2,165 research outputs found
Remote pivot decoupler pylon: Wing/store flutter suppressor
A device for suspending a store from an aerodynamic support surface, such an an aircraft wing, and more specifically, for improving upon singlet pivot decoupler pylons by reducing both frequency of active store, alignment, and alignment system space and power requirements. Two links suspend a lower pylon/rack section and releasable attached store from an upper pylon section mounted under the wing. The links allow the lower pylon section to rotate in pitch about a remote pivot point. A leaf spring connected between the lower section and electrical alignment system servomechanism provides pitch alignment of the lower section/store combination. The servomechanism utilizes an electric servomotor to drive the gear train and reversibly move the leaf spring, thereby maintaining the pitch attitude of the store within acceptable limits. The damper strokes when the lower section rotates to damp large oscillations of store
Critical currents in weakly textured MgB2: Nonlinear transport in anisotropic heterogeneous media
A model for highly non-linear transport in heterogeneous media consisting of
anisotropic particles with a preferred orientation is proposed and applied to
the current transport in weakly textured magnesium diboride, MgB2. It
essentially explains why, unlike in conventional superconductors, a significant
macroscopic anisotropy of the critical currents can be induced by the
preparation of MgB2 tapes. The field and angular dependence of the critical
current is calculated for various degrees of texture and compared to
experimental data
Seasonal Unit Root Tests under Structural Breaks
In this paper, several seasonal unit root tests are analysed in the context of structural breaks at known time and a new break corrected test is suggested. We show that the widely used HEGY test as well as an LM variant thereof are asymptotically robust to seasonal mean shifts of finite magnitude. In finite samples, however, experiments reveal that such tests suffer from severe size distortions and power reductions when breaks are present. Hence, a new break corrected LM test is proposed in order to overcome this problem. Importantly, the correction for seasonal mean shifts bears no consequence on the limiting distributions thereby maintaining the legitimacy of canonical critical values. Moreover, although this test assumes a breakpoint a priori, it is robust in terms of misspecification of the time of the break. This asymptotic property is well reproduced in finite samples. Based on a Monte Carlo study, our new test is compared with other procedures suggested in the literature and shown to hold superior finite sample properties.Structural Breaks, Unit Roots, Seasonal Unit Root Tests
Quantum phase transitions of the Majorana toric code in the presence of finite Cooper-pair tunneling
The toric code based on Majorana fermions on mesoscopic superconducting
islands is a promising candidate for quantum information processing. In the
limit of vanishing Cooper-pair tunneling, it has been argued that the phase
transition separating the topologically ordered phase of the toric code from
the trivial one is in the universality class of (2+1)D-XY. On the other hand,
in the limit of infinitely large Cooper-pair tunneling, the phase transition is
in the universality class of (2+1)D-Ising. In this work, we treat the case of
finite Cooper-pair tunneling and address the question of how the continuous XY
symmetry breaking phase transition turns into a discrete
symmetry breaking one when the Cooper-pair tunneling rate is increased. We show
that this happens through a couple of tricritical points and first order phase
transitions. Using a Jordan-Wigner transformation, we map the problem to that
of spins coupled to quantum rotors and subsequently, propose a Landau field
theory for this model that matches the known results in the respective limits.
We calculate the effective field theories and provide the relevant critical
exponents for the different phase transitions. Our results are relevant for
predicting the stability of the topological phase in realistic experimental
implementations.Comment: 5 pages, 2 figure
Coulomb-assisted braiding of Majorana fermions in a Josephson junction array
We show how to exchange (braid) Majorana fermions in a network of
superconducting nanowires by control over Coulomb interactions rather than
tunneling. Even though Majorana fermions are charge-neutral quasiparticles
(equal to their own antiparticle), they have an effective long-range
interaction through the even-odd electron number dependence of the
superconducting ground state. The flux through a split Josephson junction
controls this interaction via the ratio of Josephson and charging energies,
with exponential sensitivity. By switching the interaction on and off in
neighboring segments of a Josephson junction array, the non-Abelian braiding
statistics can be realized without the need to control tunnel couplings by gate
electrodes. This is a solution to the problem how to operate on topological
qubits when gate voltages are screened by the superconductor
Proposal for an optical laser producing light at half the Josephson frequency
We describe a superconducting device capable of producing laser light in the
visible range at half of the Josephson generation frequency with the optical
phase of the light locked to the superconducting phase difference. It consists
of two single-level quantum dots embedded into a p-n semiconducting
heterostructure and surrounded by a cavity supporting a resonant optical mode.
We study decoherence and spontaneous switching in the device.Comment: 4+3 pages, 3+1 figure
Design and Fabrication of the NASA Decoupler Pylon for the F-16 Aircraft
The NASA Decoupler Pylon is a passive means of suppressing wing-store flutter. The feasibility of demonstrating this concept on the F-16 aircraft was established through model wind tunnel tests and analyses. As a result of these tests and studies a ship set of Decoupler Pylons was designed and fabricated for a flight test demonstration on the F-16 aircraft. Basic design criteria were developed during the analysis study pertaining to pylon pitch stiffness, alignment system requirements, and damping requirements. A design was developed which utilized an electrical motor for the pylon alignment system. The design uses a four pin, two link pivot design which results in a remote pivot located at the center of gravity of the store when the store is in the aligned position. The pitch spring was fabricated from a tapered constant stress cantilevered beam. The pylon has the same external lines as the existing production pylon and is designed to use a MAU-12 ejection rack which is the same as the one used with the production pylon. The detailed design and fabrication was supported with a complete ground test of the pylon prior to shipment to NASA
From Majorana Fermions to Topological Order
We consider a system consisting of a 2D network of links between Majorana
fermions on superconducting islands. We show that the fermionic Hamiltonian
modeling this system is topologically-ordered in a region of parameter space.
In particular we show that Kitaev's toric code emerges in fourth-order
perturbation theory. By using a Jordan-Wigner transformation we can map the
model onto a family of signed 2D Ising models in a transverse field where the
signs (FM or AFM) are determined by additional gauge bits. Our mapping allows
an understanding of the non-perturbative regime and the phase transition to a
non-topological phase. We discuss the physics behind a possible implementation
of this model and argue how it can be used for topological quantum computation
by adiabatic changes in the Hamiltonian.Comment: 4+4 pages, 5 figures. v2 has a new reference and a few new comments.
In v3: yet another new reference and Supplementary Material is renamed
Appendix. In v4: several typos are corrected, to appear in Phys. Rev. Let
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