Dissipationless Spin Current Research Articles

Spin current through an ESR quantum dot: A real-time study

The spin transport in a strongly interacting spin-pump nanodevice is studied using the time-dependent variational-matrix-product-state approach. The precession magnetic field generates a dissipationless spin current through the quantum dot. We compute the real-time spin current away from the equilibrium condition. Both transient and stationary states are reached in the simulation. The essentially exact results are compared with those from the Hartree-Fock approximation (HFA). It is found that correlation effect on the physical quantities at quasisteady state are captured well by the HFA for small interaction strength. However the HFA misses many features in the real-time dynamics. Results reported here may shed light on the understanding of the ultrafast processes as well as the interplay of the nonequilibrium and strongly correlated effect in the transport properties.

QUANTUM SPIN CURRENT INDUCED THROUGH OPTICAL DIPOLE TRANSITION PROCESS IN SEMICONDUCTORS

We propose a scheme to generate quantum spin current via linear optical process. By interacting a three-level system based on the spin states of charged particles (electrons or holes in semiconductor, etc) with the angular momentum states of an optical field, we show that the dynamics of charged particles equipped with different spin polarizations are governed by different (opposite) additional effective magnetic fields. In this way, a pure dissipationless quantum spin current can be generated. No spin-orbit interaction (e.g., Rashba or Dresselhaus term) is needed in this scheme. We also calculate the effect of nonmagnetic impurities on the created spin currents and show that the vertex correction of the spin hall conductivity in the ladder approximation is exactly zero. Paper presented at NTU Spintronics Workshop, Singapore, May 2006.

Toward dissipationless spin transport in semiconductors

Spin-based electronics promises a radical alternative to charge-based electronics, namely the possibility of logic operations with much lower power consumption than equivalent charge-based logic operations. In this paper we review three potential means of dissipationless spin transport in semiconductors with and without spin-orbit coupling: the use of spin currents, propagating modes, and orbital currents. Spin and orbital currents induced by electric fields obey a fundamentally different law than charge transport, which is dissipative. Dissipationless spin currents occur in materials with strong spin-orbit coupling, such as GaAs, while orbital currents occur in materials with weak spin-orbit coupling, such as Si, but with degenerate bands characterized by an atomic orbital index. Spin currents have recently been observed experimentally. Propagating modes are the coupled spin-charge movement that occurs in semiconductors with spin-orbit coupling. In contrast to normal charge transport, which is diffusive, the spin-charge mode can exhibit propagating transport, with low energy loss over relatively large distances (>100 µm), by funneling energy between the spin and the charge component through the spin-orbit coupling channel. This opens the possibility for spin-based transport without either spin injection or spin detection. The schemes discussed in this paper are analyzed in comparison with schemes based on molecular electronics phenomena, dilute magnetic semiconductors, etc.

Spin Current due to Spinlike Andreev Reflection

We propose theoretically a spinlike Andreev reflection that may occur at the interface of a semimetal and a doped excitonic insulator. In contrast to the usual Andreev reflection, this process does not produce the charge current, but gives rise to a pure spin current. Further investigation reveals the existence of a spinlike Josephson phenomenon between two excitonic insulators with ferromagnetism, which results in a dissipationless spin current. We stress that the present spin current satisfies the conservation condition and is quite different from the spin Hall current induced from spin-orbital interactions.

Spin-Hall Insulator

Recent theories predict dissipationless spin current induced by an electric field in doped semiconductors. Nevertheless, the charge current is still dissipative in these systems. In this work, we theoretically predict the dissipationless spin-Hall effect, without any accompanying charge current, in some classes of band insulators, including zero-gap semiconductors such as HgTe and narrow-gap semiconductors such as PbTe. This effect is similar to the quantum-Hall effect in that all the states below the gap contribute and there occurs no dissipation. However, the spin-Hall conductance is not quantized even in two dimensions. This is the first example of a nontrivial topological structure in a band insulator without any magnetic field.

Dissipationless spin current in anisotropicp-doped semiconductors

Recently, dissipationless spin current has been predicted for $p$-doped semiconductors with spin-orbit coupling. Here we investigate the effect of the breaking of spherical symmetry on the dissipationless spin current, and obtain values of the intrinsic spin Hall conducitivity for realistic semiconductor band structures with cubic symmetry.

SU(2)non-Abelian holonomy and dissipationless spin current in semiconductors

Following our previous work [S. Murakami, N. Nagaosa, S. C. Zhang, Science 301, 1348 (2003)] on the dissipationless quantum spin current, we present an exact quantum mechanical calculation of this novel effect based on the linear response theory and the Kubo formula. We show that it is possibxle to define an exactly conserved spin current, even in the presence of the spin-orbit coupling in the Luttinger Hamiltonian of p-type semiconductors. The light- and the heavy-hole bands form two Kramers doublets, and an SU(2) non-abelian gauge field acts naturally on each of the doublets. This quantum holonomy gives rise to a monopole structure in momentum space, whose curvature tensor directly leads to the novel dissipationless spin Hall effect, i.e., a transverse spin current is generated by an electric field. The result obtained in the current work gives a quantum correction to the spin current obtained in the previous semiclassical approximation.

Universal intrinsic spin Hall effect.

We describe a new effect in semiconductor spintronics that leads to dissipationless spin currents in paramagnetic spin-orbit coupled systems. We argue that in a high-mobility two-dimensional electron system with substantial Rashba spin-orbit coupling, a spin current that flows perpendicular to the charge current is intrinsic. In the usual case where both spin-orbit split bands are occupied, the intrinsic spin-Hall conductivity has a universal value for zero quasiparticle spectral broadening.

Persistent spin currents in helimagnets

We demonstrate that weak external magnetic fields generate dissipationless spin currents in the ground state of systems with spiral magnetic order. Our conclusions are based on phenomenological considerations and on microscopic mean-field theory calculations for an illustrative toy model. We speculate on possible applications of this effect in spintronic devices.

Dissipationless Spin Transport in Thin Film Ferromagnets

Metallic thin film ferromagnets generically possess spiral states that carry dissipationless spin currents. We relate the critical values of these supercurrents to micromagnetic material parameters, identify the circumstances under which the supercurrents will be most robust, and propose experiments which could reveal this new collective transport behavior.