Shi-Jie Xiong

Two-dimensional pentagonal crystals and possible spin-polarized Dirac dispersion relations

Based on first-principles calculations we show that the two-dimensional pentagonal (pt) structures, the compositions of pt-BN2, pt-C, and pt-Fe2S, are stable. As a common feature, they are composed of 3 components: 2 stretched honeycomb sublattices and 1 square sublattice, conferring flexibility of tailoring the properties peculiar to the graphene. Although the Dirac dispersion relation is removed in metallic pt-BN2 and insulating pt-C due to the hybridization of two honeycomb sublattices, it survives in pt-Fe2S because of the suppression of such hybridization between different spins. As a result, in the dispersion relation of pt-Fe2S spin-polarized and anisotropic Dirac cones occur. We suggest that such type of dispersion relation can be used to produce spin-filter effect by applying electric bias in a specific direction.

Exact ground state of several quasi-two- and three-dimensional frustrated Heisenberg models

Abstract It is proved that a product of local singlets is the exact ground state for several quasi-two- and three-dimensional (Q-2D and 3D) frustrated Heisenberg models with special parameter values.

The possible ground states of doped holes in (La1−xSrx)2CuO4

Abstract On the basis of the Anderson lattice Hamiltonian, the ground states of the doped holes in (La1−xSrx)2CuO4 are investigated by using a variational analysis. It is found that a single hole is localized within a linear cluster with length of about 8a (a being the oxygen-oxygen spacing), and the hopping of holes produces conductivity with temperature dependence exp [−(c/k B T) 1 4 ] at low temperature (critical conductivity) and T−1 exp(−v/kBT) at higher temperature (thermal-excited hopping conductivity). The hole-cluster also breaks the AF correlation between the copper sites on its two sides, the AF coherence length is then proportional to a/ x . By increasing the doping, an infinite cluster may be formed to hold all the doped holes, the AF background is resolved into isolated spin-singlets or spins, and the Fermi-liquid-like features of the holes appear. The critical doping densities for these transitions are determined. Many unusual properties of this material can be quantitatively explained.

A Datta-Das transistor and conductance switch based on a zigzag graphene nanoribbon

We theoretically investigate the transport properties of a zigzag graphene nanoribbon, where there exists the Rashba spin-orbit coupling only in the central region modulated by a gate voltage. Using the approach of nonequilibrium Green function, we show that when the two electrode regions are ferromagnetic but the central region is nonmagnetic, the device based on graphene has the behavior of the Datta-Das transistor and at the same time a giant magnetoresistance effect can be observed. Inversely, when the central region has a magnetization but the two electrode regions are nonmagnetic, the device can work as a conductance switch, because an energy gap appears in the band structure of the central graphene ribbon under the coaction of the Rashba spin-orbit coupling and the magnetization. In addition, we find that the behavior of the conductance switch of the device is robust when the weak disorder is present in the system.

Robust quantum repeater with atomic ensembles and single-photon sources

We present an efficient scheme for generating high-fidelity entanglement locally which is essential for quantum repeater protocols. Two local ``polarization entangled states of atomic ensembles

Entropy exchange, coherent information, and concurrence

u

Exact ground state of the 2D modulation-frustration Heisenberg model

and

Dispersive interactions between matter qubits and bright squeezed light

d

The bound of entanglement of superpositions with more than two components

are generated by absorbing a single photon emitted by on-demand single-photon sources, based on which high-fidelity local entanglement between four ensembles can be established efficiently through Bell-state measurement. Entanglement in basic links and entanglement connection between links are carried out by the use of two-photon interference. In addition to being robust against phase fluctuations in the quantum channels, this scheme may speed up quantum communication with higher fidelity compared with the partial read protocol [Sangouard et al., Phys. Rev. A 77, 062301 (2008)] which may generate entanglement most quickly among the previous schemes with the same ingredients.

Diffusion behavior of quasiparticles in two-dimensional disordered superconductors with s+id pairing symmetry

For a simple model we derive analytic expressions of entropy exchange and coherent information, from which relations between them and the concurrence are drawn. We find that in the quantum evolution the entropy exchange exhibits behavior opposite to that of the concurrence, whereas the coherent information shows features very similar to those of the concurrence. The meaning of this result for general systems is discussed.