504 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
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
Low-energy properties of the Kondo lattice model
We study the zero-temperature properties of the Kondo lattice model within
the dynamical mean-field theory. As impurity solver we use the numerical
renormalization group. We present results for the paramagnetic case showing the
anticipated heavy Fermion physics, including direct evidence for the appearance
of a large Fermi surface for antiferromagnetic exchange interaction. Allowing
for the formation of a Neel state, we observe at finite doping an
antiferromagnetic metal below a critical exchange interaction, which shows a
crossover from a local-moment antiferromagnet with a small Fermi surface for
weak exchange coupling to a heavy-fermion antiferromagnet with a large Fermi
surface for increasing exchange. Including lattice degrees of freedom via an
additional Holstein term we observe a significant suppression of the Kondo
effect, leading to strongly reduced lowenergy scale. For too large
electron-phonon coupling we find a complete collaps of the heavy Fermi liquid
and the formation of polarons.Comment: 11 pages, 7 figure
Dynamic susceptibilities of the single impurity Anderson model within an enhanced non-crossing approximation
The single impurity Anderson model (SIAM) is studied within an enhanced
non-crossing approximation (ENCA). This method is extended to the calculation
of susceptibilities and thoroughly tested, also in order to prepare
applications as a building block for the calculation of susceptibilities and
phase transitions in correlated lattice systems. A wide range of model
parameters, such as impurity occupancy, temperature, local Coulomb repulsion
and hybridization strength, are studied. Results for the spin and charge
susceptibilities are presented. By comparing the static quantities to exact
Bethe ansatz results, it is shown that the description of the magnetic
excitations of the impurity within the ENCA is excellent, even in situations
with large valence fluctuations or vanishing Coulomb repulsion. The description
of the charge susceptibility is quite accurate in situations where the singly
occupied ionic configuration is the unperturbed ground state; however, it seems
to overestimate charge fluctuations in the asymmetric model at too low
temperatures. The dynamic spin excitation spectra is dominated by the
Kondo-screening of the impurity spin through the conduction band, i.e. the
formation of the local Kondo-singlet. A finite local Coulomb interaction U
leads to a drastic reduction of the charge response via processes involving the
doubly occupied impurity state. In the asymmetric model, the charge
susceptibility is enhanced for excitation energies smaller than the Kondo scale
T_K due to the influence of valence fluctuations.Comment: 16 pages, 13 figure
Anomalous Normal-State Properties of High-T Superconductors -- Intrinsic Properties of Strongly Correlated Electron Systems?
A systematic study of optical and transport properties of the Hubbard model,
based on Metzner and Vollhardt's dynamical mean-field approximation, is
reviewed. This model shows interesting anomalous properties that are, in our
opinion, ubiquitous to single-band strongly correlated systems (for all spatial
dimensions greater than one), and also compare qualitatively with many
anomalous transport features of the high-T cuprates. This anomalous
behavior of the normal-state properties is traced to a ``collective single-band
Kondo effect,'' in which a quasiparticle resonance forms at the Fermi level as
the temperature is lowered, ultimately yielding a strongly renormalized Fermi
liquid at zero temperature.Comment: 27 pages, latex, 13 figures, Invited for publication in Advances in
Physic
Zeros of the Partition Function and Pseudospinodals in Long-Range Ising Models
The relation between the zeros of the partition function and spinodal
critical points in Ising models with long-range interactions is investigated.
We find the spinodal is associated with the zeros of the partition function in
four-dimensional complex temperature/magnetic field space. The zeros approach
the real temperature/magnetic field plane as the range of interaction
increases.Comment: 20 pages, 9 figures, accepted to PR
Band Calculation for Ce-compounds on the basis of Dynamical Mean Field Theory
The band calculation scheme for electron compounds is developed on the
basis of the dynamical mean field theory (DMFT) and the LMTO method. The
auxiliary impurity problem is solved by a method named as NCAv', which
includes the correct exchange process of the virtual
excitation as the vertex correction to the non-crossing approximation (NCA) for
the fluctuation. This method leads to the correct magnitude
of the Kondo temperature, , and makes it possible to carry out
quantitative DMFT calculation including the crystalline field (CF) and the
spin-orbit (SO) splitting of the self-energy. The magnetic excitation spectra
are also calculated to estimate . It is applied to Ce metal and CeSb
at T=300 K as the first step. In Ce metal, the hybridization intensity (HI)
just below the Fermi energy is reduced in the DMFT band. The photo-emission
spectra (PES) have a conspicuous SO side peak, similar to that of experiments.
is estimated to be about 70 K in -Ce, while to be about
1700 K in -Ce. In CeSb, the double-peak-like structure of PES is
reproduced. In addition, which is not so low is obtained because HI
is enhanced just at the Fermi energy in the DMFT band.Comment: 30pages, 18 figure
Unified description of Fermi and non-Fermi liquid behavior in a conserving slave boson approximation for strongly correlated impurity models
We show that the presence of Fermi or non-Fermi liquid behavior in the SU(N)
x SU(M) Anderson impurity models may be read off the infrared threshold
exponents governing the spinon and holon dynamics in a slave boson
representation of these models. We construct a conserving T-matrix
approximation which recovers the exact exponents with good numerical accuracy.
Our approximation includes both coherent spin flip scattering and charge
fluctuation processes. For the single-channel case the tendency to form bound
states drastically modifies the low energy behavior. For the multi-channel case
in the Kondo limit the bound state contributions are unimportant.Comment: 4 pages, Latex, 3 postscript figures included Final version with
minor changes in wording, to appear in Phys.Rev.Let
Investigation of the Two-Particle-Self-Consistent Theory for the Single-Impurity Anderson Model and an Extension to the Case of Strong Correlation
The two-particle-self-consistent theory is applied to the single-impurity
Anderson model. It is found that it cannot reproduce the small energy scale in
the strong correlation limit. A modified scheme to overcome this difficulty is
proposed by introducing an appropriate vertex correction explicitly. Using the
same vertex correction, the self-energy is investigated, and it is found that
under certain assumptions it reproduces the result of the modified perturbation
theory which interpolates the weak and the strong correlation limits.Comment: 5 pages, 7 figures, submitted to J. Phys. Soc. Jp
Exact Criterion for Determining Clustering vs. Reentrant Melting Behavior for Bounded Interaction Potentials
We examine in full generality the phase behavior of systems whose constituent
particles interact by means of potentials which do not diverge at the origin,
are free of attractive parts and decay fast enough to zero as the interparticle
separation r goes to infinity. By employing a mean field-density functional
theory which is shown to become exact at high temperatures and/or densities, we
establish a criterion which determines whether a given system will freeze at
all temperatures or it will display reentrant melting and an upper freezing
temperature.Comment: 5 pages, 3 figures, submitted to PRL on March 29, 2000 New version:
10 pages, 9 figures, forwarded to PRE on October 16, 200
- …