11,313 research outputs found
Disordered graphene Josephson junctions
A tight-binding approach based on the Chebyshev-Bogoliubov-de Gennes method
is used to describe disordered single-layer graphene Josephson junctions.
Scattering by vacancies, ripples or charged impurities is included. We compute
the Josephson current and investigate the nature of multiple Andreev
reflections, which induce bound states appearing as peaks in the density of
states for energies below the superconducting gap. In the presence of single
atom vacancies, we observe a strong suppression of the supercurrent that is a
consequence of strong inter-valley scattering. Although lattice deformations
should not induce inter-valley scattering, we find that the supercurrent is
still suppressed, which is due to the presence of pseudo-magnetic barriers. For
charged impurities, we consider two cases depending on whether the average
doping is zero, i.e. existence of electron-hole puddles, or finite. In both
cases, short range impurities strongly affect the supercurrent, similar to the
vacancies scenario
Tight-binding description of intrinsic superconducting correlations in multilayer graphene
Using highly efficient GPU-based simulations of the tight-binding
Bogoliubov-de Gennes equations we solve self-consistently for the pair
correlation in rhombohedral (ABC) and Bernal (ABA) multilayer graphene by
considering a finite intrinsic s-wave pairing potential. We find that the two
different stacking configurations have opposite bulk/surface behavior for the
order parameter. Surface superconductivity is robust for ABC stacked multilayer
graphene even at very low pairing potentials for which the bulk order parameter
vanishes, in agreement with a recent analytical approach. In contrast, for
Bernal stacked multilayer graphene, we find that the order parameter is always
suppressed at the surface and that there exists a critical value for the
pairing potential below which no superconducting order is achieved. We
considered different doping scenarios and find that homogeneous doping strongly
suppresses surface superconductivity while non-homogeneous field-induced doping
has a much weaker effect on the superconducting order parameter. For multilayer
structures with hybrid stacking (ABC and ABA) we find that when the thickness
of each region is small (few layers), high-temperature surface
superconductivity survives throughout the bulk due to the proximity effect
between ABC/ABA interfaces where the order parameter is enhanced.Comment: 7 page
Theoretical analysis for critical fluctuations of relaxation trajectory near a saddle-node bifurcation
A Langevin equation whose deterministic part undergoes a saddle-node
bifurcation is investigated theoretically. It is found that statistical
properties of relaxation trajectories in this system exhibit divergent
behaviors near a saddle-node bifurcation point in the weak-noise limit, while
the final value of the deterministic solution changes discontinuously at the
point. A systematic formulation for analyzing a path probability measure is
constructed on the basis of a singular perturbation method. In this
formulation, the critical nature turns out to originate from the neutrality of
exiting time from a saddle-point. The theoretical calculation explains results
of numerical simulations.Comment: 18pages, 17figures.The version 2, in which minor errors have been
fixed, will be published in Phys. Rev.
An Abstract Plan Preparation Language
This paper presents a new planning language that is more abstract than most existing planning languages such as the Planning Domain Definition Language (PDDL) or the New Domain Description Language (NDDL). The goal of this language is to simplify the formal analysis and specification of planning problems that are intended for safety-critical applications such as power management or automated rendezvous in future manned spacecraft. The new language has been named the Abstract Plan Preparation Language (APPL). A translator from APPL to NDDL has been developed in support of the Spacecraft Autonomy for Vehicles and Habitats Project (SAVH) sponsored by the Explorations Technology Development Program, which is seeking to mature autonomy technology for application to the new Crew Exploration Vehicle (CEV) that will replace the Space Shuttle
A Formal Framework for the Analysis of Algorithms That Recover From Loss of Separation
We present a mathematical framework for the specification and verification of state-based conflict resolution algorithms that recover from loss of separation. In particular, we propose rigorous definitions of horizontal and vertical maneuver correctness that yield horizontal and vertical separation, respectively, in a bounded amount of time. We also provide sufficient conditions for independent correctness, i.e., separation under the assumption that only one aircraft maneuvers, and for implicitly coordinated correctness, i.e., separation under the assumption that both aircraft maneuver. An important benefit of this approach is that different aircraft can execute different algorithms and implicit coordination will still be achieved, as long as they all meet the explicit criteria of the framework. Towards this end we have sought to make the criteria as general as possible. The framework presented in this paper has been formalized and mechanically verified in the Prototype Verification System (PVS)
Short-time dynamics of finite-size mean-field systems
We study the short-time dynamics of a mean-field model with non-conserved
order parameter (Curie-Weiss with Glauber dynamics) by solving the associated
Fokker-Planck equation. We obtain closed-form expressions for the first moments
of the order parameter, near to both the critical and spinodal points, starting
from different initial conditions. This allows us to confirm the validity of
the short-time dynamical scaling hypothesis in both cases. Although the
procedure is illustrated for a particular mean-field model, our results can be
straightforwardly extended to generic models with a single order parameter.Comment: accepted for publication in JSTA
How to hide a secret direction
We present a procedure to share a secret spatial direction in the absence of
a common reference frame using a multipartite quantum state. The procedure
guarantees that the parties can determine the direction if they perform joint
measurements on the state, but fail to do so if they restrict themselves to
local operations and classical communication (LOCC). We calculate the fidelity
for joint measurements, give bounds on the fidelity achievable by LOCC, and
prove that there is a non-vanishing gap between the two of them, even in the
limit of infinitely many copies. The robustness of the procedure under particle
loss is also studied. As a by-product we find bounds on the probability of
discriminating by LOCC between the invariant subspaces of total angular
momentum N/2 and N/2-1 in a system of N elementary spins.Comment: 4 pages, 1 figur
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