagnetic p-n junctions. Several phenomena with possible spintronic applications are predicted, including spin-voltaic effect, spin valve effect, exponential and giant magnetoresistance. It is demonstrated that only nonequilibrium spin can be injected across the space-charge region of a p-n junction, so that there is no spin injection (or extraction) at low bias.

A solar cell illuminated by circularly-polarized light generates charge and spin currents. We show that the spin polarization of the current significantly exceeds the spin polarization of the carrier density for the majority carriers. Based on this principle we propose a semiconductor spin-polarized solar battery and substantiate our proposal using analytical arguments and numerical modeling., 4 pages, 3 figures

Selected problems of fundamental importance for spintronics and spin-polarized transport are reviewed, some of them with a special emphasis on their applications in quantum computing and coherent control of quantum dynamics. The role of the solid-state environment in the decoherence of electron spins is discussed. In particular, the limiting effect of the spin-orbit interaction on spin relaxation of conduction electrons is carefully examined in the light of recent theoretical and experimental progress. Most of the proposed spintronic devices involve spin-polarized transport across interfaces in various hybrid structures. The specific example discussed here, of a magnetic semiconductor/superconductor interface, displays many intricacies which a complex spin-dependent interface introduces in the spin-polarized transport. It is proposed that pairs of entangled electrons in a superconductor (Cooper pairs) can be transfered to a non-superconducting region, and consequently separated for a transport study of the spin entanglement. Several important theoretical proposals for quantum computing are based on electronic and nuclear spin entanglement in a solid. Physical requirements for these proposals to be useful are discussed and some alternative views are presented. Finally, a recent discovery of optical control of nuclear spins in semiconductors is reviewed and placed in the context of a long-standing search for electronic control of nuclear dynamics., 6 pages, uses IEEEtran.sty, review paper to be presented at INTERMAG 2000, to be published in IEEE Transactions on Magnetics

We review from a theoretical perspective the emerging field of spintronics where active control of spin transport and dynamics in electronic materials may provide novel device application possibilities. In particular, we discuss the quantum mechanical principles underlying spintronics applications, emphasizing the formidable challenges involving spin decoherence and spin injection facing any eventual device fabrication. We provide a critical assessment of the current status of the field with special attention to possible device applications., Comment: 4 pages, no figures. Invited talk presented at The 58th Annual Device Research Conference (June 19-21, 2000, Denver, CO); to appear in IEEE Proceedings

For decades, silicon has been the dominant material for conventional, charge-based electronics. A new twist makes silicon ripe to enter the domain of spintronics, where the new currency is electron spin.

We present a method to determine the nodal structure of the energy gap of unconventional superconductors such as high $T_c$ materials. We show how nonlinear electrodynamics phenomena in the Meissner regime, arising from the presence of lines on the Fermi surface where the superconducting energy gap is very small or zero, can be used to perform ``node spectroscopy'', that is, as a sensitive bulk probe to locate the angular position of those lines. In calculating the nonlinear supercurrent response, we include the effects of orthorhombic distortion and $a-b$ plane anisotropy. Analytic results presented demonstrate a systematic way to experimentally distinguish order parameters of different symmetries, including cases with mixed symmetry (for example, $d+s$ and $s+id$). We consider, as suggested by various experiments, order parameters with predominantly $d$-wave character, and describe how to determine the possible presence of other symmetries. The nonlinear magnetic moment displays a distinct behavior if nodes in the gap are absent but regions with small, finite, values of the energy gap exist., Comment: 18 pages, Revtex, 9 postscript figures. Submitted to Phys. Rev B

We present a method to compute the magnetic moment of a bulk, finite-size, three-dimensional, anisotropic superconductor. Our numerically implemented perturbative procedure is based on a solution of the nonlinear Maxwell- London equations, where we include the nonlinear relation between current and gauge invariant velocity. The method exploits the small ratio of penetration depth to sample size. We show how to treat the open boundary conditions over an infinite domain and the continuity requirement at the interface. We demonstrate how our method substantially reduces the computational work required and discuss its implementation to an oblate spheroid. The numerical solution is obtained from a finite difference method. We briefly discuss the relevance of this work to similar problems in other fields., Comment: 43 pages RevTex ms and four postscript figures. To appear in Journal of Computational Physics

We consider the nonlinear transverse magnetic moment that arises in the Meissner state of superconductors with strongly anisotropic order parameter. We compute this magnetic moment as a function of applied field and geometry, assuming d-wave pairing, for realistic samples, finite in all three dimensions, of high temperature superconducting materials. Return currents, shape effects and the anisotropy of the penetration depth are all included. We numerically solve the nonlinear Maxwell-London equations for a finite system. The effect, which is a probe of the order parameter symmetry in the bulk of the sample, should be readily measurable if pairing is in a d-wave state. Failure to observe it would set a lower bound to the s-wave component., RevTex, 15 pages, six Postscript Figures