1,026 research outputs found
Spectral properties of locally correlated electrons in a BCS superconductor
We present a detailed study of the spectral properties of a locally
correlated site embedded in a BCS superconducting medium. To this end the
Anderson impurity model with superconducting bath is analysed by numerical
renormalisation group (NRG) calculations. We calculate one and two-particle
dynamic response function to elucidate the spectral excitation and the nature
of the ground state for different parameter regimes with and without
particle-hole symmetry. The position and weight of the Andreev bound states is
given for all relevant parameters. We also present phase diagrams for the
different ground state parameter regimes. This work is also relevant for
dynamical mean field theory extensions with superconducting symmetry breaking.Comment: 22 pages, 12 figure
Sum-rule Conserving Spectral Functions from the Numerical Renormalization Group
We show how spectral functions for quantum impurity models can be calculated
very accurately using a complete set of ``discarded'' numerical renormalization
group eigenstates, recently introduced by Anders and Schiller. The only
approximation is to judiciously exploit energy scale separation. Our derivation
avoids both the overcounting ambiguities and the single-shell approximation for
the equilibrium density matrix prevalent in current methods, ensuring that
relevant sum rules hold rigorously and spectral features at energies below the
temperature can be described accurately.Comment: 4 pages + 1 page appendix, 2 figure
The Kondo crossover in shot noise of a single quantum dot with orbital degeneracy
We investigate out of equilibrium transport through an orbital Kondo system
realized in a single quantum dot, described by the multiorbital impurity
Anderson model. Shot noise and current are calculated up to the third order in
bias voltage in the particle-hole symmetric case, using the renormalized
perturbation theory. The derived expressions are asymptotically exact at low
energies. The resulting Fano factor of the backscattering current is
expressed in terms of the Wilson ratio and the orbital degeneracy as
at zero temperature. Then,
for small Coulomb repulsions , we calculate the Fano factor exactly up to
terms of order , and also carry out the numerical renormalization group
calculation for intermediate in the case of two- and four-fold degeneracy
(). As increases, the charge fluctuation in the dot is suppressed,
and the Fano factor varies rapidly from the noninteracting value to the
value in the Kondo limit , near the crossover region
, with the energy scale of the hybridization .Comment: 10 pages, 4 figure
Anderson impurity in pseudo-gap Fermi systems
We use the numerical renormalization group method to study an Anderson
impurity in a conduction band with the density of states varying as rho(omega)
\propto |omega|^r with r>0. We find two different fixed points: a local-moment
fixed point with the impurity effectively decoupled from the band and a
strong-coupling fixed point with a partially screened impurity spin. The
specific heat and the spin-susceptibility show powerlaw behaviour with
different exponents in strong-coupling and local-moment regime. We also
calculate the impurity spectral function which diverges (vanishes) with
|omega|^{-r} (|\omega|^r) in the strong-coupling (local moment) regime.Comment: 8 pages, LaTeX, 4 figures includes as eps-file
Numerical Renormalization Group Calculations for the Self-energy of the impurity Anderson model
We present a new method to calculate directly the one-particle self-energy of
an impurity Anderson model with Wilson's numerical Renormalization Group method
by writing this quantity as the ratio of two correlation functions. This way of
calculating Sigma(z) turns out to be considerably more reliable and accurate
than via the impurity Green's function alone. We show results for the
self-energy for the case of a constant coupling between impurity and conduction
band (ImDelta = const) and the effective Delta(z) arising in the Dynamical Mean
Field Theory of the Hubbard model. Implications to the problem of the
metal-insulator transition in the Hubbard model are also discussed.Comment: 18 pages, 9 figures, submitted to J. Phys.: Condens. Matte
The Strong Coupling Fixed-Point Revisited
In recent work we have shown that the Fermi liquid aspects of the strong
coupling fixed point of the s-d and Anderson models can brought out more
clearly by interpreting the fixed point as a renormalized Anderson model,
characterized by a renormalized level , resonance width,
, and interaction , and a simple prescription for their
calculation was given using the numerical renormalization group (NRG). These
three parameters completely specify a renormalized perturbation theory (RPT)
which leads to exact expressions for the low temperature behaviour. Using a
combination of the two techniques, NRG to determine ,
, and , and then substituting these in the RPT
expressions gives a very efficient and accurate way of calculating the low
temperature behaviour of the impurity as it avoids the necessity of subtracting
out the conduction electron component. Here we extend this approach to an
Anderson model in a magnetic field, so that , ,
and become dependent on the magnetic field. The de-renormalization
of the renormalized quasiparticles can then be followed as the magnetic field
strength is increased. Using these running coupling constants in a RPT
calculation we derive an expression for the low temperature conductivity for
arbitrary magnetic field strength.Comment: Contribution to JPSJ volume commemorating the 40th anniversary of the
publication of Kondo's original pape
A Numerical Renormalization Group approach to Green's Functions for Quantum Impurity Models
We present a novel technique for the calculation of dynamical correlation
functions of quantum impurity systems in equilibrium with Wilson's numerical
renormalization group. Our formulation is based on a complete basis set of the
Wilson chain. In contrast to all previous methods, it does not suffer from
overcounting of excitation. By construction, it always fulfills sum rules for
spectral functions. Furthermore, it accurately reproduces local thermodynamic
expectation values, such as occupancy and magnetization, obtained directly from
the numerical renormalization group calculations.Comment: 13 pages, 7 figur
The bosonic Kondo effect
The Kondo effect is associated with the formation of a many-body ground state
that contains a quantum-mechanical entanglement between a (localized) fermion
and the free fermions. We show that a bosonic version of the Kondo effect can
occur in degenerate atomic Fermi gases near the Feshbach resonance. We also
discuss how this bosonic Kondo effect can be observed experimentally.Comment: 4 pages, 2 figures, some references added, some removed. More
comments adde
Two Anderson impurities in a 2D host with Rashba spin-orbit interaction
We have studied the two-dimensional two-impurity Anderson model with
additional Rashba spin-orbit interaction by means of the modified perturbation
theory. The impurity Green's functions we have constructed exactly reproduce
the first four spectral moments. We discuss the height and the width of the
even/odd Kondo peaks as functions of the inter-impurity distance and the Rashba
energy (the strength of the Rashba spin-orbit interaction). For small
impurity separations the Kondo temperature shows a non-monotonic dependence on
being different in the even and the odd channel. We predict that the
Kondo temperature has only almost linear dependence on and not an
exponential increase with Comment: To be published in Phys. Rev.
Renormalization group approaches to strongly correlated electron systems
In recent years the numerical renormalization group (NRG) method has been extended to the calculation of dynamic response functions and transport properties of magnetic impurity models. The approach can now be applied more widely to lattice models of strongly correlated electron systems by the use of dynamical mean field theory (DMFT), in which the lattice problem is transformed into one for an e ective impurity with an additional self-consistency constraint. We review these developments and assess the potential for further applications of this approach. We also discuss an alternative approach to renormalization, renormalized perturbation theory, in which the leading asymptotically exact results for the low temperature regime for a number of magnetic impurity models can be obtained within nite order perturbation theory
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