117 research outputs found
Quantum Corrals, Eigenmodes and Quantum Mirages in s-wave Superconductors
We study the electronic structure of magnetic and non-magnetic quantum
corrals embedded in s-wave superconductors. We demonstrate that a quantum
mirage of an impurity bound state peak can be projected from the occupied into
the empty focus of a non-magnetic quantum corral via the excitation of the
corral's eigenmodes. We observe an enhanced coupling between magnetic
impurities inside the corral, which can be varied through oscillations in the
corral's impurity potential. Finally, we discuss the form of eigenmodes in
magnetic quantum corrals.Comment: 4 pages, 4 figure
Bethe ansatz approach to thermodynamics of superconducting magnetic alloys
We derive thermodynamic Bethe ansatz equations for a model describing an
Anderson impurity embedded in a BCS superconductor. The equations
are solved analytically in the zero-temperature limit, T=0. It is shown that
the impurities depress superconductivity in the Kondo limit, however at T=0 the
system remains in the superconducting state for any impurity concentration. In
the mixed-valence regime, an impurity contribution to the density of states of
the system near the Fermi level overcompensates a Cooper pairs weakening, and
superconductivity is enhanced.Comment: 4 pages, RevTex, to appear in PR
Bound States for a Magnetic Impurity in a Superconductor
We discuss a solvable model describing an Anderson like impurity in a BCS
superconductor. The model can be mapped onto an Ising field theory in a
boundary magnetic field, with the Ising fermions being the quasi-particles of
the Bogoliubov transformation in BCS theory. The reflection S-matrix exhibits
Andreev scattering, and the existence of bound states of the quasi-particles
with the impurity lying inside the superconducting gap.Comment: 7 pages, Plain Te
Nonvanishing Local Moment in Triplet Superconductors
The Kondo effect in a -wave superconductor is studied by
applying the Wilson's numerical renormalization group method. In this type of
superconductor with a full energy gap like a s-wave one, the ground state is
always a spin doublet, while a local spin is shrunk by the Kondo effect. The
calculated magnetic susceptibility indicates that the spin of the ground state
is generated by the orbital effect of the -wave Cooper
pairs. The effect of spin polarization of the triplet superconductor is also
discussed.Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jp
Quantum Interference between Impurities: Creating Novel Many-Body States in s-wave Superconductors
We demonstrate that quantum interference of electronic waves that are
scattered by multiple magnetic impurities in an s-wave superconductor gives
rise to novel bound states. We predict that by varying the inter-impurity
distance or the relative angle between the impurity spins, the states' quantum
numbers, as well as their distinct frequency and spatial dependencies, can be
altered. Finally, we show that the superconductor can be driven through
multiple local crossovers in which its spin polarization, , changes
between and 1.Comment: 4 pages, 4 figure
Spin and orbital effects of Cooper pairs coupled to a single magnetic impurity
The Kondo effect strongly depends on spin and orbital degrees of freedom of
unconventional superconductivity. We focus on the Kondo effect in the -wave and -wave superconductors to compare the
magnetic properties of the spin-triplet and spin-singlet Cooper pairs. The
difference appears when both of the paired electrons couple to a local spin
directly. For the -wave, the ground state is always a spin doublet
for a local spin, and it is always a spin singlet for
. The latter is due to uniaxial spin anisotropy of the triplet
Cooper pair. For the -wave, the interchange of ground
states occurs, which resembles a competition between the Kondo effect and the
superconducting energy gap in s-wave superconductors. Thus the internal degrees
of freedom of Cooper pairs give a variety to the Kondo effect.Comment: 7 pages, 6 figures, RevTex, to be published in Phys. Rev.
Etude comparative de deux cryptosystèmes: L’AES versus l’attracteur Chaotique
Le transfert croissant de données multimédias engendre des besoins en termes de sécurité d’informations. Pour répondre aux normes de confidentialité, plusieurs algorithmes de cryptage ont été développés ces dernières années. En raison de l’utilisation massive des données images dans divers domaines (médicale, industrielle, …), il est nécessaire d’utiliser et d’adapter ces techniques de sécurité à ce type de données. Dans ce contexte, une étude comparative de deux algorithmes de cryptage appliqués aux images est réalisée. Le premier est le cryptosystème classique dit Advanced Encryption Standard (AES) et le deuxième issu des signaux chaotiques basés sur une carte logistique afin d’évaluer leurs robustesse en termes de sécurité
Gap States in Dilute Magnetic Alloy Superconductors
We study states in the superconducting gap induced by magnetic impurities
using self-consistent quantum Monte Carlo with maximum entropy and formally
exact analytic continuation methods. The magnetic impurity susceptibility has
different characteristics for T_{0} \alt T_{c0} and T_{0} \agt T_{c0}
(: Kondo temperature, : superconducting transition temperature)
due to the crossover between a doublet and a singlet ground state. We
systematically study the location and the weight of the gap states and the gap
parameter as a function of and the concentration of the
impurities.Comment: 4 pages in ReVTeX including 4 encapsulated Postscript figure
L’Amélioration de la Sécurité du Chiffrement Algébrique Modulaire par les Générateurs de Fibonacci
Le Chiffre de Hill est l'un des algorithmes à clés symétriques qui a montré des avantages pour le cryptage de données. Cependant l'algorithme original est vulnérable aux «Attaque à Texte Clair Choisi». Un autre revers pour le Cryptage des images est qu’il révèle certaines tendances et ne cache pas toutes les caractéristiques de l'image (images avec des fonds homogènes). Pour surmonter ces problèmes, dans cet article, nous proposons une modification de l’algorithme de Hill en incluant une translation et en introduisant à chaque étape du processus du Cryptage des clés dynamiques Pseudo-Aléatoirement. La partie expérimentale de cet article prouve que la variante proposée donne un meilleur cryptage pour tous types d'images et montre bien une amélioration par rapport à l’algorithme conventionnel
Superconductivity in the Two-Band Hubbard Model in Infinite Dimensions
We study a two-band Hubbard model in the limit of infinite dimensions, using
a combination of analytical methods and Monte-Carlo techniques. The normal
state is found to display various metal to insulators transitions as a function
of doping and interaction strength. We derive self-consistent equations for the
local Green's functions in the presence of superconducting long-range order,
and extend previous algorithms to this case. We present direct numerical
evidence that in a specific range of parameter space, the normal state is
unstable against a superconducting state characterized by a strongly frequency
dependent order-parameter.Comment: 12 pages (14 figures not included, available upon request), Latex,
LPTENS Preprint 93/1
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