arXiv subjects

Ming-Hao Liu

Publications and source records attributed to Ming-Hao Liu.

69 records · Page 4Linked to original sources

arXiv2011

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.

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arXiv2010

Anomalous spin Hall effects in Dresselhaus (110) quantum wells

Anomalous spin Hall effects that belong to the intrinsic type in Dresselhaus (110) quantum wells are discussed. For the out-of-plane spin component, antisymmetric current-induced spin polarization induces opposite spin Hall accumulation, even though there is no spin-orbit force due to Dresselhaus (110) coupling. A surprising feature of this spin Hall induction is that the spin accumulation sign does not change upon bias reversal. Contribution to the spin Hall accumulation from the spin Hall induction and the spin deviation due to intrinsic spin-orbit force as well as extrinsic spin scattering, can be straightforwardly distinguished simply by reversing the bias. For the inplane component, inclusion of a weak Rashba coupling leads to a new type of $S_y$ intrinsic spin Hall effect solely due to spin-orbit-force-driven spin separation.

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arXiv2009

Upstanding Rashba spin in honeycomb lattices: Electrically reversible surface spin polarization

The spin-split states subject to Rashba spin-orbit coupling in two-dimensional systems have long been accepted as pointing inplane and perpendicular to the corresponding wave vectors. This is in general true for free electron model, but exceptions do exist elsewhere. Within the tight-binding model, we unveil the unusual upstanding behavior of those Rashba spins around $\bar{K}$ and $\bar{K}^{\prime}$ points in honeycomb lattices. Our calculation (i) explains the recent experiment of the Tl/Si(111)-$(1\times1)$ surface alloy [Phys. Rev. Lett. \textbf{102}, 096805 (2009)], where abrupt upstanding spin states near $\bar{K}$ are observed, and (ii) predicts an electrically reversible out-of-plane surface spin polarization.

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arXiv2008

Current-induced spin polarization in spin-orbit-coupled two-dimensional electron systems

Current-induced spin polarization (CISP) is rederived in ballistic spin-orbit-coupled electron systems, based on equilibrium statistical mechanics. A simple and useful picture is correspondingly proposed to help understand the CISP and predict the polarization direction. Nonequilibrium Landauer-Keldysh formalism is applied to demonstrate the validity of the statistical picture, taking the linear Rashba-Dresselhaus [001] two-dimensional system as a specific example. Spin densities induced by the CISP in semiconductor heterostructures and in metallic surface states are compared, showing that the CISP increases with the spin splitting strength and hence suggesting that the CISP should be more observable on metal and semimetal surfaces due to the discovered strong Rashba splitting. An application of the CISP designed to generate a spin-Hall pattern in the inplane, instead of the out-of-plane, component is also proposed.

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arXiv2008

Nonequilibrium spin transport on Au(111) surfaces

The well-known experimentally observed \textit{sp}-derived Au(111) Shockley surface states with Rashba spin splitting are perfectly fit by an effective tight-binding model, considering a two-dimensional hexagonal lattice with $p_{z}$-orbital and nearest neighbor hopping only. The extracted realistic band parameters are then imported to perform the Landauer-Keldysh formalism to calculate nonequilibrium spin transport in a two-terminal setup sandwiching a Au(111) surface channel. Obtained results show strong spin density on the Au(111) surface and demonstrate (i) intrinsic spin-Hall effect, (ii) current-induced spin polarization, and (iii) Rashba spin precession, all of which have been experimentally observed in semiconductor heterostructures, but not in metallic surface states. We therefore urge experiments in the latter for these spin phenomena.

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arXiv2007

Intrinsic spin-Hall accumulation in honeycomb lattices: Band structure effects

Local spin and charge densities on a two-dimensional honeycomb lattice are calculated by the Landauer-Keldysh formalism (LKF). Through the empirical tight-binding method, we show how the realistic band structure can be brought into the LKF. Taking the Bi(111) surface, on which strong surface states and Rashba spin-orbit coupling are present [Phys. Rev. Lett. 93, 046403 (2004)], as a numeric example, we show typical intrinsic spin-Hall accumulation (ISHA) patterns thereon. The Fermi-energy-dependence of the spin and charge transport in two-terminal nanostructure samples is subsequently analyzed. By changing E_{F}, we show that the ISHA pattern is nearly isotropic (free-electron-like) only when E_{F} is close to the band bottom, and is sensitive/insensitive to E_{F} for the low/high bias regime with such E_{F}. With E_{F} far from the band bottom, band structure effects thus enter the ISHA patterns and the transport direction becomes significant.

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arXiv2007

Local spin density in two-dimensional electron gas with hexagonal boundary

The intrinsic spin-Hall effect in hexagon-shaped samples is investigated. To take into account the spin-orbit couplings and to fit the hexagon edges, we derive the triangular version of the tight-binding model for the linear Rashba [Sov. Phys. Solid State 2, 1109 (1960)] and Dresselhaus [Phys. Rev. 100, 580 (1955)] [001] Hamiltonians, which allow direct application of the Landauer-Keldysh non-equilibrium Green function formalism to calculating the local spin density within the hexagonal sample. Focusing on the out-of-plane component of spin, we obtain the geometry-dependent spin-Hall accumulation patterns, which are sensitive to not only the sample size, the spin-orbit coupling strength, the bias strength, but also the lead configurations. Contrary to the rectangular samples, the accumulation pattern can be very different in our hexagonal samples. Our present work provides a fundamental description of the geometry effect on the intrinsic spin-Hall effect, taking the hexagon as the specific case. Moreover, broken spin-Hall symmetry due to the coexistence of the Rashba and Dresselhaus couplings is also discussed. Upon exchanging the two coupling strengths, the accumulation pattern is reversed, confirming the earlier predicted sign change in spin-Hall conductivity.

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arXiv2007

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.

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arXiv2007

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.

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arXiv2006

Persistent spin helix in Rashba-Dresselhaus two-dimensional electron systems

A persistent spin helix (PSH) in spin-orbit-coupled two-dimensional electron systems was recently predicted to exist in two cases: [001] quantum wells (QWs) with equal coupling strengths of the Rashba and the Dresselhaus interactions (RD), and Dresselhaus-only [110] QWs. Here we present supporting results and further investigations, using our previous results [Phys. Rev. B 72, 153305 (2005)]. Refined PSH patterns for both RD [001] and Dresselhaus [110] QWs are shown, such that the feature of the helix is clearly seen. We also discuss the time dependence of spin to reexamine the origin of the predicted persistence of the PSH. For the RD [001] case, we further take into account the random Rashba effect, which is much more realistic than the constant Rashba model. The distorted PSH pattern thus obtained suggests that such a PSH may be more observable in the Dresselhaus [110] QWs, if the dopants cannot be regularly enough distributed.

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arXiv2006

Nonuniform Rashba-Dresselhaus spin precession along arbitrary paths

Electron spin precession in nonuniform Rashba-Dresselhaus two-dimensional electron systems along arbitrary continuous paths is investigated. We derive an analytical formula to describe the spin vectors (expectation values of the injected spin) in such conditions using a contour-integral method. The obtained formalism is capable of dealing with the nonuniformity of the Rashba spin-orbit field due to the inherent random distribution of the ionized dopants, and can be applied to curved one-dimensional quantum wires. Interesting examples are given, and the modification to the spin precession pattern in a Rashba-Dresselhaus channel when taking the random Rashba field into account is shown.

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arXiv2006

Spin precession in inversion-asymmetric two-dimensional systems

We present a theoretical method to calculate the expectation value of spin in an inversion-asymmetric two-dimensional (2D) system with respect to an arbitrarily spin-polarized electron state, injected via an ideal point contact. The 2D system is confined in a [001]-grown quantum well, where both the Rashba and the Dresselhaus spin-orbit couplings are taken into account. The obtained analytical results allow more concrete description of the spatial behaviors of the spin precession caused by individually the Rashba and the Dresselhaus terms. Applying the calculation on the Datta-Das spin-FET, whose original design considers only the Rashba effect inside the channel, we investigate the possible influence due to the Dresselhaus spin-orbit coupling. Concluded solution is the choice of +-[1+-10], in particular [110], as the channel direction.

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arXiv2006

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.

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arXiv2006

Datta-Das transistor: Significance of channel direction, size-dependence of source contacts, and boundary effects

We analyze the spin expectation values for injected spin-polarized electrons (spin vectors) in a [001]-grown Rashba-Dresselhaus two-dimensional electron gas (2DEG). We generalize the calculation for point spin injection in semi-infinite 2DEGs to finite-size spin injection in bounded 2DEGs. Using the obtained spin vector formula, significance of the channel direction for the Datta-Das transistor is illustrated. Numerical results indicate that the influence due to the finite-size injection is moderate, while the channel boundary reflection may bring unexpected changes. Both effects are concluded to decrease when the spin-orbit coupling strength is strong. Hence [110] is a robust channel direction and is therefore the best candidate for the design of the Datta-Das transistor.

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arXiv2004

Spin precession due to spin-orbit coupling in a two-dimensional electron gas with spin injection via ideal quantum point contact

We present the analytical result of the expectation value of spin resulting from an injected spin polarized electron into a semi-infinitely extended 2DEG plane with [001] growth geometry via ideal quantum point contact. Both the Rashba and Dresselhaus spin-orbit couplings are taken into account. A pictorial interpretation of the spin precession along certain transport directions is given. The spin precession due to the Rashba term is found to be especially interesting since it behaves simply like a windshield wiper which is very different from the ordinary precession while that due to the Dresselhaus term is shown to be crystallographic-direction-dependent. Some crystallographic directions with interesting and handleable behavior of spin precession are found and may imply certain applicability in spintronic devices.

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