Spin Currents In Semiconductors Research Articles

Rashba semiconductor as spin Hall material: Experimental demonstration of spin pumping in wurtziten-GaN:Si

Pure spin currents in semiconductors are essential for implementation in the next generation of spintronic elements. Heterostructures of III-nitride semiconductors are currently employed as central building blocks for lighting and high-power devices. Moreover, the long relaxation times and the spin-orbit coupling (SOC) in these materials indicate them as privileged hosts for spin currents and related phenomena. Spin pumping is an efficient mechanism for the inception of spin current and its conversion into charge current in nonmagnetic metals and semiconductors with Rashba SOC. We report on the generation at room temperature (RT) of pure spin current in nonmagnetic degenerate $n$-GaN:Si from a magnetic permalloy layer driven to ferromagnetic resonance (FMR) conditions. The FMR signal and the generated Hall voltages due to spin Hall effect, inverse spin Hall effect (ISHE), and spurious mechanisms are detected simultaneously. No spin-pumping-induced voltage could be measured below RT, despite the persistence of FMR signal. After eliminating the spurious (not due to ISHE) components contributing to the generated voltage, we find for $n$-GaN:Si a spin Hall angle ${\ensuremath{\theta}}_{\mathrm{SH}}=3.03\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, exceeding by one order of magnitude those reported for other semiconductors, pointing at III-nitrides as particularly efficient spin current generators.

Ultrafast and Gigantic Spin Injection in Semiconductors.

The injection of spin currents in semiconductors is one of the big challenges of spintronics. Motivated by the ultrafast demagnetization and spin injection into metals, we propose an alternative femtosecond route based on the laser excitation of superdiffusive spin currents in a ferromagnet such as Ni. Our calculations show that even though only a fraction of the current crosses the Ni-Si interface, the laser-induced creation of strong transient electrical fields at a ferromagnet-semiconductor interface allows for the injection of chargeless spin currents with record spin polarizations of 80%. Beyond that they are pulsed on the time scale of 100fs which opens the door for new experiments and ultrafast spintronics.

Optical effects of spin currents in semiconductors

A spin current has novel linear and second-order nonlinear optical effects due to its symmetry properties. With the symmetry analysis and the eight-band microscopic calculation we have systematically investigated the interaction between a spin current and a polarized light beam (or the "photon spin current") in direct-gap semiconductors. This interaction is rooted in the intrinsic spin-orbit coupling in valence bands and does not rely on the Rashba or Dresselhaus effect. The light-spin current interaction results in an optical birefringence effect of the spin current. The symmetry analysis indicates that in a semiconductor with inversion symmetry, the linear birefringence effect vanishes and only the circular birefringence effect exists. The circular birefringence effect is similar to the Faraday rotation in magneto-optics but involves no net magnetization nor breaking the time-reversal symmetry. Moreover, a spin current can induce the second-order nonlinear optical processes due to the inversion-symmetry breaking. These findings form a basis of measuring a pure spin current where and when it flows with the standard optical spectroscopy, which may provide a toolbox to explore a wealth of physics connecting the spintronics and photonics.

Measurement of pure spin currents in semiconductors

A new field has emerged called spintronics (spin-based electronics), where the spin of electrons rather than the charge carries information. This may offer opportunities for a new generation of electronic devices. Measuring spin currents in semiconductors plays a key role in spintronics. In this review article, starting from the basic properties of spin currents and the current experimental techniques for spin current measurements, we briefly introduce our theory on optical effects of spin currents in the direct-gap semiconductors, i.e., the Faraday rotation and nonlinear optics, which provide a direct measurement of pure spin currents in semiconductors.

Proposal for the direct optical detection of pure spin currents in semiconductors

We suggest a different practical scheme for the direct detection of pure spin current by using the two-color Faraday rotation of optical quantum interference process (QUIP) in a semiconductor system. We demonstrate theoretically that the Faraday rotation of QUIP depends sensitively on the spin orientation and wave vector of the carriers, and can be tuned by the relative phase and the polarization direction of the $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ laser beams. By adjusting these parameters, the magnitude and direction of the spin current can be detected.

Spin Currents in Semiconductors, Metals, and Insulators

Recent theoretical developments on the physics of spin currents in semiconductors, metals and insulators are reviewed with the stress on the role of relativistic spin–orbit interaction. The key concept is the interplay between the electric field and spin current, which might enable the spintronics without magnetic field/magnets. We present this principle discussing spin Hall effect and multiferroic behavior as two realizations.

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 & Co. KGaA, Weinheim)

Injection of ballistic pure spin currents in semiconductors by a single-color linearly polarized beam

We demonstrate the injection and control of pure spin currents in [110]-oriented GaAs quantum wells at room temperature by one-photon absorption of a single linearly polarized optical pulse. These currents result from the interference of absorption processes associated with the right and left circularly polarized components of the pulse, with the current direction determined by their relative phase. The current generation process differs from the circular photogalvanic effect, which relies only on the intensity of circularly polarized beams. By using spatially resolved pump-probe techniques, we obtain signatures for the currents by measuring the resulting spin separations of 1--4 nm. The separation decreases with increasing excitation fluence, consistent with a reduction in the momentum relaxation time with increasing carrier density.

Theory of Spin Hall Conductivity inn-Doped GaAs

We develop a theory of extrinsic spin currents in semiconductors, resulting from spin-orbit coupling at charged scatterers, which leads to skew-scattering and side-jump contributions to the spin-Hall conductivity. Applying the theory to bulk n-GaAs, without any free parameters, we find spin currents that are in reasonable agreement with experiments by Kato et al. [Science 306, 1910 (2004)].

Spin symmetry and spin current of helicity eigenstates of the Luttinger Hamiltonian

A general spin symmetry argument is proposed for spin currents in semiconductors. In particular, due to the symmetry with respect to spin polarization of the helicity eigenstates of the Luttinger Hamiltonian for a hole-doped semiconductor, the spin polarized flux from a single helicity eigenstate induced by an external electric field, is canceled exactly when all such contributions from eigenstates that are degenerate in energy are summed. Thus, the net spin current predicted by Murakami et al. (Science 301 (2003) 1348), cannot be produced by such a Hamiltonian. Possible symmetry breaking mechanisms which may generate a spin current are discussed.

Quantum interference control of ballistic pure spin currents in semiconductors.

We demonstrate all-optical quantum interference injection and control of a ballistic pure spin current (without an accompanying charge current) in GaAs/AlGaAs quantum wells, consisting of spin-up electrons traveling in one direction and spin-down electrons traveling in the opposite direction. This current is generated through quantum interference of one- and two-photon absorption of approximately 100 fs phase-locked pulses that have orthogonal linear polarizations. We use a spatially resolved pump-probe technique to measure carrier movement of approximately 10 nm. Results agree with recent theoretical predictions.

Probing spin currents in semiconductors

A novel device for spin-current detection in semiconductors, based on the extraordinary Hall effect, is proposed. When a spin current is injected into a nonmagnetic semiconductor, it is found that a transverse electric field, or Hall voltage, is generated. The Hall voltage is proportional to the spin current in the nonmagnetic semiconductor. The proposed device can be used as a tool to investigate the fundamental spin-diffusion length and spin-dependent Hall coefficients in a semiconductor.

Optically injected spin currents in semiconductors.

We show that quantum interference of one and two photon absorption from a two color field allows one to optically inject ballistic spin currents in unbiased semiconductors. The spin currents can be generated with or without an accompanying electrical current and can be controlled using the relative phase of the two colors. We characterize the injected spin currents using symmetry arguments and an eight-band Kane model.