Engineering direct-indirect band gap transition in wurtzite GaAs nanowires through size and uniaxial strain

Electronic structures of wurtzite GaAs nanowires in the [0001] direction were studied using first-principles calculations. It was found that the band gap of GaAs nanowires experience a direct-to-indirect transition when the diameter of the nanowires is smaller than ~28 {\AA}. For those thin GaAs nanowires with an indirect band gap, it was found that the gap can be tuned to be direct if a moderate external uniaxial strain is applied. Both tensile and compressive strain can trigger the indirect-to-direct gap transition. The critical strains for the gap-transition are determined by the energy crossover of two states in conduction bands.Comment: 4 pages, 4 figure

Space-Based Gravity Detector for a Space Laboratory

A space-based superconducting gravitational low-frequency wave detector is considered. Sensitivity of the detector is sufficient to use the detector as a partner of other contemporary low-frequency detectors like LIGO and LISA. This device can also be very useful for experimental study of other effects predicted by theories of gravitation.Comment: 4 pages, 4 figures

Full electrical control of Charge and Spin conductance through Interferometry of Edge States in Topological Insulators

We investigate electron interferometry of edge states in Topological Insulators. We show that, when inter-boundary coupling is induced at two quantum point contacts of a four terminal setup, both Fabry-P\'erot-like and Aharonov-Bohm-like loop processes arise. These underlying interference effects lead to a full electrically controllable system, where the magnitude of charge and spin linear conductances can be tuned by gate voltages, without applying magnetic fields. In particular we find that, under appropriate conditions, inter-boundary coupling can lead to negative values of the conductance. Furthermore, the setup also allows to selectively generate pure charge or pure spin currents, by choosing the voltage bias configuration.Comment: 12 pages, 5 figures (expanded discussion section, corrected typos

Wormhole Effect in a Strong Topological Insulator

An infinitely thin solenoid carrying magnetic flux Phi (a `Dirac string') inserted into an ordinary band insulator has no significant effect on the spectrum of electrons. In a strong topological insulator, remarkably, such a solenoid carries protected gapless one-dimensional fermionic modes when Phi=hc/2e. These modes are spin-filtered and represent a distinct bulk manifestation of the topologically non-trivial insulator. We establish this `wormhole' effect by both general qualitative considerations and by numerical calculations within a minimal lattice model. We also discuss the possibility of experimental observation of a closely related effect in artificially engineered nanostructures.Comment: 4 pages, 3 figures. For related work and info visit http://www.physics.ubc.ca/~fran

Field-effect mobility enhanced by tuning the Fermi level into the band gap of Bi2Se3

By eliminating normal fabrication processes, we preserve the bulk insulating state of calcium-doped Bi2Se3 single crystals in suspended nanodevices, as indicated by the activated temperature dependence of the resistivity at low temperatures. We perform low-energy electron beam irradiation (<16 keV) and electrostatic gating to control the carrier density and therefore the Fermi level position in the nanodevices. In slightly p-doped Bi2-xCaxSe3 devices, continuous tuning of the Fermi level from the bulk valence band to the band-gap reveals dramatic enhancement (> a factor of 10) in the field-effect mobility, which suggests suppressed backscattering expected for the Dirac fermion surface states in the gap of topological insulators

Ergodic Capacity Analysis of Remote Radio Head Associations in Cloud Radio Access Networks

Characterizing user to Remote Radio Head (RRH) association strategies in cloud radio access networks (C-RANs) is critical for performance optimization. In this letter, the single nearest and N--nearest RRH association strategies are presented, and the corresponding impact on the ergodic capacity of C-RANs is analyzed, where RRHs are distributed according to a stationary point process. Closed-form expressions for the ergodic capacity of the proposed RRH association strategies are derived. Simulation results demonstrate that the derived uplink closed-form capacity expressions are accurate. Furthermore, the analysis and simulation results show that the ergodic capacity gain is not linear with either the RRH density or the number of antenna per RRH. The ergodic capacity gain from the RRH density is larger than that from the number of antennas per RRH,which indicates that the association number of the RRH should not be bigger than 4 to balance the performance gain and the implementation cost.Comment: 4 pages, 2 figures, accepted by IEEE Wireless Communication Letter

Cooperative Non-Orthogonal Multiple Access in 5G Systems

Non-orthogonal multiple access (NOMA) has recently received considerable attention as a promising candidate for 5G systems. A key feature of NOMA is that users with better channel conditions have prior information about the messages of the other users. This prior knowledge is fully exploited in this paper, where a cooperative NOMA scheme is proposed. Outage probability and diversity order achieved by this cooperative NOMA scheme are analyzed, and an approach based on user pairing is also proposed to reduce system complexity in practice

Intrinsic rotation drive by collisionless trapped electron mode turbulence

Both the parallel residual stress and parallel turbulent acceleration driven by electrostatic collisionsless trapped electron mode (CTEM) turbulence are calculated analytically using gyrokinetic theory. Quasilinear results show that the parallel residual stress contributes an outward flux of co-current rotation for normal magnetic shear and turbulence intensity profile increasing outward. This may induce intrinsic counter-current rotation or flattening of the co-current rotation profile. The parallel turbulent acceleration driven by CTEM turbulence vanishes, due to the absence of a phase shift between density fluctuation and ion pressure fluctuation. This is different from the case of ion temperature gradient (ITG) turbulence, for which the turbulent acceleration can provide co-current drive for normal magnetic shear and turbulence intensity profile increasing outward. Its order of magnitude is predicted to be the same as that of the divergence of the residual stress [Lu Wang and P.H. Diamond, Phys. Rev. Lett. {\bf 110}, 265006 (2013)]. A possible connection of these theoretical results to experimental observations of electron cyclotron heating effects on toroidal rotation is discussed.Comment: Accepted by Phys. Plasma