arXiv subjects

Shun-Qing Shen

Publications and source records attributed to Shun-Qing Shen.

At least 145 records · Page 8Linked to original sources

arXiv2006

Spin transverse force and intrinsic quantum transverse transport

The spin-orbit coupling may generate spin transverse force on moving electron spin, which gives a heuristic picture for the quantum transverse transport of electron. A relation between the spin and anomalous Hall conductance and spin force was established, and applied to several systems. It was predicted that the sign change of anomalous Hall conductance can occur in diluted magnetic semiconductors of narrow band and can be applied to identify intrinsic mechanism experimentally.

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arXiv2006

The SU(3) bosons and the spin nematic state on the spin-1 bilinear-biquadratic triangular lattice

A bond-operator mean-field theory in the SU(3) bosons representation is developed to describe the antiferro-nematic phase of the spin-1 bilinear-biquadratic model. The calculated static structure factors reveal delicately that the antiferro-nematic state may exhibit both the ferro- and antiferro-quadruple long-range orders, which is reminiscent of the ferrimagnets or the canted antiferromagnets. This result may influence the spin wave theory concerned with this phase. Possible relevance of this unconventional state to the quasi-two-dimensional triangular material NiGa2S4 is addressed.

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arXiv2005

Spin-Hall effect: Back to the Beginning on a Higher Level

The phenomena of the spin-Hall effect, initially proposed over three decades ago in the context of asymmetric Mott skew scattering, was revived recently by the proposal of a possible intrinsic spin-Hall effect originating from a strongly spin-orbit coupled band structure. This new proposal has generated an extensive debate and controversy over the past two years. The purpose of this workshop, held at the Asian Pacific Center for Theoretical Physics, was to bring together many of the leading groups in this field to resolve such issues and identify future challenges. We offer this short summary to clarify the now settled issues on some of the more controversial aspects of the debate and help refocus the research efforts in new and important avenues.

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arXiv2005

Transverse electric current induced by optically injected spin current in cross-shaped InGaAs/InAlAs system

We examine electric response of a linearly polarized light normally shed on a cross-shaped quasi 2-dimensional InGaAs/InAlAs system with structure inversion asymmetry. The photo-excited conduction electrons carry a pure spin current with in-plane spin polarization due to the Rashba spin-orbit interaction. We use Landauer-Büttiker formalism to show that this spin current induces two inward or outward transverse charge currents, which are observable in experiments. This effect may serve as an experimental probe of certain types of spin current.

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arXiv2005

Spin transverse force on spin current in an electric field

As a relativistic quantum mechanical effect, it is shown that the electric field exerting a transverse force on an electron spin 1/2 only if the electron is moving and the spin is polarized along the electric field. The spin force, analogue to the Lorentz for on an electron charge in a magnetic field, is perpendicular to the electric field and the spin current polarized anong the electric field. This spin-dependent force can be used to understand the zitterbewegung of electron wave packet with spin-orbit coupling and is relevant to the generation of the charge Hall effect driven by the spin current in semiconductors.

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arXiv2005

Spin and orbital valence bond solids in a one-dimensional spin-orbital system: Schwinger boson mean field theory

A generalized one-dimensional $SU(2)\times SU(2)$ spin-orbital model is studied by Schwinger boson mean-field theory (SBMFT). We explore mainly the dimer phases and clarify how to capture properly the low temperature properties of such a system by SBMFT. The phase diagrams are exemplified. The three dimer phases, orbital valence bond solid (OVB) state, spin valence bond solid (SVB) state and spin-orbital valence bond solid (SOVB) state, are found to be favored in respectively proper parameter regions, and they can be characterized by the static spin and pseudospin susceptibilities calculated in SBMFT scheme. The result reveals that the spin-orbit coupling of $SU(2)\times SU(2)$ type serves as both the spin-Peierls and orbital-Peierles mechanisms that responsible for the spin-singlet and orbital-singlet formations respectively.

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arXiv2005

Charge Hall effect driven by spin-dependent chemical potential gradients and Onsager relations in mesoscopic systems

We study theoretically the spin-Hall effect as well as its reciprocal phenomenon (a transverse charge current driven by a spin-dependent chemical potential gradient) in electron and hole finite size mesoscopic systems. The Landauer-Buttiker-Keldysh formalism is used to model samples with mobilities and Rashba coupling strengths which are experimentally accessible and to demonstrate the appearance of measurable charge currents induced by the spin-dependent chemical potential gradient in the reciprocal spin-Hall effect. We also demonstrate that within the mesoscopic coherent transport regime the Onsager relations are fulfilled for the disorder averaged conductances for electron and hole mesoscopic systems.

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arXiv2005

Edge Spin Current and Spin Polarization in Quantum Hall Regime

We study the edge spin current and spin polarization in a two-dimensional electron gas with spin-orbit coupling in the presence of a perpendicular magnetic field. The edge effect removes the degeneracy of Landau levels due to the competition between the Rashba coupling and the Zeeman splitting, which exists in the bulk. Large spin edge current and non-linear response to an electric field are found when the two levels are degenerated in the bulk. It is found that he spin current is proportional to spin polarization along the y-direction in an finite magnetic field, which provides a way to extract the spin current in the system.

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arXiv2005

Spin resolved Hall effect driven by spin-orbit coupling

Spin and electric Hall currents are calculated numerically in a two-dimensional mesoscopic system with Rashba and Dresselhaus spin-orbit coupling by means of the Landauer-Buttiker formalism. It is found that both electric and spin Hall currents circulate when two spin-orbit couplings coexist, while the electric Hall conductance vanishes if either one is absent. The electric and spin Hall conductances are suppressed in strong disorder, but survive in weak disorder. Physically it can be understood that the spinomotive transverse "force" generated by spin-orbit coupling is responsible for the formation of the spin Hall current and the lack of transverse reflection symmetry is the origin of the electric Hall current.

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arXiv2005

Contractor renormalization group theory of the SU($N$) chains and ladders

Contractor renormalization group (CORE) method is applied to the SU($N$) chain and ladders in this paper. In our designed schemes, we show that these two classes of systems can return to their original form of Hamiltonian after CORE transformation. Successive iteration of the transformation leads to a fixed point so that the ground state energy and the energy gap to the ground state can be deduced. The result of SU($N$) chain is compared with the one by Bethe ansatz method. The transformation on spin-1/2 ladders gives a finite gap in the excited energy spectra to the ground state in an intuitive way. The application to SU(3) ladders is also discussed.

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arXiv2004

Two-dimensional gapless spin liquids in frustrated SU(N) quantum magnets

A class of the symmetrically frustrated SU(N) models is constructed for quantum magnets based on the generators of SU(N) group. The total Hamiltonian lacks SU(N) symmtry. A mean field theory in the quasi-particle representation is developed for spin liquid states. Numerical solutions in two dimension indicate that the ground states are gapless and the quasi-particles are Dirac particles. The mechanism may be helpful in exploring the spin liquid phases in the spin-1 bilinear-biquadratic model and the spin-orbital model in higher dimensions.

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arXiv2004

Effects of spin imbalance on the electric-field driven quantum dissipationless spin current in $p$-doped Semiconductors

It was proposed recently by Murakami et al. [Science \textbf{301}, 1348(2003)] that in a large class of $p$-doped semiconductors, an applied electric field can drive a quantum dissipationless spin current in the direction perpendicular to the electric field. In this paper we investigate the effects of spin imbalance on this intrinsic $spin$ Hall effect. We show that in a real sample with boundaries, due to the presence of spin imbalance near the edges of the sample, the spin Hall conductivity is not a constant but a sensitively $position$-$dependent$ quantity, and due to this fact, in order to take the effects of spin imbalance properly into account, a microscopic calculation of both the quantum dissipationless spin Hall current and the spin accumulation on an equal footing is thus required. Based on such a microscopic calculation, a detailed discussion of the effects of spin imbalance on the intrinsic spin Hall effect in thin slabs of $p$-doped semiconductors are presented.

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arXiv2004

Resonant spin Hall conductance in quantum Hall systems lacking bulk and structural inversion symmetry

Following a previous work [Shen, Ma, Xie and Zhang, Phys. Rev. Lett. 92, 256603 (2004)] on the resonant spin Hall effect, we present detailed calculations of the spin Hall conductance in two-dimensional quantum wells in a strong perpendicular magnetic field. The Rashba coupling, generated by spin-orbit interaction in wells lacking bulk inversion symmetry, introduces a degeneracy of Zeeman-split Landau levels at certain magnetic fields. This degeneracy, if occuring at the Fermi energy, will induce a resonance in the spin Hall conductance below a characteristic temperature of order of the Zeeman energy. At very low temperatures, the spin Hall current is highly non-ohmic. The Dresselhaus coupling due to the lack of structure inversion symmetry partially or completely suppresses the spin Hall resonance. The condition for the resonant spin Hall conductance in the presence of both Rashba and Dresselhaus couplings is derived using a perturbation method. In the presence of disorder, we argue that the resonant spin Hall conductance occurs when the two Zeeman split extended states near the Fermi level becomes degenerate due to the Rashba coupling and that the the quantized charge Hall conductance changes by 2e^2/h instead of e^2/h as the magnetic field changes through the resonant field.

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arXiv2004

Spin Hall effect and Berry phase in two dimensional electron gas

The spin Hall effect is investigated in a high mobility two dimensional electron system with the spin-orbital coupling of both the Rashba and the Dresselhaus types. A spin current perpendicular to the electric field is generated by either the Rashba or the Dresselhaus coupling. The spin Hall conductance is independent of the stength of the coupling, but its sign is determined by the relative ratio of the two couplings. The direction of spin current is controllable by tuning the magnitude of the surface electric field perpendicular to the two dimensional plane via adjusting the Rashba coupling. It is observed that the spin Hall conductance has a close relation to the Berry phase of conduction electrons.

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arXiv2004

Controllable quantum spin precession by Aharonov-Casher phase in conducting ring

We investigate quantum spin transport in a structure of conducting ring, embedded in textured electric field, with two leads, and obtain an exact solution for the problem. The spin precession induced by the Aharonov-Casher phase is studied. It is shown that the spin-polarized current and its polarizability can be controlled by the electric field. As a result the polarizability is a function of the geometric phase which originates from the spin-orbital interaction in the ring. 72.25.-b, 03.65.Vf

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arXiv2004

Influences of spin accumulation on the intrinsic spin Hall effect in two dimensional electron gases with Rashba spin-orbit coupling

In a two dimensional electron gas with Rashba spin-orbit coupling, the external electric field may cause a spin Hall current in the direction perpendicular to the electric field. This effect was called the intrinsic spin Hall effect. In this paper, we investigate the influences of spin accumulation on this intrinsic spin Hall effect. We show that due to the existence of boundaries in a real sample, the spin Hall current generated by the intrinsic spin Hall effect will cause spin accumulation near the edges of the sample, and in the presence of spin accumulation, the spin Hall conductivity will not have a universal value. The influences of spin accumulation on the intrinsic spin Hall effect in narrow strips of two dimensional electron gases with Rashba spin-orbit coupling are investigated in detail.

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arXiv2004

Conductance modulation in spin field-efect transistors under finite bias voltages

The conductance modulations in spin field-effect transistors under finite bias voltages were studied. It was shown that when a finite bias voltage is applied between two terminals of a spin field-effect transistor, the spin precession states of injected spin-polarized electrons in the semiconductor channel of the device will depend not only the gate-voltage controlled Rashba spin-orbit coupling but also depend on the bias voltage and, hence, the conductance modulation in the device due to Rashba spin-orbit coupling may also depend sensitively on the bias voltage.

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arXiv2004

Charge ordering and phase separation in the infinite dimensional extended Hubbard model

We study the extended Hubbard model with both on-site (U) and nearest neighbor (V) Coulomb repulsion using the exact diagonalization method within the dynamical mean field theory. For a fixed U (U=2.0), the T-n phase-diagrams are obtained for V=1.4 and V=1.2, at which the ground states of n=1/2 system is charge-ordered and charge-disordered, respectively. In both cases, robust charge order is found at finite temperature and in an extended filling regime around n=1/2. The order parameter changes non-monotonously with temperature. For V=1.4, phase separation between charge-ordered and charge-disordered phases is observed in the low temperature and n < 0.5 regime. It is described by an "S"-shaped structure of the n-/mu curve. For V=1.2, the ground state is charge-disordered, and a reentrant charge-ordering transition is observed for 0.42 < n < 0.68. Relevance of our results to experiments for doped manganites is discussed.

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