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Shou-Cheng Zhang

Publications and source records attributed to Shou-Cheng Zhang.

18 recordsLinked to original sources
PubMed2006

Quantum spin Hall effect and topological phase transition in HgTe quantum wells.

We show that the quantum spin Hall (QSH) effect, a state of matter with topological properties distinct from those of conventional insulators, can be realized in mercury telluride-cadmium telluride semiconductor quantum wells. When the thickness of the quantum well is varied, the electronic state changes from a normal to an "inverted" type at a critical thickness d(c). We show that this transition is a topological quantum phase transition between a conventional insulating phase and a phase exhibiting the QSH effect with a single pair of helical edge states. We also discuss methods for experimental detection of the QSH effect.

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PubMed2006

Helical liquid and the edge of quantum spin Hall systems.

The edge states of the recently proposed quantum spin Hall systems constitute a new symmetry class of one-dimensional liquids dubbed the "helical liquid," where the spin orientation is determined by the direction of electron motion. We prove a no-go theorem which states that a helical liquid with an odd number of components cannot be constructed in a purely 1D lattice system. In a helical liquid with an odd number of components, a uniform gap in the ground state can appear when the time-reversal symmetry is spontaneously broken by interactions. On the other hand, a correlated two-particle backscattering term by an impurity can become relevant while keeping the time-reversal invariance.

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PubMed2006

Quantum spin Hall effect.

The quantum Hall liquid is a novel state of matter with profound emergent properties such as fractional charge and statistics. The existence of the quantum Hall effect requires breaking of the time reversal symmetry caused by an external magnetic field. In this work, we predict a quantized spin Hall effect in the absence of any magnetic field, where the intrinsic spin Hall conductance is quantized in units of 2(e/4pi). The degenerate quantum Landau levels are created by the spin-orbit coupling in conventional semiconductors in the presence of a strain gradient. This new state of matter has many profound correlated properties described by a topological field theory.

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PubMed2005

Spin-orbit coupling-induced magnetic phase in the d-density-wave phase of La(2-x)Ba(x)CuO4 superconductors.

We study the effects of spin-orbit coupling in the d-density wave (DDW) phase. In the low-temperature orthorhombic phase of La(2-x)Ba(x)CuO4, we find that spin-orbit coupling induces ferromagnetic moments in the DDW phase, which are polarized along the [110] direction with a considerable magnitude. This effect does not exist in the superconducting phase. On the other hand, if the d-density wave order does not exist at zero field, a magnetic field along the [110] direction always induces such a staggered orbital current. We discuss experimental constraints on the DDW states in light of our theoretical predictions.

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PubMed2005

Maxwell equation for coupled spin-charge wave propagation.

We show that the dissipationless spin current in the ground state of the Rashba model gives rise to a reactive coupling between the spin and charge propagation, which is formally identical to the coupling between the electric and the magnetic fields in the (2 + 1)-dimensional Maxwell equation. This analogy leads to a remarkable effect of fractionalization of spin packets (FSP) where a density packet can spontaneously split into two counterpropagation packets, each carrying the opposite spin. In a certain parameter regime, the coupled spin and charge wave propagates like a transverse "photon." We propose both optical and purely electronic experiments to detect the FSP effect.

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PubMed2005

Orbitronics: the intrinsic orbital current in p-doped silicon.

The spin Hall effect depends crucially on the intrinsic spin-orbit coupling of the energy band. Because of the smaller spin-orbit coupling in silicon, the spin Hall effect is expected to be much reduced. We show that an electric field in p-doped silicon can induce a dissipationless orbital current in a fashion reminiscent of the spin Hall effect. The vertex correction from impurity scattering vanishes and the effect is robust against disorder. The orbital Hall effect leads to accumulation of local orbital momentum at the edge of the sample, and can be detected by the Kerr effect.

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PubMed2005

Intrinsic spin Hall effect in the two-dimensional hole gas.

We show that two types of spin-orbit coupling in the 2 dimensional hole gas, with and without inversion symmetry breaking, contribute to the intrinsic spin-Hall effect. Furthermore, the vertex correction due to impurity scattering vanishes in both cases, in sharp contrast to the case of usual Rashba coupling in the electron band. Recently, the spin-Hall effect in a hole doped GaAs semiconductor has been observed experimentally by Wunderlich et al. [ Phys. Rev. Lett. 94, 047204 (2005).]. From the fact that the lifetime broadening is smaller than the spin splitting, and the fact impurity vertex corrections vanish in this system, we argue that the observed spin-Hall effect should be in the intrinsic regime.

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PubMed2005

Magic doping fractions for high-temperature superconductors.

We report hole-doping dependence of the in-plane resistivity rho(ab) in a cuprate superconductor La(2-x)Sr(x)CuO4, carefully examined using a series of high-quality single crystals. Our detailed measurements find a tendency towards charge ordering at particular rational hole-doping fractions of 1/16, 3/32, 1/8, and 3/16. This observation appears to suggest a specific form of charge order and is most consistent with the recent theoretical prediction of the checkerboard-type ordering of the Cooper pairs at rational doping fractions x = (2m+1)/2n, with integers m and n.

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PubMed2004

Pair density wave in the pseudogap state of high temperature superconductors.

Recent scanning tunneling microscopy experiments of Bi(2)Sr(2)CaCu(2)O(8+delta) have shown evidence of real-space organization of electronic states at low energies in the pseudogap state [Science 303, 1995 (2004)]]. We argue based on symmetry considerations as well as model calculations that the experimentally observed modulations are due to a density wave of d-wave Cooper pairs without global phase coherence. We show that scanning tunneling microscopy measurements can distinguish a pair density wave from more typical electronic modulations such as those due to charge density wave ordering or scattering from an on site periodic potential.

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PubMed2004

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.

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PubMed2004

Dynamic generation of spin-orbit coupling.

Spin-orbit coupling plays an important role in determining the properties of solids, and is crucial for spintronics device applications. Conventional spin-orbit coupling arises microscopically from relativistic effects described by the Dirac equation, and is described as a single particle band effect. In this work, we propose a new mechanism in which spin-orbit coupling can be generated dynamically in strongly correlated, nonrelativistic systems as the result of Fermi surface instabilities in higher angular momentum channels. Various spin-orbit couplings can emerge in these new phases, and their magnitudes can be continuously tuned by temperature or other quantum parameters.

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PubMed2004

Quantitative test of SO(5) symmetry in the vortex state of Nd(1.85)Ce(0.15)CuO4.

By numerically solving models with competing superconducting and antiferromagnetic orders, we study the magnetic field dependence of the antiferromagnetic moment in both the weak and strong field regimes. Through a comparison with the neutron scattering results of Kang et al. and Matsuura et al. on Nd(1.85)Ce(0.15)CuO4, we conclude that this system is close to a SO(5) symmetric critical point. We also make a quantitative prediction on increasing the upper critical field B(c2) and the superconducting transition temperature T(c) by applying an in-plane magnetic field.

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PubMed2003

Eight-dimensional quantum Hall effect and "Octonions".

We construct a generalization of the quantum Hall effect where particles move in an eight-dimensional space under an SO(8) gauge field. The underlying mathematics of this particle liquid is that of the last normed division algebra, the octonions. Two fundamentally different liquids with distinct configuration spaces can be constructed, depending on whether the particles carry spinor or vector SO(8) quantum numbers. One of the liquids lives on a 20-dimensional manifold with an internal component of SO(7) holonomy, whereas the second liquid lives on a 14-dimensional manifold with an internal component of G2 holonomy.

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PubMed2003

Dissipationless quantum spin current at room temperature.

Although microscopic laws of physics are invariant under the reversal of the arrow of time, the transport of energy and information in most devices is an irreversible process. It is this irreversibility that leads to intrinsic dissipations in electronic devices and limits the possibility of quantum computation. We theoretically predict that the electric field can induce a substantial amount of dissipationless quantum spin current at room temperature, in hole-doped semiconductors such as Si, Ge, and GaAs. On the basis of a generalization of the quantum Hall effect, the predicted effect leads to efficient spin injection without the need for metallic ferromagnets. Principles found here could enable quantum spintronic devices with integrated information processing and storage units, operating with low power consumption and performing reversible quantum computation.

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PubMed2003

Antiferromagnetic order as the competing ground state in electron-doped Nd1.85Ce0.15CuO4.

Superconductivity in the high-transition-temperature (high-T(c)) copper oxides competes with other possible ground states. The physical explanation for superconductivity can be constrained by determining the nature of the closest competing ground state, and establishing if that state is universal among the high-T(c) materials. Antiferromagnetism has been theoretically predicted to be the competing ground state. A competing ground state is revealed when superconductivity is destroyed by the application of a magnetic field, and antiferromagnetism has been observed in hole-doped materials under the influence of modest fields. None of the previous experiments have revealed the quantum phase transition from the superconducting state to the antiferromagnetic state, because they failed to reach the upper critical field B(c2). Here we report the results of transport and neutron-scattering experiments on electron-doped Nd1.85Ce0.15CuO4 (refs 13, 14), where B(c2) can be reached. The applied field reveals a static, commensurate, anomalously conducting long-range ordered antiferromagnetic state, in which the induced moment scales approximately linearly with the field strength until it saturates at B(c2). This and previous experiments on the hole-doped materials therefore establishes antiferromagnetic order as a competing ground state in the high-T(c) copper oxide materials, irrespective of electron or hole doping.

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PubMed2003

Exact microscopic wave function for a topological quantum membrane.

The higher dimensional quantum Hall liquid constructed recently supports stable topological membrane excitations. Here we introduce a microscopic interacting Hamiltonian and present its exact ground state wave function. We show that this microscopic ground state wave function describes a topological quantum membrane. We also construct variational wave functions for excited states using the noncommutative algebra on the four sphere. Our approach introduces a nonperturbative method to quantize topological membranes.

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