251 research outputs found
pi-Junction behavior and Andreev bound states in Kondo quantum dots with superconducting leads
We investigate the temperature- and coupling-dependent transport through
Kondo dot contacts with symmetric superconducting s-wave leads. For finite
temperature T we use a superconducting extension of a selfconsistent auxiliary
boson scheme, termed SNCA, while at T=0 a perturbative renormalization group
treatment is applied. The finite-temperature phase diagram for the 0--pi
transition of the Josephson current in the junction is established and related
to the phase-dependent position of the subgap Kondo resonance with respect to
the Fermi energy. The conductance of the contact is evaluated in the zero-bias
limit. It approaches zero in the low-temperature regime, however, at finite T
its characteristics are changed through the coupling- and temperature-dependent
0--pi transition.Comment: 12 pages, 12 figure
Kondo effect in a magnetic field and the magnetoresistivity of Kondo alloys
The effect of a magnetic field on the spectral density of a
Kondo impurity is investigated at zero and finite temperatures by using
Wilson's numerical renormalization group method. A splitting of the total
spectral density is found for fields larger than a critical value
, where is the Kondo scale. The splitting
correlates with a peak in the magnetoresistivity of dilute magnetic alloys
which we calculate and compare with the experiments on
. The linear magnetoconductance of quantum
dots exhibiting the Kondo effect is also calculated.Comment: 4 pages, 4 eps figure
Kinks in the electronic dispersion of the Hubbard model away from half filling
We study kinks in the electronic dispersion of a generic strongly correlated
system by dynamic mean-field theory (DMFT). The focus is on doped systems away
from particle-hole symmetry where valence fluctuations matter potentially.
Three different algorithms are compared to asses their strengths and
weaknesses, as well as to clearly distinguish physical features from
algorithmic artifacts. Our findings extend a view previously established for
half-filled systems where kinks reflect the coupling of the fermionic
quasiparticles to emergent collective modes, which are identified here as spin
fluctuations. Kinks are observed when strong spin fluctuations are present and,
additionally, a separation of energy scales for spin and charge excitations
exists. Both criteria are met by strongly correlated systems close to a
Mott-insulator transition. The energies of the kinks and their doping
dependence fit well to the kinks in the cuprates, which is surprising in view
of the spatial correlations neglected by DMFT.Comment: 13 pages, 15 figure
Diagrammatic method for investigating universal behavior of impurity systems
The universal behavior of magnetic impurities in a metal is proved with the
help of skeleton diagrams. The energy scales are derived from the structure of
the skeleton diagrams. A minimal set of skeleton diagrams is sorted out that
scales exactly. For example, the non-crossing approximation for the Anderson
impurity model can describe the crossover phenomenon. The universal
Wilson-number is calculated within the non-crossing approximation. The method
allows for an assessment of various approximations for impurity Hamiltonians.Comment: 21 pages, 3 figure
Anderson impurity model at finite Coulomb interaction U: generalized Non-crossing Approximation
We present an extension of the non-crossing approximation (NCA), which is
widely used to calculate properties of Anderson impurity models in the limit of
infinite Coulomb repulsion , to the case of finite . A
self-consistent conserving pseudo-particle representation is derived by
symmetrizing the usual NCA diagrams with respect to empty and doubly occupied
local states. This requires an infinite summation of skeleton diagrams in the
generating functional thus defining the ``Symmetrized finite-U NCA'' (SUNCA).
We show that within SUNCA the low energy scale (Kondo temperature) is
correctly obtained, in contrast to other simpler approximations discussed in
the literature.Comment: 7 pages, 6 figure
Heavy-Fermions in a Transition-Metal Compound:
The recent discovery of heavy-Fermion properties in Lithium Vanadate and the
enormous difference in its properties from the properties of Lithium Titanate
as well as of the manganite compounds raise some puzzling questions about
strongly correlated Fermions. These are disscussed as well as a solution to the
puzzles provided.Comment: late
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