Non-Abelian spin-orbit gauge: Persistent spin helix and quantum square ring

We re-express the Rashba and Dresselhaus interactions as non-Abelian spin-orbit gauges and provide a new perspective in understanding the persistent spin helix [Phys. Rev. Lett. 97, 236601 (2006)]. A spin-orbit interacting system can be transformed into a free electron gas in the equal-strength Rashba-Dresselhaus [001] linear model, the Dresselhaus [110] linear model, and a one-dimensional system. A general tight-binding Hamiltonian for non-uniform spin-orbit interactions and hoppings along arbitrary directions, within the framework of finite difference method, is obtained. As an application based on this Hamiltonian, a quantum square ring in contact with two ideal leads is found to exhibit four states, insulating, spin-filtering, spin-flipping, and spin-keeping states.Comment: 7 pages, 3 figure

Birth and Early Growth of Entanglement by sd Exchange with Gate-Voltage-Controllable Destiny

We investigate bipartite entanglement between two distant parties, \textit{A} and \textit{B}, comprising local magnetic impurities (or qudits) induced by the quench through \textit{sd} exchange in a field-effect-transistor geometry. A wave-function-based time-dependent formalism is employed by including non-dissipative responses that allow for the control of entanglement via gate voltages. Our study focuses on the birth and early growth of entanglement, by introducing environment support states that render site- and layer-resolved logarithmic negativity (LN) and mutual information (MI). In the minimal set, where party \textit{A} (\textit{B}) consists of a qubit, we identify entanglement sudden deaths (ESDs), which are explained by a visualization picture analyzing the density matrix. Vibrating electron currents facilitate the birth of entanglement, while they are not required for its growth and subsistence. The LN emerges near the edge layers in \textit{A} and \textit{B}, while MI shows up outside these two parties within the spacing layer. The MI is born earlier than the LN. When a gate voltage large enough to disjoint part of the system is applied within the spacing region, it partially suppresses the entanglement, quantified by the LN. This suppression does not appear immediately after the presence of the disjoint voltage. Applying this disjoint voltage to the site(s) hosting the qudit(s) helps prevent the site- and layer-resolved LN from encountering ESDs. The local impurities in parties \textit{A} and \textit{B} are initially of opposite spin directions in an unentangled state, as can be prepared by two of our proposed protocols. However, the features described above do not depend on the chosen protocols.Comment: 10 figure

Quantum World-line Monte Carlo Method with Non-binary Loops and Its Application

A quantum world-line Monte Carlo method for high-symmetrical quantum models is proposed. Firstly, based on a representation of a partition function using the Matsubara formula, the principle of quantum world-line Monte Carlo methods is briefly outlined and a new algorithm using non-binary loops is given for quantum models with high symmetry as SU(N). The algorithm is called non-binary loop algorithm because of non-binary loop updatings. Secondary, one example of our numerical studies using the non-binary loop updating is shown. It is the problem of the ground state of two-dimensional SU(N) anti-ferromagnets. Our numerical study confirms that the ground state in the small N <= 4 case is a magnetic ordered Neel state, but the one in the large N >= 5 case has no magnetic order, and it becomes a dimer state.Comment: 14 pages, 5 figures, Invited talk at the 18th Annual Workshop on Recent Developments of Computer Simulation Studies in Condensed Matter Physics, Athens, 7-11 March, 200

Rashba Spin Interferometer

A spin interferometer utilizing the Rashba effect is proposed. The novel design is composed of a one-dimensional (1D) straight wire and a 1D half-ring. By calculating the norm of the superposed wave function, we derive analytical expressions to describe the spin interference spectrum as a function of the Rashba coupling strength. Presented spin interference results are identified to include (i) the quantum-mechanical 4pi rotation effect, (ii) geometric effect, and (iii) Shubnikov-de Haas-like beating effect.Comment: 3 pages, 3 figures, appears in the proceedings of the 10th Joint MMM/Intermag Conferenc

Microwave-driven ferromagnet--topological-insulator heterostructures: The prospect for giant spin battery effect and quantized charge pump devices

We study heterostructures where a two-dimensional topological insulator (TI) is attached to two normal metal (NM) electrodes while an island of a ferromagnetic insulator (FI) with precessing magnetization covers a portion of its lateral edges to induce time-dependent exchange field underneath via the magnetic proximity effect. When the FI island covers both lateral edges, such device pumps pure spin current in the absence of any bias voltage, thereby acting as an efficient spin battery with giant output current even at very small microwave power input driving the precession. When only one lateral edge is covered by the FI island, both charge and spin current are pumped into the NM electrodes. We delineate conditions for the corresponding conductances (current-to-microwave-frequency ratio) to be quantized in a wide interval of precession cone angles, which is robust with respect to weak disorder and can be further extended by changes in device geometry.Comment: 7 pages, 7 color figures, PDFLaTe

Broken spin-Hall accumulation symmetry by magnetic field and coexisted Rashba and Dresselhaus interactions

The spin-Hall effect in the two-dimensional electron gas (2DEG) generates symmetric out-of-plane spin Sz accumulation about the current axis in the absence of external magnetic field. Here we employ the real space Landauer-Keldysh formalism [B. K. Nikolic et al., Phys. Rev. Lett. 95, 046601 (2005); Phys. Rev. B 73, 075303 (2006)] by considering a four-terminal setup to investigate the circumstances in which this symmetry is broken. For the absence of Dresselhaus interaction, starting from the applied out-of-plane B corresponding to Zeeman splitting energy 0 - 0.5 times the Rashba hopping energy tR, the breaking process is clearly seen. The influence of the Rashba interaction on the magnetization of the 2DEG is studied herein. For coexisted Rashba tR and Dresselhaus tD spin-orbit couplings in the absence of B, interchanging tR and tD reverses the entire accumulation pattern.Comment: 3 pages, 2 figures, appears in the proceedings of 10th MMM/INTERMAG conferenc

Precessionless spin transport wire confined in quasi-two-dimensional electron systems

We demonstrate that in an inversion-asymmetric two-dimensional electron system 2DES with both Rashba and Dresselhaus spin-orbit couplings taken into account, certain transport directions on which no spin precession occurs can be found when the injected spin is properly polarized. By analyzing the expectation value of spin with respect to the injected electron state on each space point in the 2DES, we further show that the adjacent regions with technically reachable widths along these directions exhibit nearly conserved spin. Hence a possible application in semiconductor spintronics, namely, precessionless spin transport wire, is proposed.Comment: 3 pages, 4 figures, to be appeared in Journal of Applied Physics, Proceedings of the 50th MMM Conferenc

Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach

We study spin and charge currents pumped by precessing magnetization of a single ferromagnetic layer within F|I|N or F|I|F (F-ferromagnet; I-insulator; N-normal-metal) multilayers of nanoscale thickness attached to two normal metal electrodes with no applied bias voltage between them. Both simple one-dimensional model, consisting of a single precessing spin and a potential barrier as the "sample," and realistic three-dimensional devices are investigated. In the rotating reference frame, where the magnetization appears to be static, these junctions are mapped onto a four-terminal dc circuit whose effectively half-metallic ferromagnetic electrodes are biased by the frequency $\hbar \omega/e$ of microwave radiation driving magnetization precession at the ferromagnetic resonance (FMR) conditions. We show that pumped spin current in F|I|F junctions, diminished behind the tunnel barrier and increased in the opposite direction, is filtered into charge current by the second $F$ layer to generate dc pumping voltage of the order of $\sim 1$ $\mu$V (at FMR frequency $\sim 10$ GHz) in an open circuit. In F|I|N devices, several orders of magnitude smaller charge current and the corresponding dc voltage appear concomitantly with the pumped spin current due to barrier induced asymmetry in the transmission coefficients connecting the four electrodes in the rotating frame picture of pumping.Comment: 8 pages, 5 figure

Inverse quantum spin Hall effect generated by spin pumping from precessing magnetization into a graphene-based two-dimensional topological insulator

We propose a multiterminal nanostructure for electrical probing of the quantum spin Hall effect (QSHE) in two-dimensional (2D) topological insulators. The device consists of a ferromagnetic (FM) island with precessing magnetization that pumps (in the absence of any bias voltage) pure spin current symmetrically into the left and right adjacent 2D TIs modeled as graphene nanoribbons with the intrinsic spin-orbit (SO) coupling. The QSH regime of the six-terminal TI|FM|TI nanodevice, attached to two longitudinal and four transverse normal metal electrodes, is characterized by the SO-coupling-induced energy gap, chiral spin-filtered edge states within finite length TI regions, and quantized spin Hall conductance when longitudinal bias voltage is applied, despite the presence of the FM island. The same unbiased device, but with precessing magnetization of the central FM island, blocks completely pumping of total spin and charge currents into the longitudinal electrodes while generating DC transverse charge Hall currents. Although these transverse charge currents are not quantized, their induction together with zero longitudinal charge current is a unique electrical response of TIs to pumped pure spin current that cannot be mimicked by SO-coupled but topologically trivial systems. In the corresponding two-terminal inhomogeneous TI|FM|TI nanostructures, we image spatial profiles of local spin and charge currents within TIs which illustrate transport confined to chiral spin-filtered edges states while revealing concomitantly the existence of interfacial spin and charge currents flowing around TI|FM interfaces and penetrating into the bulk of TIs over some short distance.Comment: 11 pages, 8 figures; published expanded version with new figures on spatial profiles of local spin and charge current

Spin and charge transport in U-shaped one-dimensional channels with spin-orbit couplings

A general form of the Hamiltonian for electrons confined to a curved one-dimensional (1D) channel with spin-orbit coupling (SOC) linear in momentum is rederived and is applied to a U-shaped channel. Discretizing the derived continuous 1D Hamiltonian to a tight-binding version, the Landauer-Keldysh formalism (LKF) for nonequilibrium transport can be applied. Spin transport through the U-channel based on the LKF is compared with previous quantum mechanical approaches. The role of a curvature-induced geometric potential which was previously neglected in the literature of the ring issue is also revisited. Transport regimes between nonadiabatic, corresponding to weak SOC or sharp turn, and adiabatic, corresponding to strong SOC or smooth turn, is discussed. Based on the LKF, interesting charge and spin transport properties are further revealed. For the charge transport, the interplay between the Rashba and the linear Dresselhaus (001) SOCs leads to an additional modulation to the local charge density in the half-ring part of the U-channel, which is shown to originate from the angle-dependent spin-orbit potential. For the spin transport, theoretically predicted eigenstates of the Rashba rings, Dresselhaus rings, and the persistent spin-helix state are numerically tested by the present quantum transport calculation.Comment: 16 pages, 7 figure