Liviu P. Zârbo

Spin Hall effect transistor.

In a Spin Hall The spin Hall effect, in which an electrical current causes accumulation of electron spins of opposite signs in the direction transverse to the current flow, provides a promising avenue of research in exploiting the spin degree of freedom in electronic devices. However, implementing the effect in a device is challenging. Wunderlich et al. (p. 1801) combine the concept of the spin Hall effect with that of a spin transistor, and build a nonmagnetic device in a which a spin current, injected by optical means, is “stripped” of its charge component, goes through a spin-modulation layer, and is detected using the inverse spin Hall effect. Such manipulation of the spin current may help in future spintronic applications. Manipulation of the spin degree of freedom of electrons is used to build a spin transistor without magnetic materials. The field of semiconductor spintronics explores spin-related quantum relativistic phenomena in solid-state systems. Spin transistors and spin Hall effects have been two separate leading directions of research in this field. We have combined the two directions by realizing an all-semiconductor spin Hall effect transistor. The device uses diffusive transport and operates without electrical current in the active part of the transistor. We demonstrate a spin AND logic function in a semiconductor channel with two gates. Our study shows the utility of the spin Hall effect in a microelectronic device geometry, realizes the spin transistor with electrical detection directly along the gated semiconductor channel, and provides an experimental tool for exploring spin Hall and spin precession phenomena in an electrically tunable semiconductor layer.

An antidamping spin–orbit torque originating from the Berry curvature

Magnetization switching at the interface between ferromagnetic and paramagnetic metals, controlled by current-induced torques, could be exploited in magnetic memory technologies. Compelling questions arise regarding the role played in the switching by the spin Hall effect in the paramagnet and by the spin-orbit torque originating from the broken inversion symmetry at the interface. Of particular importance are the antidamping components of these current-induced torques acting against the equilibrium-restoring Gilbert damping of the magnetization dynamics. Here, we report the observation of an antidamping spin-orbit torque that stems from the Berry curvature, in analogy to the origin of the intrinsic spin Hall effect. We chose the ferromagnetic semiconductor (Ga,Mn)As as a material system because its crystal inversion asymmetry allows us to measure bare ferromagnetic films, rather than ferromagnetic-paramagnetic heterostructures, eliminating by design any spin Hall effect contribution. We provide an intuitive picture of the Berry curvature origin of this antidamping spin-orbit torque as well as its microscopic modelling. We expect the Berry curvature spin-orbit torque to be of comparable strength to the spin-Hall-effect-driven antidamping torque in ferromagnets interfaced with paramagnets with strong intrinsic spin Hall effect.

Nonequilibrium Spin Hall Accumulation in Ballistic Semiconductor Nanostructures

We demonstrate that flow of longitudinal unpolarized current through a ballistic two-dimensional electron gas with Rashba spin-orbit coupling will induce nonequilibrium spin accumulation which has opposite sign for the two lateral edges and it is, therefore, the principal observable signature of the spin Hall effect in two-probe semiconductor nanostructures. The magnitude of its out-of-plane component is gradually diminished by static disorder, while it can be enhanced by an in-plane transverse magnetic field. Moreover, our prediction of the longitudinal component of the spin Hall accumulation, which is insensitive to the reversal of the bias voltage, offers a smoking gun to differentiate experimentally between the extrinsic, intrinsic, and mesoscopic spin Hall mechanisms.

Spin-orbit driven ferromagnetic resonance

Ferromagnetic resonance is the most widely used technique for characterizing ferromagnetic materials. However, its use is generally restricted to wafer-scale samples or specific micro-magnetic devices, such as spin valves, which have a spatially varying magnetization profile and where ferromagnetic resonance can be induced by an alternating current owing to angular momentum transfer. Here we introduce a form of ferromagnetic resonance in which an electric current oscillating at microwave frequencies is used to create an effective magnetic field in the magnetic material being probed, which makes it possible to characterize individual nanoscale samples with uniform magnetization profiles. The technique takes advantage of the microscopic non-collinearity of individual electron spins arising from spin-orbit coupling and bulk or structural inversion asymmetry in the band structure of the sample. We characterize lithographically patterned (Ga,Mn)As and (Ga,Mn)(As,P) nanoscale bars, including broadband measurements of resonant damping as a function of frequency, and measurements of anisotropy as a function of bar width and strain. In addition, vector magnetometry on the driving fields reveals contributions with the symmetry of both the Dresselhaus and Rashba spin-orbit interactions.

Spatial distribution of local currents of massless Dirac fermions in quantum transport through graphene nanoribbons

We employ the formalism of bond currents, expressed in terms of the nonequilibrium Green functions, to image the charge flow between two sites of the honeycomb lattice of graphene ribbons of few nanometers width. In sharp contrast to nonrelativistic electrons, current density profiles of quantum transport at energies close to the Dirac point in clean zigzag graphene nanoribbons (ZGNR) differs markedly from the profiles of charge density peaked at the edges due to zero-energy localized edge states. For transport through the lowest propagating mode induced by these edge states, edge vacancies do not affect current density peaked in the center of ZGNR. Furthermore, the long-range potential of a single impurity placed in the center of ZGNR acts to reduce local current around it while concurrently increasing the current density along the zigzag edge, so that ZGNR conductance remains perfect G=2e2/h.

Spin gating electrical current

The level of the chemical potential is a fundamental parameter of the electronic structure of a physical system, which consequently plays an important role in defining the properties of active electrical devices. We directly measure the chemical potential shift in the relativistic band structure of the ferromagnetic semiconductor (Ga,Mn)As, controlled by changes in its magnetic order parameter. Our device comprises a non-magnetic aluminum single electron channel capacitively coupled to the (Ga,Mn)As gate electrode. The chemical potential shifts of the gate are directly read out from the shifts in the Coulomb blockade oscillations of the single electron transistor. The experiments introduce a concept of spin gating electrical current. In our spin transistor spin manipulation is completely removed from the electrical current carrying channel.

Piezospin polarization of currents in nanostructures.

Torsional oscillations of a free-standing semiconductor beam are shown to cause spin-dependent oscillating potentials that spin-polarize an applied charge current in the presence of intentional or disorder scattering potentials. We propose several realizations of mechanical spin generators and manipulators based on this piezo-spintronic effect.

Extrinsically vs. intrinsically driven spin Hall effect in disordered mesoscopic multiterminal bars

We show that pure spin Hall current, flowing out of a four-terminal phase-coherent two-dimensional electron gas (2DEG) within inversion asymmetric semiconductor heterostructure, contains contributions from both the extrinsic mechanisms (spin-orbit dependent scattering off impurities) and the intrinsic ones (due to the Rashba coupling). While the extrinsic contribution vanishes in the weakly and strongly disordered limits, and the intrinsic one dominates in the quasiballistic limit, in the crossover transport regime the spin Hall conductance, exhibiting sample-to-sample large fluctuations and sign change, is not simply reducible to either of the two mechanisms, which can be relevant for interpretation of experiments on dirty 2DEGs [V. Sih et al., Nature Phys. 1, 31 (2005)].

Spin and charge shot noise in mesoscopic spin Hall systems

Injection of unpolarized charge current through the longitudinal leads of a four-terminal two-dimensional electron gas with the Rashba spin-orbit (SO) coupling and SO scattering off extrinsic impurities is responsible not only for the pure spin Hall current in the transverse leads, but also for non-equilibrium random time-dependent current fluctuations. We employ the scattering approach to current-current correlations in multiterminal nanoscale conductors to analyze the shot noise of transverse pure spin Hall current and zero charge current, or transverse spin current and non-zero charge Hall current, driven by unpolarized or spin-polarized injected longitudinal charge current, respectively. Since any spin-flip acts as an additional source of noise, we argue that these shot noises provide a unique experimental tool to differentiate between intrinsic and extrinsic SO mechanisms underlying the spin Hall effect in paramagnetic devices.