44 research outputs found
Ferromagnetism in the Hubbard Model on fcc-type lattices
The Hubbard model on fcc-type lattices is studied in the dynamical mean-field
theory of infinite spatial dimensions. At intermediate interaction strength
finite temperature Quantum Monte Carlo calculations yield a second order phase
transition to a highly polarized, metallic ferromagnetic state. The Curie
temperatures are calculated as a function of electronic density and interaction
strength. A necessary condition for ferromagnetism is a density of state with
large spectral weight near one of the band edges.Comment: typos corrected, references updated, discussion extente
The asymmetric single-impurity Anderson model - the modified perturbation theory
We investigate the single-impurity Anderson model by means of the recently
introduced modified perturbation theory. This approximation scheme yields
reasonable results away from the symmetric case. The agreement with exactly
known results for the symmetric case is checked, and results for the
non-symmetric case are presented. With decreasing conduction band occupation,
the breakdown of the screening of the local moment is observed. In the
crossover regime between Kondo limit and mixed-valence regime, an enhanced
zero-temperature susceptibility is found.Comment: 7 pages, 7 figures, to appear in Physica
Disorder-enhanced delocalization and local-moment quenching in a disordered antiferromagnet
The interplay of disorder and spin-fluctuation effects in a disordered
antiferromagnet is studied. In the weak-disorder regime (W \le U), while the
energy gap decreases rapidly with disorder, the sublattice magnetization,
including quantum corrections, is found to remain essentially unchanged in the
strong correlation limit. Magnon energies and Neel temperature are enhanced by
disorder in this limit. A single paradigm of disorder-enhanced delocalization
qualitatively accounts for all these weak disorder effects. Vertex corrections
and magnon damping, which appear only at order (W/U)^4, are also studied. With
increasing disorder a crossover is found at W \sim U, characterized by a rapid
decrease in sublattice magnetization due to quenching of local moments, and
formation of spin vacancies. The latter suggests a spin-dilution behavior,
which is indeed observed in softened magnon modes, lowering of Neel
temperature, and enhanced transverse spin fluctuations.Comment: 12 pages, includes 8 postscript figures. To appear in Physical Review
B. References adde
Disorder and Impurities in Hubbard-Antiferromagnets
We study the influence of disorder and randomly distributed impurities on the
properties of correlated antiferromagnets. To this end the Hubbard model with
(i) random potentials, (ii) random hopping elements, and (iii) randomly
distributed values of interaction is treated using quantum Monte Carlo and
dynamical mean-field theory. In cases (i) and (iii) weak disorder can lead to
an enhancement of antiferromagnetic (AF) order: in case (i) by a
disorder-induced delocalization, in case (iii) by binding of free carriers at
the impurities. For strong disorder or large impurity concentration
antiferromagnetism is eventually destroyed. Random hopping leaves the local
moment stable but AF order is suppressed by local singlet formation. Random
potentials induce impurity states within the charge gap until it eventually
closes. Impurities with weak interaction values shift the Hubbard gap to a
density off half-filling. In both cases an antiferromagnetic phase without
charge gap is observed.Comment: 16 pages, 9 figures, latex using vieweg.sty (enclosed); typos
corrected, references updated; to appear in "Advances in Solid State
Physics", Vol. 3
Strong-coupling scenario of a metamagnetic transition
We investigate the periodic Anderson model in the presence of an external
magnetic field, using dynamical mean-field theory in combination with the
modified perturbation theory. A metamagnetic transition is observed which
exhibits a massive change in the electronic properties. These are discussed in
terms of the quasiparticle weight and densities of states. The results are
compared with the experimental results of the metamagnetic transition in
CeRu_2Si_2.Comment: 5 pages, 3 figures, to appear in PR
Dynamical mean-field study of ferromagnetism in the periodic Anderson model
The ferromagnetic phase diagram of the periodic Anderson model is calculated
using dynamical mean-field theory in combination with the modified perturbation
theory. Concentrating on the intermediate valence regime, the phase boundaries
are established as function of the total electron density, the position of the
atomic level and the hybridization strength. The main contribution to the
magnetic moment stems from the f-electrons. The conduction band polarization
is, depending on the system parameters either parallel or antiparallel to the
f-magnetization. By investigating the densities of states, one observes that
the change of sign of the conduction band polarization is closely connected to
the hybridization gap, which is only apparent in the case of almost complete
polarization of the f-electrons. Finite-temperature calculations are also
performed, the Curie temperature as function of electron density and f-level
position are determined. In the intermediate-valence regime, the phase
transitions are found to be of second order.Comment: 12 pages, 11 figures, accepted by Phys. Rev.
Structural brain anomalies in patients with FOXG1 syndrome and in Foxg1+/- mice
Objective FOXG1 syndrome is a rare neurodevelopmental disorder associated with heterozygous FOXG1 variants or chromosomal microaberrations in 14q12. The study aimed at assessing the scope of structural cerebral anomalies revealed by neuroimaging to delineate the genotype and neuroimaging phenotype associations. Methods We compiled 34 patients with a heterozygous (likely) pathogenic FOXG1 variant. Qualitative assessment of cerebral anomalies was performed by standardized re-analysis of all 34 MRI data sets. Statistical analysis of genetic, clinical and neuroimaging data were performed. We quantified clinical and neuroimaging phenotypes using severity scores. Telencephalic phenotypes of adult Foxg1+/- mice were examined using immunohistological stainings followed by quantitative evaluation of structural anomalies. Results Characteristic neuroimaging features included corpus callosum anomalies (82%), thickening of the fornix (74%), simplified gyral pattern (56%), enlargement of inner CSF spaces (44%), hypoplasia of basal ganglia (38%), and hypoplasia of frontal lobes (29%). We observed a marked, filiform thinning of the rostrum as recurrent highly typical pattern of corpus callosum anomaly in combination with distinct thickening of the fornix as a characteristic feature. Thickening of the fornices was not reported previously in FOXG1 syndrome. Simplified gyral pattern occurred significantly more frequently in patients with early truncating variants. Higher clinical severity scores were significantly associated with higher neuroimaging severity scores. Modeling of Foxg1 heterozygosity in mouse brain recapitulated the associated abnormal cerebral morphology phenotypes, including the striking enlargement of the fornix. Interpretation Combination of specific corpus callosum anomalies with simplified gyral pattern and hyperplasia of the fornices is highly characteristic for FOXG1 syndrome.Peer reviewe
Magnetic correlations in the two dimensional Anderson-Hubbard model
Magnetic correlations in the two dimensional Anderson-Hubbard model / R. T. Scalettar ; M. Ulmke. - In: Physical review. B. 55. 1997. S. 4149-415