905 research outputs found
Superconductivity in the Kondo lattice model
We study the Kondo lattice model with additional attractive interaction
between the conduction electrons within the dynamical mean-field theory using
the numerical renormalization group to solve the effective quantum impurity
problem. In addition to normal-state and magnetic phases we also allow for the
occurrence of a superconducting phase. In the normal phase we observe a very
sensitive dependence of the low-energy scale on the conduction-electron
interaction. We discuss the dependence of the superconducting transition on the
interplay between attractive interaction and Kondo exchange.Comment: Submitted to ICM 2009 Conference Proceeding
A Fermi Sea of Heavy Electrons (a Kondo Lattice) is Never a Fermi Liquid
I demonstrate a contradiction which arises if we assume that the Fermi
surface in a heavy electron metal represents a finite jump in occupancy
An Enhanced Perturbational Study on Spectral Properties of the Anderson Model
The infinite- single impurity Anderson model for rare earth alloys is
examined with a new set of self-consistent coupled integral equations, which
can be embedded in the large expansion scheme ( is the local spin
degeneracy). The finite temperature impurity density of states (DOS) and the
spin-fluctuation spectra are calculated exactly up to the order . The
presented conserving approximation goes well beyond the -approximation
({\em NCA}) and maintains local Fermi-liquid properties down to very low
temperatures. The position of the low lying Abrikosov-Suhl resonance (ASR) in
the impurity DOS is in accordance with Friedel's sum rule. For its shift
toward the chemical potential, compared to the {\em NCA}, can be traced back to
the influence of the vertex corrections. The width and height of the ASR is
governed by the universal low temperature energy scale . Temperature and
degeneracy -dependence of the static magnetic susceptibility is found in
excellent agreement with the Bethe-Ansatz results. Threshold exponents of the
local propagators are discussed. Resonant level regime () and intermediate
valence regime () of the model are thoroughly
investigated as a critical test of the quality of the approximation. Some
applications to the Anderson lattice model are pointed out.Comment: 19 pages, ReVTeX, no figures. 17 Postscript figures available on the
WWW at http://spy.fkp.physik.th-darmstadt.de/~frithjof
Charge gaps and quasiparticle bands of the ionic Hubbard model
The ionic Hubbard model on a cubic lattice is investigated using analytical
approximations and Wilson's renormalization group for the charge excitation
spectrum. Near the Mott insulating regime, where the Hubbard repulsion starts
to dominate all energies, the formation of correlated bands is described. The
corresponding partial spectral weights and local densities of states show
characteristic features, which compare well with a hybridized-band picture
appropriate for the regime at small , which at half-filling is known as a
band insulator. In particular, a narrow charge gap is obtained at half-filling,
and the distribution of spectral quasi-particle weight reflects the fundamental
hybridization mechanism of the model
Renormalization Group Approach to Spectral Properties of the Two-Channel Anderson Impurity Model
The impurity Green function and dynamical susceptibilties for the two-channel
Anderson impurity model are calculated. An exact expression for the self-energy
of the impurity Green function is derived. The imaginary part of the
self-energy scales as \sqrt{|\w/T_K|} for serving as a hallmark for
non-Fermi behavior. The many-body resonance is pinned to a universal value
at \w=0. Its shape becomes increasingly more symmetric for
the Kondo-regimes of the model. The dynamical susceptibilities are governed by
two energy scales and and approach a constant value for \w\to 0,
whereas relation \chi''(\w)\propto \w holds for the single channel model.Comment: 4 pages, 4 figure, revte
The Hubbard Model at Infinite Dimensions: Thermodynamic and Transport Properties
We present results on thermodynamic quantities, resistivity and optical
conductivity for the Hubbard model on a simple hypercubic lattice in infinite
dimensions. Our results for the paramagnetic phase display the features
expected from an intuitive analysis of the one-particle spectra and
substantiate the similarity of the physics of the Hubbard model to those of
heavy fermion systems. The calculations were performed using an approximate
solution to the single-impurity Anderson model, which is the key quantity
entering the solution of the Hubbard model in this limit. To establish the
quality of this approximation we compare its results, together with those
obtained from two other widely used methods, to essentially exact quantum Monte
Carlo results.Comment: 29 pages, 16 figure
Conserving approximations in direct perturbation theory: new semianalytical impurity solvers and their application to general lattice problems
For the treatment of interacting electrons in crystal lattices approximations
based on the picture of effective sites, coupled in a self-consistent fashion,
have proven very useful. Particularly in the presence of strong local
correlations, a local approach to the problem, combining a powerful method for
the short ranged interactions with the lattice propagation part of the
dynamics, determines the quality of results to a large extent. For a
considerable time the non crossing approximation (NCA) in direct perturbation
theory, an approach originally developed by Keiter for the Anderson impurity
model, built a standard for the description of the local dynamics of
interacting electrons. In the last couple of years exact methods like the
numerical renormalization group (NRG) as pioneered by Wilson, have surpassed
this approximation as regarding the description of the low energy regime. We
present an improved approximation level of direct perturbation theory for
finite Coulomb repulsion U, the crossing approximation one (CA1) and discuss
its connections with other generalizations of NCA. CA1 incorporates all
processes up to fourth order in the hybridization strength V in a
self-consistent skeleton expansion, retaining the full energy dependence of the
vertex functions. We reconstruct the local approach to the lattice problem from
the point of view of cumulant perturbation theory in a very general way and
discuss the proper use of impurity solvers for this purpose. Their reliability
can be tested in applications to e.g. the Hubbard model and the
Anderson-lattice model. We point out shortcomings of existing impurity solvers
and improvements gained with CA1 in this context.
This paper is dedicated to the memory of Hellmut Keiter.Comment: 45 pages, 22 figure
From ferromagnetism to spin-density wave: Magnetism in the two channel periodic Anderson model
The magnetic properties of the two-channel periodic Anderson model for
uranium ions, comprised of a quadrupolar and a magnetic doublet are
investigated through the crossover from the mixed-valent to the stable moment
regime using dynamical mean field theory. In the mixed-valent regime
ferromagnetism is found for low carrier concentration on a hyper-cubic lattice.
The Kondo regime is governed by band magnetism with small effective moments and
an ordering vector \q close to the perfect nesting vector. In the stable
moment regime nearest neighbour anti-ferromagnetism dominates for less than
half band filling and a spin density wave transition for larger than half
filling. is governed by the renormalized RKKY energy scale \mu_{eff}^2
^2 J^2\rho_0(\mu).Comment: 4 pages, RevTeX, 3 eps figure
Self-Consistent Perturbation Theory for Thermodynamics of Magnetic Impurity Systems
Integral equations for thermodynamic quantities are derived in the framework
of the non-crossing approximation (NCA). Entropy and specific heat of 4f
contribution are calculated without numerical differentiations of thermodynamic
potential. The formulation is applied to systems such as PrFe4P12 with
singlet-triplet crystalline electric field (CEF) levels.Comment: 3 pages, 2 figures, proc. ASR-WYP-2005 (JAERI
Investigation of on-site inter-orbital single electron hoppings in general multi-orbital systems
A general multi-orbital Hubbard model, which includes on-site inter-orbital
electron hoppings, is introduced and studied. It is shown that the on-site
inter-orbital single electron hopping is one of the most basic interactions.
Two electron spin-flip and pair-hoppings are shown to be correlation effects of
higher order than the on-site inter-orbital single hopping. It is shown how the
double and higher hopping interactions can be well-defined for arbitrary
systems. The two-orbital Hubbard model is studied numerically to demonstrate
the influence of the single electron hopping effect, leading to a change of the
shape of the bands and a shrinking of the difference between the two bands.
Inclusion of the on-site inter-orbital hopping suppresses the so-called
orbital-selective Mott transition.Comment: 5 pages, 3 figure
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