155,825 research outputs found
The tunneling conductance between a superconducting STM tip and an out-of-equilibrium carbon nanotube
We calculate the current and differential conductance for the junction
between a superconducting (SC) STM tip and a Luttinger liquid (LL). For an
infinite single-channel LL, the SC coherence peaks are preserved in the
tunneling conductance for interactions weaker than a critical value, while for
strong interactions (g <0.38), they disappear and are replaced by cusp-like
features. For a finite-size wire in contact with non-interacting leads, we find
however that the peaks are restored even for extremely strong interactions. In
the presence of a source-drain voltage the peaks/cusps split, and the split is
equal to the voltage. At zero temperature, even very strong interactions do not
smear the two peaks into a broader one; this implies that the recent
experiments of Y.-F. Chen et. al. (Phys. Rev. Lett. 102, 036804 (2009)) do not
rule out the existence of strong interactions in carbon nanotubes.Comment: 8 pages, 3 figure
Numerical analysis of one-dimensional waves in generalized thermoelasticity
Classical thermoelasticity theory is based on Fourier\u27s Law of heat conduction, which, when combined with the other fundamental field equations, leads to coupled hyperbolic-parabolic governing equations. These equations imply that thermal effects are to be felt instantaneously, far away from the external thermomechanieal load. Therefore, this theory admits infinite speeds of propagation of thermoelastic disturbances. This paradox becomes especially evident in problems involving very short time intervals, or high rates. of heat flux. Since infinite wave speeds are physically unrealistic in some situations, and since experiments have shown the existence of wavetype thermoelastic interactions, like in the observation of thermal pulses in dielectric crystals, generalized thermoelasticity theories have been developed. This thesis concentrates on one generalized thermoelasticity theory, proposed by Green and Lindsay, in which a generalized thermoelastic coupling constant, e, and two relaxation times, t0 and t, account for finite speed thermoelastic waves . A numerical analysis of an exact analytical solution, involving an instantaneous plane source of heat in an infinite body, is performed. The analysis reveals two finite speed wave fronts for each of the four fields: displacement, stress, temperature, and heat flux. The results are complimentary to previous analysis, and improve upon them, because a large range of parameters is involved, and the exact solution to the problem has been used
Statistical mechanics of image restoration and error-correcting codes
We develop a statistical-mechanical formulation for image restoration and
error-correcting codes. These problems are shown to be equivalent to the Ising
spin glass with ferromagnetic bias under random external fields. We prove that
the quality of restoration/decoding is maximized at a specific set of parameter
values determined by the source and channel properties. For image restoration
in mean-field system a line of optimal performance is shown to exist in the
parameter space. These results are illustrated by solving exactly the
infinite-range model. The solutions enable us to determine how precisely one
should estimate unknown parameters. Monte Carlo simulations are carried out to
see how far the conclusions from the infinite-range model are applicable to the
more realistic two-dimensional case in image restoration.Comment: 20 pages, 9 figures, ReVTe
Nonequilibrium-induced metal-superconductor quantum phase transition in graphene
We study the effects of dissipation and time-independent nonequilibrium drive
on an open superconducting graphene. In particular, we investigate how
dissipation and nonequilibrium effects modify the semi-metal-BCS quantum phase
transition that occurs at half-filling in equilibrium graphene with attractive
interactions. Our system consists of a graphene sheet sandwiched by two
semi-infinite three-dimensional Fermi liquid reservoirs, which act both as a
particle pump/sink and a source of decoherence. A steady-state charge current
is established in the system by equilibrating the two reservoirs at different,
but constant, chemical potentials. The nonequilibrium BCS superconductivity in
graphene is formulated using the Keldysh path integral formalism, and we obtain
generalized gap and number density equations valid for both zero and finite
voltages. The behaviour of the gap is discussed as a function of both
attractive interaction strength and electron densities for various
graphene-reservoir couplings and voltages. We discuss how tracing out the
dissipative environment (with or without voltage) leads to decoherence of
Cooper pairs in the graphene sheet, hence to a general suppression of the gap
order parameter at all densities. For weak enough attractive interactions we
show that the gap vanishes even for electron densities away from half-filling,
and illustrate the possibility of a dissipation-induced metal-superconductor
quantum phase transition. We find that the application of small voltages does
not alter the essential features of the gap as compared to the case when the
system is subject to dissipation alone (i.e. zero voltage).Comment: 13 pages, 8 figure
On the use of the proximity force approximation for deriving limits to short-range gravitational-like interactions from sphere-plane Casimir force experiments
We discuss the role of the proximity force approximation in deriving limits
to the existence of Yukawian forces - predicted in the submillimeter range by
many unification models - from Casimir force experiments using the sphere-plane
geometry. Two forms of this approximation are discussed, the first used in most
analyses of the residuals from the Casimir force experiments performed so far,
and the second recently discussed in this context in R. Decca et al. [Phys.
Rev. D 79, 124021 (2009)]. We show that the former form of the proximity force
approximation overestimates the expected Yukawa force and that the relative
deviation from the exact Yukawa force is of the same order of magnitude, in the
realistic experimental settings, as the relative deviation expected between the
exact Casimir force and the Casimir force evaluated in the proximity force
approximation. This implies both a systematic shift making the actual limits to
the Yukawa force weaker than claimed so far, and a degree of uncertainty in the
alpha-lambda plane related to the handling of the various approximations used
in the theory for both the Casimir and the Yukawa forces. We further argue that
the recently discussed form for the proximity force approximation is
equivalent, for a geometry made of a generic object interacting with an
infinite planar slab, to the usual exact integration of any additive two-body
interaction, without any need to invoke approximation schemes. If the planar
slab is of finite size, an additional source of systematic error arises due to
the breaking of the planar translational invariance of the system, and we
finally discuss to what extent this may affect limits obtained on power-law and
Yukawa forces.Comment: 11 page, 5 figure
Aerodynamic noise from rigid trailing edges with finite porous extensions
This paper investigates the effects of finite flat porous extensions to
semi-infinite impermeable flat plates in an attempt to control trailing-edge
noise through bio-inspired adaptations. Specifically the problem of sound
generated by a gust convecting in uniform mean steady flow scattering off the
trailing edge and permeable-impermeable junction is considered. This setup
supposes that any realistic trailing-edge adaptation to a blade would be
sufficiently small so that the turbulent boundary layer encapsulates both the
porous edge and the permeable-impermeable junction, and therefore the
interaction of acoustics generated at these two discontinuous boundaries is
important. The acoustic problem is tackled analytically through use of the
Wiener-Hopf method. A two-dimensional matrix Wiener-Hopf problem arises due to
the two interaction points (the trailing edge and the permeable-impermeable
junction). This paper discusses a new iterative method for solving this matrix
Wiener-Hopf equation which extends to further two-dimensional problems in
particular those involving analytic terms that exponentially grow in the upper
or lower half planes. This method is an extension of the commonly used "pole
removal" technique and avoids the needs for full matrix factorisation.
Convergence of this iterative method to an exact solution is shown to be
particularly fast when terms neglected in the second step are formally smaller
than all other terms retained. The final acoustic solution highlights the
effects of the permeable-impermeable junction on the generated noise, in
particular how this junction affects the far-field noise generated by
high-frequency gusts by creating an interference to typical trailing-edge
scattering. This effect results in partially porous plates predicting a lower
noise reduction than fully porous plates when compared to fully impermeable
plates.Comment: LaTeX, 20 pp., 19 graphics in 6 figure
BB Potentials in Quenched Lattice QCD
The potentials between two B-mesons are computed in the heavy-quark limit
using quenched lattice QCD at . Non-zero central
potentials are clearly evident in all four spin-isospin channels, (I,s_l) =
(0,0) , (0,1) , (1,0) , (1,1), where s_l is the total spin of the light degrees
of freedom. At short distance, we find repulsion in the channels and
attraction in the I=s_l channels. Linear combinations of these potentials that
have well-defined spin and isospin in the t-channel are found, in three of the
four cases, to have substantially smaller uncertainties than the potentials
defined with the s-channel (I,s_l), and allow quenching artifacts from single
hairpin exchange to be isolated. The BB*\pi coupling extracted from the
long-distance behavior of the finite-volume t-channel potential is found to be
consistent with quenched calculations of the matrix element of the isovector
axial-current. The tensor potentials in both of the s_l = 1 channels are found
to be consistent with zero within calculational uncertainties.Comment: 30 page
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