Electron Spin Currents Research Articles

Magnon transport through a quantum dot: Conversion to electronic spin and charge currents

We consider a single-level quantum dot coupled to magnetic insulators (magnonic reservoirs) and magnetic metals (electronic reservoirs). The whole system is in an external magnetic field. In a general case, the system includes two magnonic and two electronic reservoirs, but we also present results for some specific situations, where only two or three reservoirs are effectively connected to the dot. The main objective is the analysis of the conversion of magnon current to electronic spin and charge currents, and {\it vice versa}. We consider the limiting case of large Coulomb energy in the dot (Coulomb blockade), as well as the case when the Coulomb energy is finite and double occupancy is allowed.

Unconventional scaling and significant enhancement of the spin Seebeck effect in multilayers

Thermal spin pumping constitutes a novel mechanism for generation of spin currents; however their weak intensity constitutes a major roadblock for its usefulness. We report a phenomenon that produces a huge spin current in the central region of a multilayer system, resulting in a giant spin Seebeck effect in a structure formed by repetition of ferromagnet/metal bilayers. The result is a consequence of the interconversion of magnon and electron spin currents at the multiple interfaces. This work opens the possibility to design thin film heterostructures that may boost the application of thermal spin currents in spintronics.

Signatures of electron-magnon interaction in charge and spin currents through magnetic tunnel junctions: A nonequilibrium many-body perturbation theory approach

We develop a numerically exact scheme for resumming certain classes of Feynman diagrams in the self-consistent perturbative expansion for the electron and magnon self-energies in the nonequilibrium Green function formalism applied to a coupled electron-magnon (e-m) system driven out of equilibrium by the applied finite bias voltage. Our scheme operates with the electronic and magnonic GFs and the corresponding self-energies viewed as matrices in the Keldysh space, rather than conventionally extracting their retarded and lesser components, which greatly simplifies translation of diagrams into compact mathematical expressions and their computational implementation. This is employed to understand the effect of inelastic e-m scattering on charge and spin current vs bias voltage ${V}_{b}$ in F/I/F (F-ferromagnet; I-insulating barrier) magnetic tunnel junctions (MTJs), which are modeled on a quasi-one-dimensional (quasi-1D) tight-binding lattice for the electronic subsystem and quasi-1D Heisenberg model for the magnonic subsystem. For this purpose, we evaluate the Fock diagram for the electronic self-energy and the electron-hole polarization bubble diagram for the magnonic self-energy. The respective electronic and magnonic GF lines within these diagrams are the fully interacting ones, thereby requiring to solve the ensuing coupled system of nonlinear integral equations self-consistently. Despite using the quasi-1D model and treating e-m interaction in many-body fashion only within a small active region consisting of few lattice sites around the F/I interface, our analysis captures essential features of the so-called zero-bias anomaly observed [V. Drewello, J. Schmalhorst, A. Thomas, and G. Reiss, Phys. Rev. B 77, 014440 (2008)] in both MgO- and ${\mathrm{AlO}}_{x}\ensuremath{-}$based realistic 3D MTJs where the second derivative ${d}^{2}I/d{V}_{b}^{2}$ (i.e., inelastic electron tunneling spectrum) of charge current exhibits sharp peaks of opposite sign on either side ${V}_{b}=0$. We show that this is closely related to a substantially modified magnonic density of states (DOS) after the e-m interaction is turned on---the magnonic bandwidth over which DOS is nonzero becomes broadened, thereby making e-m scattering at arbitrary small bias voltage possible, while DOS also acquires peaks (on the top of a continuous background) signifying the formation of quasibound states of magnons dressed by the cloud of electron-hole pair excitations. We also demonstrate that the sum of electronic spin currents in all of the semi-infinite leads attached to the active region quantifies the loss of spin angular momentum carried away from the active region by the magnonic spin current.

The electron–phonon interaction from fundamental local gauge symmetries in solids

The elastic properties of solids are described in close analogy with General Relativity, by locally gauging the translational group of space–time. Electron interactions with the crystal lattice are thus generated by enforcing full gauge invariance, with the introduction of a gauge field. Elementary excitations are associated with the local gauge, contrasting to the usual interpretation as Goldstone bosons emerging from global symmetry breaking. In the linear limit of the theory, the gauge field displays elastic waves, that we identify with acoustic phonons, when the field is quantized. Coupling with the electronic part of the system yields the standard electron–phonon interaction. If spin–orbit effects are included, unusual couplings emerge between the strain field and the electronic spin current, leading to novel physics that may be relevant for spintronic applications.

Nonlinear detection of spin currents in graphene with non-magnetic electrodes

The degree to which an electrical current is spin polarized is usually determined by how easily it travels across an interface with a magnetic contact. By using nonlinear interactions between spin and charge in graphene, the polarization of spin currents can be measured without magnetic contacts. The abilities to inject and detect spin carriers are fundamental for research on transport and manipulation of spin information1,2. Pure electronic spin currents have been recently studied in nanoscale electronic devices using a non-local lateral geometry, both in metallic systems 3 and in semiconductors4. To unlock the full potential of spintronics we must understand the interactions of spin with other degrees of freedom. Such interactions have been explored recently, for example, by using spin Hall5,6,7 or spin thermoelectric effects6,8,9. Here we present the detection of non-local spin signals using non-magnetic detectors, through an as-yet-unexplored nonlinear interaction between spin and charge. In analogy to the Seebeck effect10, where a heat current generates a charge potential, we demonstrate that a spin current in a paramagnet leads to a charge potential, if the conductivity is energy dependent. We use graphene11 as a model system to study this effect, as recently proposed12. The physical concept demonstrated here is generally valid, opening new possibilities for spintronics.

Optically rewritable patterns of nuclear magnetization in gallium arsenide

The control of nuclear spin polarization is important to the design of materials and algorithms for spin-based quantum computing and spintronics. Towards that end, it would be convenient to control the sign and magnitude of nuclear polarization as a function of position within the host lattice. Here we show that, by exploiting different mechanisms for electron-nuclear interaction in the optical pumping process, we are able to control and image the sign of the nuclear polarization as a function of distance from an irradiated GaAs surface. This control is achieved using a crafted combination of light helicity, intensity and wavelength, and is further tuned via use of NMR pulse sequences. These results demonstrate all-optical creation of micron scale, rewritable patterns of positive and negative nuclear polarization in a bulk semiconductor without the need for ferromagnets, lithographic patterning techniques, or quantum-confined structures.

Two-photon indirect optical injection and two-color coherent control in bulk silicon

Using an empirical pseudopotential description of electron states and an adiabatic bond charge model for phonon states in bulk silicon, we theoretically investigate two-photon indirect optical injection of carriers and spins and two-color coherent control of the motion of the injected carriers and spins. For two-photon indirect carrier and spin injection, we identify the selection rules of band edge transitions, the injection in each conduction band valley, and the injection from each phonon branch at 4 K and 300 K. At 4 K, the TA phonon-assisted transitions dominate the injection at low photon energies, and the TO phonon-assisted at high photon energies. At 300 K, the former dominates at all photon energies of interest. The carrier injection shows anisotropy and linear-circular dichroism with respect to light propagation direction. For light propagating along the $<001>$ direction, the carrier injection exhibits valley anisotropy, and the injection into the $Z$ conduction band valley is larger than that into the $X/Y$ valleys. For $\sigma^-$ light propagating along the $<001>$ ($<111>$) direction, the degree of spin polarization gives a maximum value about 20% (6%) at 4 K and -10% (20%) at 300 K, and at both temperature shows abundant structure near the injection edges due to contributions from different phonon branches. Forthe two-color coherent current injection with an incident optical field composed of a fundamental frequency and its second harmonic, the response tensors of the electron (hole) charge and spin currents are calculated at 4 K and 300 K. We show the current control for three different polarization scenarios. The spectral dependence of the maximum swarm velocity shows that the direction of charge current reverses under increase in photon energy.

Electron and spin transport in adiabatic quantum pump based on armchair graphene nanoribbons

The phenomenon of adiabatic quantum pump in double-barrier structures based on armchair graphene nanoribbons has been theoretically analyzed within the framework of the tight binding approximation. An analytical expression for a bilinear response is obtained that is valid at small Fermi energies. Using the effect of proximity to a ferromagnetic dielectric, it is possible to obtain both electron and pure spin currents. Dependences of the generated electron and spin currents on the Fermi energy have been numerically calculated. The validity of the adiabatic approximation is assessed based on the dwell time for electron travelling through the system.

Transfer of spin angular momentum from Cs vapor to nearby Cs salts through laser-induced spin currents

Optical pumping of alkali-metal atoms in vapor cells causes spin currents to flow to the cell walls where excess angular momentum accumulates in the wall nuclei. Experiments reported here indicate that the substantial enhancement of the nuclear-spin polarization of salts at the cell walls is primarily due to the nuclear-spin current, with a lesser contribution from the electron-spin current of the vapor.

Nonlocal Andreev reflection with Rashba spin—orbital interaction in a triple-quantum-dot ring

We theoretically studied the nonlocal Andreev reflection with Rashba spin—orbital interaction in a triple-quantum-dot (QD) ring, which is introduced as Rashba spin—orbital interaction to act locally on one component quantum dot. It is found that the electronic current and spin current are sensitive to the systematic parameters. The interdot spin-flip term does not play a leading role in causing electronic and spin currents. Otherwise the spin precessing term leads to shift of the peaks of the the spin-up and spin-down electronic currents in different directions and results in the spin current. Moreover, the spin-orbital interaction suppresses the nonlocal Andreev reflection, so we cannot obtain the pure spin current.

Nonlocal Andreev reflection and spin current in a three-terminal Aharonov–Bohm interferometer

This paper theoretically reports the nonlocal Andreev reflection and spin current in a normal metal-ferromagnetic metal-superconducting Aharonov–Bohm interferometer. It is found that the electronic current and spin current are sensitive to systematic parameters, such as the gate voltage of quantum dots and the external magnetic flux. The electronic current in the normal metal lead results from two competing processes: quasiparticle transmission and nonlocal Andreev reflection. The appearance of zero spin-up electronic current (or spin-down electronic current) signals the existence of nonlocal Andreev reflection, and the presence of zero electronic current results in the appearance of pure spin current.

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.

A New Spin on the Doppler Effect

Direct measurements can now be made of electron spin currents, which play a key role in advanced memory applications.

Drift-diffusion crossover and the intrinsic spin diffusion lengths in semiconductors

We study the propagation of electron spin density polarization and spin currents in n-doped semiconductors within the two-component drift-diffusion model in an applied electric field (E). The drift and diffusion contributions to the spin currents are examined, which shows how the spin current could be enhanced. We find that there is a crossover field (Ex), where the drift and diffusion contribute equally to the spin current in the downstream direction. This suggests a possible way to identify whether the process for a given E would be in the drift or diffusion regime. We derive the expression for Ex and show that the intrinsic spin diffusion length in a semiconductor can be calculated directly from Ex. The results will be useful in obtaining transport properties of the carriers’ spin in semiconductors. This investigation, however, highlights the need for further experiments to be conducted to measure Ex in semiconductors.

Enhancing One-Dimensional Charge Transport through Intermolecular π-Electron Delocalization: Conductivity Improvement for Organic Nanobelts

Effective π-electron delocalization within a PTCDI nanobelt, for which the one-dimensional molecular arrangement is dominated by the cofacial π−π stacking, has been characterized by both electron spin resonance (ESR) spectrometry and current−voltage (I−V) measurement. The long-range π-electron delocalization enables dramatic enhancement of electrical conductivity of the nanobelt through external-charge doping. When immersed in saturated hydrazine vapor, about 3 orders of magnitude increase in conductivity was achieved for the PTCDI nanobelt, implying potential application of the nanomaterials in electrical sensing of reducing gaseous species such as organic amines.

Generating Spin Currents in Semiconductors with the Spin Hall Effect

We investigate electrically induced spin currents generated by the spin Hall effect in GaAs structures that distinguish edge effects from spin transport. Using Kerr rotation microscopy to image the spin polarization, we demonstrate that the observed spin accumulation is due to a transverse bulk electron spin current, which can drive spin polarization nearly 40 microns into a region in which there is minimal electric field. Using a model that incorporates the effects of spin drift, we determine the transverse spin drift velocity from the magnetic field dependence of the spin polarization.

Coherence control of electron spin currents in semiconductors

AbstractWe provide an overview of some of our recent work on the use of one color and two color optical techniques to generate and control electronic spin currents in semiconductors for which a spin–orbit interaction exists. The generation process relies on the quantum interference between different absorption pathways, such as that between single and two photon absorption or those involving different polarization states of a monochromatic beam. For different crystal orientations and/or beam polarizations it is possible to generate a spin current with or without an electric current, and an electrical current with or without a spin current. In our experiments, which are conducted either at 80 K or 295 K, we typically employ nominally 100 fs pulses centered near 1500 and 750 nm. The currents generated are quasi‐ballistic and the carriers typically move distances of ∼1–10 nm, determined by the momentum relaxation time, which is of the order of 100 fs. The transient characteristics of spin‐polarized electrical currents generated in strained GaAs at room temperature by ∼100 fs pulses is detected by the emitted THz radiation. Pure spin currents can be detected by taking advantage of the accumulation of up and down spins on opposite sides of tightly focused pump beams. The spin states are detected through differential transmission measurements of tightly focused right and left circularly polarized, near‐band‐edge probe pulses, delayed by several picoseconds from the pump pulses to allow carrier thermalization to occur. By spatial scanning across the differential spin profiles and determining the amplitude of the response we are able to translate this into nm spatial resolution of spin displacement. Finally, the ability to generate ballistic currents using purely optical techniques allows us to generate transverse Hall‐like currents, with transverse charge currents generated from pure spin currents and transverse spin currents generated from pure charge currents. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)