996 research outputs found
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
Possible Relevance of Odd Frequency Pairing to Heavy Fermion Superconductivity
What is the character of the gapless quasiparticles in heavy fermion
superconductors (HFSC)? We discuss an odd-frequency pairing interpretation of
HFSC which leads to a two component model for the quasiparticle excitations. In
this picture, line zeroes of unpaired electrons may coexist with gapless
surfaces of paired electrons, with vanishing spin and charge coherence factors
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
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
"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
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
Complete plastid genomes from \u3ci\u3eOphioglossum californicum, Psilotum nudum,\u3c/i\u3e and \u3ci\u3eEquisetum hyemale\u3c/i\u3e reveal an ancestral land plant genome structure and resolve the position of Equisetales among monilophytes
Background: Plastid genome structure and content is remarkably conserved in land plants. This widespread conservation has facilitated taxon-rich phylogenetic analyses that have resolved organismal relationships among many land plant groups. However, the relationships among major fern lineages, especially the placement of Equisetales, remain enigmatic.
Results: In order to understand the evolution of plastid genomes and to establish phylogenetic relationships among ferns, we sequenced the plastid genomes from three early diverging species: Equisetum hyemale (Equisetales), Ophioglossum californicum (Ophioglossales), and Psilotum nudum (Psilotales). A comparison of fern plastid genomes showed that some lineages have retained inverted repeat (IR) boundaries originating from the common ancestor of land plants, while other lineages have experienced multiple IR changes including expansions and inversions. Genome content has remained stable throughout ferns, except for a few lineage-specific losses of genes and introns. Notably, the losses of the rps16 gene and the rps12i346 intron are shared among Psilotales, Ophioglossales, and Equisetales, while the gain of a mitochondrial atp1 intron is shared between Marattiales and Polypodiopsida. These genomic structural changes support the placement of Equisetales as sister to Ophioglossales + Psilotales and Marattiales as sister to Polypodiopsida. This result is augmented by some molecular phylogenetic analyses that recover the same relationships, whereas others suggest a relationship between Equisetales and Polypodiopsida.
Conclusions: Although molecular analyses were inconsistent with respect to the position of Marattiales and Equisetales, several genomic structural changes have for the first time provided a clear placement of these lineages within the ferns. These results further demonstrate the power of using rare genomic structural changes in cases where molecular data fail to provide strong phylogenetic resolution
Inelastic Neutron scattering in CeSi_{2-x}Ga_x ferromagnetic Kondo lattice compounds
Inelastic neutron scattering investigation on ferromagnetic Kondo lattice
compounds belonging to CeSi_{2-x}Ga_{x}, x = 0.7, 1.0 and 1.3, system is
reported. The thermal evolution of the quasielastic response shows that the
Kondo interactions dominate over the RKKY interactions with increase in Ga
concentration from 0.7 to 1.3. This is related to the increase in k-f
hybridization with increasing Ga concentration. The high energy response
indicates the ground state to be split by crystal field in all three compounds.
Using the experimental results we have calculated the crystal field parameters
in all three compounds studied here.Comment: 12 Pages Revtex, 2 eps figures
Complete plastid genomes from Ophioglossum californicum, Psilotum nudum, and Equisetum hyemale reveal an ancestral land plant genome structure and resolve the position of Equisetales among monilophytes
BACKGROUND: Plastid genome structure and content is remarkably conserved in land plants. This widespread conservation has facilitated taxon-rich phylogenetic analyses that have resolved organismal relationships among many land plant groups. However, the relationships among major fern lineages, especially the placement of Equisetales, remain enigmatic. RESULTS: In order to understand the evolution of plastid genomes and to establish phylogenetic relationships among ferns, we sequenced the plastid genomes from three early diverging species: Equisetum hyemale (Equisetales), Ophioglossum californicum (Ophioglossales), and Psilotum nudum (Psilotales). A comparison of fern plastid genomes showed that some lineages have retained inverted repeat (IR) boundaries originating from the common ancestor of land plants, while other lineages have experienced multiple IR changes including expansions and inversions. Genome content has remained stable throughout ferns, except for a few lineage-specific losses of genes and introns. Notably, the losses of the rps16 gene and the rps12i346 intron are shared among Psilotales, Ophioglossales, and Equisetales, while the gain of a mitochondrial atp1 intron is shared between Marattiales and Polypodiopsida. These genomic structural changes support the placement of Equisetales as sister to Ophioglossales + Psilotales and Marattiales as sister to Polypodiopsida. This result is augmented by some molecular phylogenetic analyses that recover the same relationships, whereas others suggest a relationship between Equisetales and Polypodiopsida. CONCLUSIONS: Although molecular analyses were inconsistent with respect to the position of Marattiales and Equisetales, several genomic structural changes have for the first time provided a clear placement of these lineages within the ferns. These results further demonstrate the power of using rare genomic structural changes in cases where molecular data fail to provide strong phylogenetic resolution
Complete plastid genomes from \u3ci\u3eOphioglossum californicum, Psilotum nudum,\u3c/i\u3e and \u3ci\u3eEquisetum hyemale\u3c/i\u3e reveal an ancestral land plant genome structure and resolve the position of Equisetales among monilophytes
Background: Plastid genome structure and content is remarkably conserved in land plants. This widespread conservation has facilitated taxon-rich phylogenetic analyses that have resolved organismal relationships among many land plant groups. However, the relationships among major fern lineages, especially the placement of Equisetales, remain enigmatic.
Results: In order to understand the evolution of plastid genomes and to establish phylogenetic relationships among ferns, we sequenced the plastid genomes from three early diverging species: Equisetum hyemale (Equisetales), Ophioglossum californicum (Ophioglossales), and Psilotum nudum (Psilotales). A comparison of fern plastid genomes showed that some lineages have retained inverted repeat (IR) boundaries originating from the common ancestor of land plants, while other lineages have experienced multiple IR changes including expansions and inversions. Genome content has remained stable throughout ferns, except for a few lineage-specific losses of genes and introns. Notably, the losses of the rps16 gene and the rps12i346 intron are shared among Psilotales, Ophioglossales, and Equisetales, while the gain of a mitochondrial atp1 intron is shared between Marattiales and Polypodiopsida. These genomic structural changes support the placement of Equisetales as sister to Ophioglossales + Psilotales and Marattiales as sister to Polypodiopsida. This result is augmented by some molecular phylogenetic analyses that recover the same relationships, whereas others suggest a relationship between Equisetales and Polypodiopsida.
Conclusions: Although molecular analyses were inconsistent with respect to the position of Marattiales and Equisetales, several genomic structural changes have for the first time provided a clear placement of these lineages within the ferns. These results further demonstrate the power of using rare genomic structural changes in cases where molecular data fail to provide strong phylogenetic resolution
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