3,263 research outputs found
From Andreev bound states to Majorana fermions in topological wires on superconducting substrates : a story of mutation
We study the proximity effect in a topological nanowire tunnel coupled to an
s-wave superconducting substrate. We use a general Green's function approach
that allows us to study the evolution of the Andreev bound states in the wire
into Majorana fermions. We show that the strength of the tunnel coupling
induces a topological transition in which the Majorana fermionic states can be
destroyed when the coupling is very strong. Moreover, we provide a
phenomenologial study of the effects of disorder in the superconductor on the
formation of Majorana fermions. We note a non-trivial effect of a quasiparticle
broadening term which can take the wire from a topological into a
non-topological phase in certain ranges of parameters. Our results have also
direct consequences for a nanowire coupled to an inhomogenous superconductor
Waiting times of entangled electrons in normal-superconducting junctions
We consider a normal-superconducting junction in order to investigate the
effect of new physical ingredients on waiting times. First, we study the
interplay between Andreev and specular scattering at the interface on the
distribution of waiting times of electrons or holes separately. In that case
the distribution is not altered dramatically compared to the case of a single
quantum channel with a quantum point contact since the interface acts as an
Andreev mirror for holes. We then consider a fully entangled state originating
from spliting of Cooper pairs at the interface and demonstrate a significant
enhancement of the probability to detect two consecutive electrons in a short
time interval. Finally, we discuss the electronic waiting time distribution in
the more realistic situation of partial entanglement
Use of adaptive walls in 2D tests
A new method for computing the wall effects gives precise answers to some questions arising in adaptive wall concept applications: length of adapted regions, fairings with up and downstream regions, residual misadjustments effects, reference conditions. The acceleration of the iterative process convergence and the development of an efficient technology used in CERT T2 wind tunnels give in a single run the required test conditions. Samples taken from CAST 7 tests demonstrate the efficiency of the whole process to obtain significant results with considerations of tridimensional case extension
Wall effects in wind tunnels
A synthesis of current trends in the reduction and computation of wall effects is presented. Some of the points discussed include: (1) for the two-dimensional, transonic tests, various control techniques of boundary conditions are used with adaptive walls offering high precision in determining reference conditions and residual corrections. A reduction in the boundary layer effects of the lateral walls is obtained at T2; (2) for the three-dimensional tests, the methods for the reduction of wall effects are still seldom applied due to a lesser need and to their complexity; (3) the supports holding the model of the probes have to be taken into account in the estimation of perturbatory effects
Effects of finite superconducting coherence lengths and of phase gradients in topological SN and SNS junctions and rings
We study the effect of a finite proximity superconducting (SC) coherence
length in SN and SNS junctions consisting of a semiconducting topological
insulating wire whose ends are connected to either one or two s-wave
superconductors. We find that such systems behave exactly as SN and SNS
junctions made from a single wire for which some regions are sitting on top of
superconductors, the size of the topological SC region being determined by the
SC coherence length. We also analyze the effect of a non-perfect transmission
at the NS interface on the spatial extension of the Majorana fermions.
Moreover, we study the effects of continuous phase gradients in both an open
and closed (ring) SNS junction. We find that such phase gradients play an
important role in the spatial localization of the Majorana fermions
Three-dimensional effects on airfoils
The effects of boundary layer flows along the walls of wind tunnels were studied to validate the transfer of two dimensional calculations to three dimensional transonic flowfield calculations. Results from trials in various wind tunnels were examind to determine the effects of the wall boundary flow on the control surfaces of an airfoil. Models sliding along a groove in the wall of a channel at sub- and transonic speeds were examined, with the finding that with either nonuniformities in the groove, or even if the channel walls are uniform, the lateral boundary layer can cause variations in the central flow region or alter the onset of shock at the transition point. Models for the effects in both turbulence and in the absence of turbulence are formulated, and it is noted that the characteristics of individual wind tunnels must be studied to quantify any existing three dimensional effects
Off-diagonal long-range order, cycle probabilities, and condensate fraction in the ideal Bose gas
We discuss the relationship between the cycle probabilities in the
path-integral representation of the ideal Bose gas, off-diagonal long-range
order, and Bose--Einstein condensation. Starting from the Landsberg recursion
relation for the canonic partition function, we use elementary considerations
to show that in a box of size L^3 the sum of the cycle probabilities of length
k >> L^2 equals the off-diagonal long-range order parameter in the
thermodynamic limit. For arbitrary systems of ideal bosons, the integer
derivative of the cycle probabilities is related to the probability of
condensing k bosons. We use this relation to derive the precise form of the
\pi_k in the thermodynamic limit. We also determine the function \pi_k for
arbitrary systems. Furthermore we use the cycle probabilities to compute the
probability distribution of the maximum-length cycles both at T=0, where the
ideal Bose gas reduces to the study of random permutations, and at finite
temperature. We close with comments on the cycle probabilities in interacting
Bose gases.Comment: 6 pages, extensive rewriting, new section on maximum-length cycle
A Number-Theoretic Error-Correcting Code
In this paper we describe a new error-correcting code (ECC) inspired by the
Naccache-Stern cryptosystem. While by far less efficient than Turbo codes, the
proposed ECC happens to be more efficient than some established ECCs for
certain sets of parameters. The new ECC adds an appendix to the message. The
appendix is the modular product of small primes representing the message bits.
The receiver recomputes the product and detects transmission errors using
modular division and lattice reduction
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