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
Binding energy of an impurity in polar microspheres
We have examined the binding energy of a polaron bound to a hydrogenic donor impurity located in a spherical quantum dot by means of a variational technique for both finite and infinite potential models. The polaronic effect on the binding energy has been considered taking into account the ion-phonon coupling by using the Lee-Low-Pines variational method. The results we have obtained show that the binding energies are drastically affected by the dot radius, the potential barrier height and the polaronic effects.We have examined the binding energy of a polaron bound to a hydrogenic donor impurity located in a spherical quantum dot by means of a variational technique for both finite and infinite potential models. The polaronic effect on the binding energy has been considered taking into account the ion-phonon coupling by using the Lee-Low-Pines variational method. The results we have obtained show that the binding energies are drastically affected by the dot radius, the potential barrier height and the polaronic effects
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
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
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.
Numerical Renormalization Group Study of Kondo Effect in Unconventional Superconductors
Orbital degrees of freedom of a Cooper pair play an important role in the
unconventional superconductivity. To elucidate the orbital effect in the Kondo
problem, we investigated a single magnetic impurity coupled to Cooper pairs
with a () symmetry using the numerical
renormalization group method. It is found that the ground state is always a
spin doublet. The analytical solution for the strong coupling limit explicitly
shows that the orbital dynamics of the Cooper pair generates the spin 1/2 of
the ground state.Comment: 4 pages, 2 figures, JPSJ.sty, to be published in J. Phys. Soc. Jpn.
70 (2001) No. 1
Quantum phase transition in a minimal model for the Kondo effect in a Josephson junction
We propose a minimal model for the Josephson current through a quantum dot in
a Kondo regime. We start with the model that consists of an Anderson impurity
connected to two superconducting (SC) leads with the gaps
, where for the lead at left and right. We show that, when one of the SC gaps is
much larger than the others , the starting model can
be mapped exactly onto the single-channel model, which consists of the right
lead of and the Anderson impurity with an extra onsite SC gap of
. Here and are
defined with respect to the starting model, and is the level width
due to the coupling with the left lead. Based on this simplified model, we
study the ground-state properties for the asymmetric gap, , using the numerical renormalization group (NRG) method. The
results show that the phase difference of the SC gaps , which induces the Josephson current, disturbs the screening of the
local moment to destabilize the singlet ground state typical of the Kondo
system. It can also drive the quantum phase transition to a magnetic doublet
ground state, and at the critical point the Josephson current shows a
discontinuous change. The asymmetry of the two SC gaps causes a re-entrant
magnetic phase, in which the in-gap bound state lies close to the Fermi level.Comment: 23 pages, 13 figures, typos are correcte
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