Coupled Spin and Pseudo-magnetic Field in Graphene Nanoribbons

Pseudo-magnetic field becomes an experimental reality after the observation of zero-field Landau level-like quantization in strained graphene, but it is not expected that the time-reversal symmetric pseudo-magnetic fields will have any effect on the spin degree of freedom of the charge carriers. Here, we demonstrate that spin-orbit coupling (SOC) could act as a bridge between pseudo-magnetic field and spin. In quantum spin Hall (QSH) states, the direction of the spin of edge states is tied to their direction of motion because of the SOC. The pseudo-magnetic field affects the clockwise and counter-clock-wise edge currents of the QSH states, and consequently lifts the degenerate edge states of opposite spin orientation. Because of opposite signs of the pseudo-magnetic field in two valleys of graphene, the one-dimensional charge carriers at the two opposite edges have different group velocities, and in some special cases the edge states can only propagate at one edge of the nanoribbon and the group velocity at the other edge becomes zero.Comment: 4 figure

Weaving independently generated photons into an arbitrary graph state

The controlled Z (CZ) operations acting separately on pairs of qubits are commonly adopted in the schemes of generating graph states, the multi-partite entangled states for the one-way quantum computing. For this purpose, we propose a setup of cascade CZ operation on a whole group of qubits in sequence. The operation of the setup starts with entangling an ancilla photon to the first photon as qubit, and this ancilla automatically moves from one entanglement link to another in assisting the formation of a string in graph states. The generation of some special types of graph states, such as the three-dimensional ones, can be greatly simplified in this approach. The setup presented uses weak nonlinearities, but an implementation using probabilistic linear optics is also possible.Comment: 6 pages, 7 figures. Accepted by Phys. Rev.

Superconducting Vortices induced Periodic Magnetoresistance Oscillations in Single Crystal Au Nanowires

We show in this paper that it is possible to induce superconducting vortices in a gold nanowire connected to superconducting electrodes. The gold nanowire acquires superconductivity by the proximity effect. The differential magnetoresistance of the nanowire beyond a critical magnetic field shows uniform oscillations with increasing field with a period of \phi0/(2\pir^2) (\phi0 = h/2e is the superconducting flux quantum, r = 35 nm is the radius of the nanowire). We demonstrate that these periodic oscillations are the signatures of the sequential generation and moving of vortices across the gold nanowire

Chiral Tunnelling in Twisted Graphene Bilayer

The perfect transmission in graphene monolayer and the perfect reflection in Bernal graphene bilayer for electrons incident in the normal direction of a potential barrier are viewed as two incarnations of the Klein paradox. Here we show a new and unique incarnation of the Klein paradox. Owing to the different chiralities of the quasiparticles involved, the chiral fermions in twisted graphene bilayer shows adjustable probability of chiral tunnelling for normal incidence: they can be changed from perfect tunnelling to partial/perfect reflection, or vice versa, by controlling either the height of the barrier or the incident energy. As well as addressing basic physics about how the chiral fermions with different chiralities tunnel through a barrier, our results provide a facile route to tune the electronic properties of the twisted graphene bilayer.Comment: 4 figure

The Coexistence of van Hove Singularities and Superlattice Dirac Points in a Slightly Twisted Graphene Bilayer

We consider the electronic structure of a slightly twisted graphene bilayer and show the coexistence of van Hove singularities (VHSs) and superlattice Dirac points in a continuum approximation. The graphene-on-graphene moir\'e pattern gives rise to a periodic electronic potential, which leads to the emergence of the superlattice Dirac points due to the chiral nature of the charge carriers. Owning to the distinguishing real and reciprocal structures, the sublattice exchange even and odd structures of the twisted graphene bilayer (the two types of commensurate structures) result in two different structures of the superlattice Dirac points. We further calculate the effect of a strain on the low-energy electronic structure of the twisted graphene bilayer and demonstrate that the strain affects the position of the VHSs dramatically.Comment: 5 figures, to appear in Phys. Rev.

Magnetism and effect of anisotropy with one dimensional monatomic chain of cobalt by a Monte Carlo simulation

The magnetic properties of the one dimensional (1D) monatomic chain of Co reported in a previous experimental work are investigated by a classical Monte Carlo simulation based on the anisotropic Heisenberg model. In our simulation, the effect of the on-site uniaxial anisotropy, Ku, on each individual Co atom and the nearest neighbour exchange interaction, J, are accounted for. The normalized coercivity HC(T)/HC(TCL) is found to show a universal behaviour, HC(T)/HC(TCL) = h0(e^{TB/T}-e) in the temperature interval, TCL < T < TBCal, arising from the thermal activation effect. In the above expression, h0 is a constant, TBCal is the blocking temperature determined by the calculation, and TCL is the temperature above which the classical Monte Carlo simulation gives a good description on the investigated system. The present simulation has reproduced the experimental features, including the temperature dependent coercivity, HC(T), and the angular dependence of the remanent magnetization, MR(phi,theta), upon the relative orientation (phi,theta) of the applied field H. In addition, the calculation reveals that the ferromagnetic-like open hysteresis loop is a result of a slow dynamical process at T < TBCal. The dependence of the dynamical TBCal on the field sweeping rate R, the on-site anisotropy constant Ku, and the number of atoms in the atomic chain, N, has been investigated in detail.Comment: 20 pages, 7 figures included, J Phys Condens Matter, In Pres