2,453 research outputs found
Navier-Stokes computations for circulation control airfoils
Navier-Stokes computations of subsonic to transonic flow past airfoils with augmented lift due to rearward jet blowing over a curved trailing edge are presented. The approach uses a spiral grid topology. Solutions are obtained using a Navier-Stokes code which employs an implicit finite difference method, an algebraic turbulence model, and developments which improve stability, convergence, and accuracy. Results are compared against experiments for no jet blowing and moderate jet pressures and demonstrate the capability to compute these complicated flows
Coupling of shells in a carbon nanotube quantum dot
We systematically study the coupling of longitudinal modes (shells) in a
carbon nanotube quantum dot. Inelastic cotunneling spectroscopy is used to
probe the excitation spectrum in parallel, perpendicular and rotating magnetic
fields. The data is compared to a theoretical model including coupling between
shells, induced by atomically sharp disorder in the nanotube. The calculated
excitation spectra show good correspondence with experimental data.Comment: 8 pages, 4 figure
Tunneling Spectroscopy of Quasiparticle Bound States in a Spinful Josephson Junction
The spectrum of a segment of InAs nanowire, confined between two
superconducting leads, was measured as function of gate voltage and
superconducting phase difference using a third normal-metal tunnel probe.
Sub-gap resonances for odd electron occupancy---interpreted as bound states
involving a confined electron and a quasiparticle from the superconducting
leads, reminiscent of Yu-Shiba-Rusinov states---evolve into Kondo-related
resonances at higher magnetic fields. An additional zero bias peak of unknown
origin is observed to coexist with the quasiparticle bound states.Comment: Supplementary information available here:
https://dl.dropbox.com/u/1742676/Chang_Sup.pd
Small-World Networks: Links with long-tailed distributions
Small-world networks (SWN), obtained by randomly adding to a regular
structure additional links (AL), are of current interest. In this article we
explore (based on physical models) a new variant of SWN, in which the
probability of realizing an AL depends on the chemical distance between the
connected sites. We assume a power-law probability distribution and study
random walkers on the network, focussing especially on their probability of
being at the origin. We connect the results to L\'evy Flights, which follow
from a mean field variant of our model.Comment: 11 pages, 4 figures, to appear in Phys.Rev.
Nonequilibrium Cotunneling through a Three-Level Quantum Dot
We calculate the nonlinear cotunneling conductance through a quantum dot with
3 electrons occupying the three highest lying energy levels. Starting from a
3-orbital Anderson model, we apply a generalized Schrieffer-Wolff
transformation to derive an effective Kondo model for the system. Within this
model we calculate the nonequilibrium occupation numbers and the corresponding
cotunneling current to leading order in the exchange couplings. We identify the
inelastic cotunneling thresholds and their splittings with applied magnetic
field, and make a qualitative comparison to recent experimental data on carbon
nanotube and InAs quantum-wire quantum dots. Further predictions of the model
like cascade resonances and a magnetic-field dependence of the orbital level
splitting are not yet observed but within reach of recent experimental work on
carbon nanotube and InAs nanowire quantum dots.Comment: 12 pages, 13 figure
Disruption management in passenger railway transportation.
This paper deals with disruption management in passengerrailway transportation. In the disruption management process, manyactors belonging to different organizations play a role. In this paperwe therefore describe the process itself and the roles of thedifferent actors.Furthermore, we discuss the three main subproblems in railwaydisruption management: timetable adjustment, and rolling stock andcrew re-scheduling. Next to a general description of these problems,we give an overview of the existing literature and we present somedetails of the specific situations at DSB S-tog and NS. These arethe railway operators in the suburban area of Copenhagen, Denmark,and on the main railway lines in the Netherlands, respectively.Since not much research has been carried out yet on OperationsResearch models for disruption management in the railway context,models and techniques that have been developed for related problemsin the airline world are discussed as well.Finally, we address the integration of the re-scheduling processesof the timetable, and the resources rolling stock and crew.
Accuracy, Scalability, and Efficiency of Mixed-Element USM3D for Benchmark Three-Dimensional Flows
The unstructured, mixed-element, cell-centered, finite-volume flow solver USM3D is enhanced with new capabilities including parallelization, line generation for general unstructured grids, improved discretization scheme, and optimized iterative solver. The paper reports on the new developments to the flow solver and assesses the accuracy, scalability, and efficiency. The USM3D assessments are conducted using a baseline method and the recent hierarchical adaptive nonlinear iteration method framework. Two benchmark turbulent flows, namely, a subsonic separated flow around a three-dimensional hemisphere-cylinder configuration and a transonic flow around the ONERA M6 wing are considered
A Semiconductor Nanowire-Based Superconducting Qubit
We introduce a hybrid qubit based on a semiconductor nanowire with an
epitaxially grown superconductor layer. Josephson energy of the transmon-like
device ("gatemon") is controlled by an electrostatic gate that depletes
carriers in a semiconducting weak link region. Strong coupling to an on-chip
microwave cavity and coherent qubit control via gate voltage pulses is
demonstrated, yielding reasonably long relaxation times (0.8 {\mu}s) and
dephasing times (1 {\mu}s), exceeding gate operation times by two orders of
magnitude, in these first-generation devices. Because qubit control relies on
voltages rather than fluxes, dissipation in resistive control lines is reduced,
screening reduces crosstalk, and the absence of flux control allows operation
in a magnetic field, relevant for topological quantum information
Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots
We study low-temperature transport through carbon nanotube quantum dots in
the Coulomb blockade regime coupled to niobium-based superconducting leads. We
observe pronounced conductance peaks at finite source-drain bias, which we
ascribe to elastic and inelastic cotunneling processes enhanced by the
coherence peaks in the density of states of the superconducting leads. The
inelastic cotunneling lines display a marked dependence on the applied gate
voltage which we relate to different tunneling-renormalizations of the two
subbands in the nanotube. Finally, we discuss the origin of an especially
pronounced sub-gap structure observed in every fourth Coulomb diamond
Transport signatures of quasiparticle poisoning in a Majorana island
We investigate effects of quasiparticle poisoning in a Majorana island with
strong tunnel coupling to normal-metal leads. In addition to the main Coulomb
blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage,
consistent with transport through an excited (poisoned) state of the island.
Comparison to a simple model yields an estimate of parity lifetime for the
strongly coupled island (~ 1 {\mu}s) and sets a bound for a weakly coupled
island (> 10 {\mu}s). Fluctuations in the gate-voltage spacing of Coulomb peaks
at high field, reflecting Majorana hybridization, are enhanced by the reduced
lever arm at strong coupling. In energy units, fluctuations are consistent with
previous measurements.Comment: includes supplementary materia
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