Direct Optical Detection of a Pure Spin Current in Semiconductor

We predict that a pure spin current in a semiconductor may induce Faraday birefringence even without magnetization. The theory is based on a derived effective interaction between the spin current and a polarized light beam, where the helicity of a photon is mapped to spin 1/2. The effective coupling between the polarized light beam and electron spin current can be realized in direct-gap semiconductors such as GaAs with inherent spin–orbit coupling in valence bands, but it involves neither the Rashba nor the Dresselhaus effect of samples. We estimate the amplitude of the Faraday rotation due to a pure spin current, and we present its incident-beam-angle dependence. We show that this Faraday birefringence can be directly measured.     

Ten Years of Spin Hall Effect

In this short review I survey the theory of the spin Hall effect in doped semiconductors and metals in the light of recent experiments on both kinds of materials. After a brief introduction to different types of spin–orbit coupling in solids, I describe in detail the three conceptually distinct mechanisms that are known to contribute to the spin Hall effect, namely “skew-scattering”, “side-jump”, and “intrinsic mechanism”. The skew-scattering mechanism is shown to be dominant in certain clean two-dimensional semiconductors in which one component of the spin is conserved. In such systems the side-jump mechanism is sub-dominant, but universal in form, and can become dominant if the electron mobility is reduced by changing the temperature. Both skew-scattering and side-jump contributions are generally reduced by spin precession, and skew-scattering is completely suppressed in the linear Rashba model in the absence of magnetic field. Different models of spin–orbit coupling can, however, sustain an intrinsic spin Hall effect. A brief summary of the present experimental situation concludes the review.     

Microscopic Origin of Electric and Magnetic Ordering in BiFeO<Subscript>3</Subscript>

The microscopic origin of electric and magnetic ordering in multiferroic materials is described. Multiferroic materials are systems with strong spin–orbit coupling. There is an electric mechanism for which ferroelectricity is generated by dynamic magnetism though vanishing of the electric current; this result is demonstrated in this paper. The multiferroic material BiFeO₃ is shown to be a Mott’s insulator. An expression describing the connection between the polarization, magnetization, and the spin–orbit coupling parameter is derived.     

Spin-Dephasing Anisotropy for Electrons in a Diffusive Quasi-1D GaAs Wire

We present a numerical study of dephasing of electron spin ensembles in a diffusive quasi-one-dimensional GaAs wire due to the D’yakonov–Perel’ spin-dephasing mechanism. For widths of the wire below the spin precession length and for equal strength of Rashba and linear Dresselhaus spin–orbit fields a strong suppression of spin-dephasing is found. This suppression of spin-dephasing shows a strong dependence on the wire orientation with respect to the crystal lattice. The relevance for realistic cases is evaluated by studying how this effect degrades for deviating strength of Rashba and linear Dresselhaus fields, and with the inclusion of the cubic Dresselhaus term.     

Charge Expulsion, Spin Meissner Effect, and Charge Inhomogeneity in Superconductors

Superconductivity occurs in systems that have a lot of negative charge: the highly negatively charged (CuO₂)⁼ planes in the cuprates, negatively charged (FeAs)⁻ planes in the iron arsenides, and negatively charged B ⁻ planes in magnesium diboride. And, in the nearly filled (with negative electrons) bands of almost all superconductors, as evidenced by their positive Hall coefficient in the normal state. No explanation for this charge asymmetry is provided by the conventional theory of superconductivity, within which the sign of electric charge plays no role. Instead, the sign of the charge carriers plays a key role in the theory of hole superconductivity, according to which metals become superconducting because they are driven to expel negative charge (electrons) from their interior. This is why NIS tunneling spectra are asymmetric, with larger current for negatively biased samples. The theory also offers a compelling explanation of the Meissner effect: as electrons are expelled towards the surface in the presence of a magnetic field, the Lorentz force imparts them with azimuthal velocity, thus generating the surface Meissner current that screens the interior magnetic field. In type II superconductors, the Lorentz force acting on expelled electrons that do not reach the surface gives rise to the azimuthal velocity of the vortex currents. In the absence of applied magnetic field, expelled electrons still acquire azimuthal velocity, due to the spin–orbit interaction, in opposite direction for spin-up and spin-down electrons: the “Spin Meissner effect.” This results in a macroscopic spin current flowing near the surface of superconductors in the absence of applied fields, of magnitude equal to the critical charge current (in appropriate units). Charge expulsion also gives rise to an interior outward-pointing electric field and to excess negative charge near the surface. In strongly type II superconductors this physics should give rise to charge inhomogeneity and spin currents throughout the interior of the superconductor, to large sensitivity to (non-magnetic) disorder and to a strong tendency to phase separation.     

Spin Dynamics in ZnO-Based Materials

In this work, we address the issue of spin relaxation and its relevance to spin detection in ZnO-based materials, by spin-polarized, time-resolved magneto-optical spectroscopy. We have found that spin relaxation is very fast, i.e. about 100 ps for donor bound excitons in wurtzite ZnO, despite of a weak spin–orbit interaction. We also reveal that alloying of ZnO with Cd enhances spin relaxation, prohibiting ZnCdO/ZnO structures for efficient optical spin detection. On the other hand, a variation in strain field induced by lattice mismatch with substrates does not seem to lead to a noticeable change in spin relaxation. The observed fast spin relaxation, together with the limitation imposed by the band structure, are thus identified as the two most important factors that limit the efficiency of optical spin detection in the studied ZnO-based materials.     

Infinite Spin Diffusion Length of Any Spin Polarization Along Direction Perpendicular to Effective Magnetic Field from Dresselhaus and Rashba Spin–Orbit Couplings with Identical Strengths in (001) GaAs Quantum Wells

In this note, we show that the latest spin grating measurement of spin helix by Koralek et al. (Nature 458:610, 2009) provides strong evidence of the infinite spin diffusion length of any spin polarization along the direction perpendicular to the effective magnetic field from the Dresselhaus and Rashba spin–orbit couplings with identical strengths in (001) GaAs quantum wells, predicted by Cheng et al. (Phys. Rev. B 75:205328, 2007).     

Noise of Spin-Polarized Currents at a Beam Splitter with Local Spin–Orbit Interaction

An electronic beam splitter with a local Rashba spin–orbit coupling can serve as a detector for spin-polarized currents. The spin–orbit coupling plays the role of a tunable spin rotator and can be controlled via a gate electrode on top of the conductor. We use spin-resolved scattering theory to calculate the zero-temperature current fluctuations (shot noise) for such a four-terminal device and show that the shot noise is proportional to the spin polarization of the source. Moreover, we analyze the effect of spin–orbit-induced intersubband coupling, leading to an additional spin rotation.     

Mini-MRI scanner and some possibilities for its use for studying living tissues

We show that a laboratory table-top MRI scanner can be used to obtain images of living tissues in the hand of a patient and to record spin–spin relaxation of protons in water and protons in lipid compounds. We have identified variations in the spin–spin relaxation rate as high as 24% due to the individual biochemical characteristics of the patients. We present the results of using the instrument for diagnosis of age-related changes in bone tissues density and estimating the variation in the spin–spin relaxation constants.     

The Rashba Effect Within the Coherent Scattering Formalism with Applications to Electron Quantum Optics

The influence of spin–orbit coupling in electron quantum optics experiments is investigated within the framework of the Landauer–Büttiker coherent scattering formalism. We begin with a brief review of our electron quantum optics toolbox: an electron intensity interferometer (Hanbury Brown and Twiss-type experiment), an electron collision analyzer (Hong–Ou–Mandel-type experiment), and a proposed Bell state analyzer. These experiments are performed or proposed in two-dimensional electron gas systems and, therefore, may be influenced by the Rashba spin–orbit coupling. To quantify this effect, we define the creation/annihilation operators for the stationary states of the Rashba spin–orbit coupling Hamiltonian and use them to derive the current operator within the Landauer–Büttiker formalism. The current is expressed as it is in the standard spin-independent case, but with the spin label replaced by a new label that we call the spin–orbit coupling label. The spin–orbit coupling effects can then be represented in a scattering matrix that relates the spin–orbit coupling stationary states in different leads. We apply this new formalism to the case of a four-port beamsplitter, and it is shown to mix states with different spin–orbit coupling labels in a manner that depends on the angle between the leads. A noise measurement after the collision of spin-polarized electrons at an electron beamsplitter provides a new experimental means to measure the Rashba parameter . It is also shown that the degree of electron bunching in an entangled-electron collision experiment is reduced by the spin–orbit coupling according the beamsplitter lead angle.     

NMR Evidence for C<Subscript>60</Subscript> Configurational Fluctuations Around Na Sites in Na<Subscript>2</Subscript>CsC<Subscript>60</Subscript>

The ²³Na nuclear magnetic resonance (NMR) spectrum, spin-lattice and spin–spin relaxation, as well as spin echo double resonance (SEDOR) are investigated in the ternary alkali fulleride compound Na₂CsC₆₀ in the temperature range of 10–300 K. The NMR line associated with the tetrahedral sodium site is split below 170 K (T and T′ lines) similarly to Rb₃C₆₀ although the crystal structures of these two materials are different. SEDOR measurements prove that the T and T′ sites are microscopically close. The merger of the two lines at about 170 K is attributed to motional narrowing resulting from a site exchange due to angular reorientations of the C₆₀ molecules. The exchange dynamics inferred from the spectra, spin–spin relaxation, and spin-lattice relaxation are all consistent and agree with inelastic neutron scattering, supporting our proposal that the observed T-T′ splitting originates from different local fullerene configurations around the tetrahedral alkaline sites.     

Metamagnetic Phase Transition in the 1D Ising Plus Dzyaloshinskii–Moriya Model

The 1D spin-1/2 Ising model with the Dzyaloshinskii–Moriya (DM) interaction is considered. A numerical analysis of the low-energy excitation spectrum and the ground-state magnetic phase diagram of the system using Lanczos method is presented. The DM interaction-dependency is calculated for the low-energy excitation spectrum, spiral-order parameter and spin–spin correlation functions. It is showed that the properties of the ferromagnetic and antiferromagnetic Ising chains are very different in presence of DM interaction. In particular a metamagnetic quantum phase transition occurs in the case of a ferromagnetic chain between the ferromagnetic and spiral phases. The existence of the metamagnetic phase transition is confirmed, using the variational matrix product states approach.     

Spin–Orbit Interaction and All-Semiconductor Spintronics

The purpose of this paper is to give a brief review and trace the present-day perspectives to exploit the spin–orbit interaction in conventional nonmagnetic semiconductor nanostructures. We demonstrate theoretically that the structures can be used to design basic elements of highspeed spintronic devices. In particular we discuss spin filtering, spin-dependent confinement, and scattering in all-semiconductor nanostructures.     

Rashba Spin–Orbit Interaction and Its Applications to Spin-Interference Effect and Spin-Filter Device

The Rashba spin–orbit interaction in InGaAs quantum wells (QW) is studied using the weak antilocalization analysis as a function of the structural inversion asymmetry (SIA). We have observed a clear cross-over from positive to negative magnetoresistance near zero-magnetic field by controlling the degree of the SIA in the QWs. This is a strong evidence of a zero-field spin splitting that is induced by the Rashba effect. The spin-interference effect in a gate-controlled mesoscopic Aharonov–Bohm ring structure is investigated in the presence of Rashba spin–orbit interaction. The oscillatory behavior appearing in ensemble averaged Fourier spectrum of h/2e oscillations as a function of gate voltage is possibly because of the Aharonov–Casher type interference. We propose a spin-filter device based on the Rashba effect using a nonmagnetic resonant tunneling diode structure. Detailed calculation using InAIAs/InGaAs heterostructures shows that the spin-filtering efficiency exceeds 99.9%.     

Probing Entanglement via Rashba-Induced Shot Noise Oscillations

We have recently calculated shot noise for entangled and spin-polarized electrons in novel beam-splitter geometries with a local Rashba orbit–spin (s-o) interaction in the incoming leads. This interaction allows for a gate-controlled rotation of the incoming electron spins. Here we present an alternate simpler route to the shot noise calculation in the above work and focus on only electron pairs. Shot noise for these shows continuous bunching and antibunching behaviors. In addition, entangled and unentangled triplets yield distinctive shot noise oscillations. Besides allowing for a direct way to identify triplet and singlet states, these oscillations can be used to extract s-o coupling constants through noise measurements. Incoming leads with spin–orbit interband mixing give rise to an additional modulation of the current noise. This extra rotation allows the design of a spin transistor with enhanced spin control.     

Dual Fermion Approach to High-Temperature Superconductivity

We present an extension of the recently proposed dual fermion approach to investigate the superconducting properties of the Hubbard model in two dimensions. From the spin–spin susceptibility, we find a reduction of the Néel temperature compared to results from dynamical mean-field theory (DMFT) calculations due to the incorporation of spatial correlations. We present results for the temperature dependence of the leading eigenvalue of the Bethe–Salpeter equation for the particle–particle channel. In agreement with previous studies, we find singlet d-wave pairing to be the dominant contribution to pairing. We further present first results for the finite-doping phase diagram obtained from the leading eigenvalue of the Bethe–Salpeter equations for the particle–hole and particle–particle channels.     

Interaction study of indigo carmine with albumin and dextran by NMR relaxation

In this study, the interaction processes between a dye (indigo carmine) and two different macromolecular models were studied with the aim to obtain physical-chemistry information about the dyeing of textiles. Two macromolecules, albumin and dextran (DX), were chosen to simulate wool and cotton fibers during the coloration procedure in water. Proton NMR selective and non-selective spin–lattice relaxation rate measurements were used to monitor the strength of the overall complexation behavior of indigo carmine toward albumin or DX. The affinity index, a quantitative parameter related to the strength of the ligand–macromolecule interaction, was determined from selective spin–lattice relaxation rate enhancements due to the bound ligand molar fraction. Moreover, this approach allowed the calculation of the equilibrium constant of the complex formation (K) between the dye and macromolecular models. NMR data suggested a higher indigo carmine–albumin complex thermodynamic stability with respect to the indigo carmine–DX adduct. These results indicate a stronger persistence of the dyeing process in wool with respect to cotton fibers, in agreement with literature data.     

Radiative lifetime of triplet excitation recombinations mediated by spin–photon interaction in semiconductors

The radiative recombination of triplet exciton, is studied here theoretically in inorganic amorphous and organic crystalline semiconductors. A new time-dependent exciton–spin–orbit–photon interaction operator derived recently is used to calculate rates of radiative recombination of triplet excitons and the corresponding radiative lifetimes. It is illustrated that the new operator gives rise to a first order non-zero term responsible for the triplet photoluminescence. Results agree quite well with the known experimental results.     

Quantum Dot in the Pseudo-gap Sate with Spin–Orbit Interaction

We study the linear conductance in quantum dot with spin–orbit interaction coupled to Fermi liquid leads with a power-low density of states. The conductance at zero temperature is calculated as a function of the power exponent from the density of state ρ(ω)∼|ω−E _F | ^r at the Fermi energy E _F and the different energy rates. The phase shift of the conduction electrons is also r-dependent. The model can be used in the study of the quantum phase transition.