We study theoretically the effects of long-range and on-site Coulomb interactions on the topological phases and transport properties of spin-orbit-coupled quasi-one-dimensional quantum wires imposed on an s-wave superconductor. The electrostatic pote
ntial and charge density distributions are computed self-consistently within the Hartree approximation. Due to the finite width of the wires and the charge repulsion, the potential and density distribute inhomogeneously in the transverse direction and tend to accumulate along the lateral edges where the hard-wall confinement is assumed. This result has profound effects on the topological phases and the differential conductance of the interacting quantum wires and their hybrid junctions with superconductors. Coulomb interactions renormalize the chemical potential, and alter the topological phases strongly by enhancing the topological regimes and producing jagged boundaries. Moreover, the multicritical points connecting different topological phases from high-index subbands are modified remarkably in striking contrast to the predictions of the two-band model. We further suggest the possible non-magnetic topological phase transitions manipulated externally with the aid of long-range interactions. Finally, the transport properties of normal-superconductor junctions are also examined and interaction impacts on the emergence of Majorana fermions and the strength of Majorana zero-bias peaks are revealed.
Different from conventional electroactive polymers, here we firstly present a new facile actuator made from aluminum alloy. The high-frequency electrically induced flapping motion was characterized under varied physical factors. This electroactuation
results from alternative processes of charge induction and discharge, which is confirmed by the existence of periodical pulse current in the circuit. The metal actuator is of great stability and can maintain several days if not for any structural fatigue. Easy fabrication, high tunable frequency and durability make it potential for implementation of actuators for sensors, microelectromechanical systems and robotics.
We firstly demonstrate physically-prepared compliant PVA hydrogel electrodes as a promising supplement in dielectric elastomer actuators. They are capable of conducting high voltages over 5 kV without electrolysis by an electrical double layer effect
. The hydrogel electrodes adhere tightly to the dielectric layer, which ensures its structural stability during actuation. All-polymeric actuators fabricated by these electrodes exhibited excellent consecutive working performance over 2960 cycles. The actuation was influenced by applied voltage, ramp rate, electrode elastic modulus and moisture content. A maximum areal strain over 78% was achieved. Tunable transparency, good biocompatibility, long lifetime, low cost and facile fabrication make PVA hydrogel electrode another promising candidate in the fields of sensors, artificial muscles and optical applications.
Resonant transmission through electronic quantum states that exist at the zero points of a magnetic field gradient inside a ballistic quantum wire is reported. Since the semiclassical motion along such a line of zero magnetic field takes place in for
m of unidirectional snake trajectories, these states have no classical equivalence. The existence of such quantum states has been predicted more than a decade ago by theoretical considerations of Reijniers and coworkers [1]. We further show how their properties depend on the amplitude of the magnetic field profile as well as on the Fermi energy.
The Andreev reflection of the normal state-superconductor junction both in monolayer and bilayer graphene with a single magnetic barrier is investigated by means of the Greens function formalism. Within the tight-binding model, we study the direction
-dependent Andreev reflection of two-dimensional graphene-superconductor junctions in the specular and retro-reflection regimes. The presence of a magnetic barrier close to the superconducting hybrid junction introduces a rich phenomenology. Such a barrier is capable of tuning the preferred angles of incidence for the Andreev reflection. In particular, it can enhance the specular reflection probability for certain angles of incidence in bilayer-based hybrid structures. When transmission is permitted, the Andreev reflection manifests itself in isolated peaks and asymmetric resonances associated with offsets and Fano-type oscillations in the transmission, respectively. Moreover, Fabry-P{e}rot oscillations in the Andreev reflection due to the interior scattering inside the magnetic barrier may appear. The impacts of magnetic barriers on the monolayer and bilayer hybrid interfaces are furthermore studied by calculating the differential conductances within the Blonder-Tinkham-Klapwijk formula.
The conductivity of armchair graphene nanoribbons in the presence of short-range impurities and edge roughness is studied theoretically using the Boltzmann transport equation for quasi-one-dimensional systems. As the number of occupied subbands incre
ases, the conductivity due to short-range impurities converges towards the two-dimensional case. Calculations of the magnetoconductivity confirm the edge-roughness-induced dips at cyclotron radii close to the ribbon width suggested by the recent quantum simulations.
The magnetoconductance of graphene nanoribbons with rough zigzag and armchair edges is studied by numerical simulations. nanoribbons with sufficiently small bulk disorder show a pronounced magnetoconductance minimum at cyclotron radii close to the ri
bbon width, in close analogy to the wire peak observed in conventional semiconductor quantum wires. In zigzag nanoribbons, this feature becomes visible only above a threshold amplitude of the edge roughness, as a consequence of the reduced current density close to the edges.
The transport through a quantum wire exposed to two magnetic spikes in series is modeled. We demonstrate that quantum dots can be formed this way which couple to the leads via magnetic barriers. Conceptually, all quantum dot states are accessible by
transport experiments. The simulations show Breit-Wigner resonances in the closed regime, while Fano resonances appear as soon as one open transmission channel is present. The system allows to tune the dots confinement potential from sub-parabolic to superparabolic by experimentally accessible parameters.
We derive analytical expressions for the conductivity of bilayer graphene (BLG) using the Boltzmann approach within the the Born approximation for a model of Gaussian disorders describing both short- and long-range impurity scattering. The range of v
alidity of the Born approximation is established by comparing the analytical results to exact tight-binding numerical calculations. A comparison of the obtained density dependencies of the conductivity with experimental data shows that the BLG samples investigated experimentally so far are in the quantum scattering regime where the Fermi wavelength exceeds the effective impurity range. In this regime both short- and long-range scattering lead to the same linear density dependence of the conductivity. Our calculations imply that bilayer and single layer graphene have the same scattering mechanisms. We also provide an upper limit for the effective, density dependent spatial extension of the scatterers present in the experiments.
The effects of Coulomb interactions on the electronic properties of bilayer graphene nanoribbons (BGNs) covered by a gate electrode are studied theoretically. The electron density distribution and the potential profile are calculated self-consistentl
y within the Hartree approximation. A comparison to their single-particle counterparts reveals the effects of interactions and screening. Due to the finite width of the nanoribbon in combination with electronic repulsion, the gate-induced electrons tend to accumulate along the BGN edges where the potential assumes a sharp triangular shape. This has a profound effect on the energy gap between electron and hole bands, which depends nonmonotonously on the gate voltage and collapses at intermediate electric fields. We interpret this behavior in terms of interaction-induced warping of the energy dispersion.
