42 research outputs found
Protracted Screening in the Periodic Anderson Model
The asymmetric infinite-dimensional periodic Anderson model is examined with
a quantum Monte Carlo simulation. For small conduction band filling, we find a
severe reduction in the Kondo scale, compared to the impurity value, as well as
protracted spin screening consistent with some recent controversial
photoemission experiments. The Kondo screening drives a ferromagnetic
transition when the conduction band is quarter-filled and both the RKKY and
superexchange favor antiferromagnetism. We also find RKKY-driven ferromagnetic
and antiferromagnetic transitions.Comment: 5 pages, LaTeX and 4 PS figure
Coherence scale of the Kondo lattice
It is shown that the large-N approach yields two energy scales for the Kondo
lattice model. The single-impurity Kondo temperature, , signals the onset
of local singlet formation, while Fermi liquid coherence sets in only below a
lower scale, . At low conduction electron density
("exhaustion" limit), the ratio is much smaller than unity, and
is shown to depend only on and not on the Kondo coupling. The physical
meaning of these two scales is demonstrated by computing several quantities as
a function of and temperature.Comment: 4 pages, 4 eps figures. Minor changes. To appear in Phys. Rev. Let
Superconducting Instability in the Periodic Anderson Model
Employing a quantum Monte Carlo simulation we find a pairing instability in
the normal state of the infinite dimensional periodic Anderson model.
Superconductivity arises from a normal state in which the screening is
protracted and which is clearly not a Fermi liquid. The phase diagram is
reentrant reflecting competition between superconductivity and Fermi liquid
formation. The estimated superconducting order parameter is even, but has nodes
as a function of frequency. This opens the possibility of a temporal node and
an effective order parameter composed of charge pairs and spin excitations.Comment: one postscript file, 6 pages including 6 figures. To appear in Phil.
Mag.
Low-temperature coherence in the periodic Anderson model: Predictions for photoemission of heavy Fermions
We present numerically exact predictions of the periodic and single-impurity
Anderson models to address photoemission experiments on heavy Fermion systems.
Unlike the single impurity model the lattice model is able to account for the
enhanced intensity, dispersion, and apparent weak temperature dependence of the
Kondo resonant peak seen in recent controversial photoemission experiments. We
present a consistent interpretation of these results as a crossover from the
impurity regime to an effective Hubbard model regime described by Nozieres.Comment: 4 pages, 3 figure
"Exhaustion" Physics in the Periodic Anderson Model using Iterated Perturbation Theory
We discuss the "exhaustion" problem in the context of the Periodic Anderson
Model using Iterated Perturbation Theory(IPT) within the Dynamical Mean Field
Theory. We find that, despite its limitations, IPT captures the exhaustion
physics, which manifests itself as a dramatic, strongly energy dependent
suppression of the effective Anderson impurity problem. As a consequence, low
energy scales in the lattice case are strongly suppressed compared to the
"Kondo scale" in the single-impurity picture. The IPT results are in
qualitative agreement with recent Quantum Monte Carlo results for the same
problem.Comment: 13 preprint pages including 1 table and 4 eps figures, replaced by
revised version, accepted for publication in Europhysics Letters, added
references and conten
Dynamic correlations in doped 1D Kondo insulator: Finite-T DMRG study
The finite-T DMRG method is applied to the one-dimensional Kondo lattice
model to calculate dynamic correlation functions. Dynamic spin and charge
correlations, S_f(omega), S_c(omega), and N_c(omega), and quasiparticle density
of states rho(omega) are calculated in the paramagnetic metallic phase for
various temperatures and hole densities. Near half filling, it is shown that a
pseudogap grows in these dynamic correlation functions below the crossover
temperature characterized by the spin gap at half filling. A sharp peak at
omega=0 evolves at low temperatures in S_f(omega) and N_c(omega). This may be
an evidence of the formation of the collective excitations, and this confirms
that the metallic phase is a Tomonaga-Luttinger liquid in the low temperature
limit.Comment: 5 pages, 6 Postscript figures, REVTe
Nonlocal dynamical correlations of strongly interacting electron systems
We introduce an extension of the dynamical mean field approximation (DMFA) which retains the causal properties and generality of the DMFA, but allows for systematic inclusion of non-local corrections. Our technique maps the problem to a self-consistently embedded cluster. The DMFA (exact result) is recovered as the cluster size goes to one (infinity). As a demonstration, we study the Falicov-Kimball model using a variety of cluster sizes. We show that the sum rules are preserved, the spectra are positive definite, and the non-local correlations suppress the CDW transition temperature. Introduction. Strongly interacting electron systems have been on the forefront of theoretical and experimental interest for several decades. This interest has intensified with the discovery of a variety of Heavy Fermion and related non Fermi liquid systems and the high-T c superconductors. In all these systems strong electronic interactions play a dominant role in the selection of at least the low temperature phase. The simplest theoretical models of strongly correlated electrons, the Hubbard model (HM) and the periodic Anderson model (PAM), have remained unsolved in more than one dimension despite a multitude of sophisticated techniques introduced since the inception of the models. With the ground breaking work by Metzner and Vollhardt [1] it was realized that these models become significantly simpler in the limit of infinite dimensions, D = ∞. Namely, provided that the kinetic energy is properly rescaled as 1/ √ D, they retain only local, though nontrivial dynamics: The self energy is constant in momentum space, though it has a complicated frequency dependence. Consequently, the HM and PAM map onto a generalized single impurity Anderson model. The thermodynamics and phase diagram have been obtained numerically by quantum Monte Carlo (QMC) and other methods
The low-energy scale of the periodic Anderson model
Wilson's Numerical Renormalization Group method is used to study the
paramagnetic ground state of the periodic Anderson model within the dynamical
mean-field approach. For the particle-hole symmetric model, which is a Kondo
insulator, we find that the lattice Kondo scale T_0 is strongly enhanced over
the impurity scale T_K; T_0/T_K ~ exp(1/3I), where I is the Schrieffer-Wolff
exchange coupling. In the metallic regime, where the conduction band filling is
reduced from one, we find characteristic signatures of Nozi\`eres exhaustion
scenario, including a strongly reduced lattice Kondo scale, a significant
suppression of the states available to screen the f-electron moment, and a
Kondo resonance with a strongly enhanced height. However, in contrast to the
quantitative predictions of Nozi\`eres, we find that the T_0 ~ T_K with a
coefficient which depends strongly on conduction band filling.Comment: 11 pages, 9 figures, submitted to Phys. Rev.