856 research outputs found
GRB observations by Fermi LAT revisited: new candidates found
We search the Fermi-LAT photon database for an extended gamma-ray emission
which could be associated with any of the 581 previously detected gamma-ray
bursts (GRBs) visible to the Fermi-LAT. For this purpose we compare the number
of photons with energies E > 100 MeV and E > 1 GeV which arrived in the first
1500 seconds after the burst from the same region, to the expected background.
We require that the expected number of false detections does not exceed 0.05
for the entire search and find the high-energy emission in 19 bursts, four of
which (GRB 081009, GRB 090720B, GRB 100911 and GRB 100728A) were previously
unreported. The first three are detected at energies above 100 MeV, while the
last one shows a statistically significant signal only above 1 GeV.Comment: Updated after referee comments, published in MNRAS Letters; 5 pages,
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Analytical approximation for single-impurity Anderson model
We have applied the recently developed dual fermion technique to the spectral
properties of single-band Anderson impurity problem (SIAM). In our approach a
series expansion is constructed in vertices of the corresponding atomic
Hamiltonian problem. This expansion contains a small parameter in two limiting
cases: in the weak coupling case (), due to the smallness of the
irreducible vertices, and near the atomic limit (), when bare
propagators are small. Reasonable results are obtained also for the most
interesting case of strong correlations (). The atomic problem of
the Anderson impurity model has a degenerate ground state, so the application
of the perturbation theory is not straightforward. We construct a special
approach dealing with symmetry-broken ground state of the renormalized atomic
problem. Formulae for the first-order dual diagram correction are obtained
analytically in the real-time domain. Most of the Kondo-physics is reproduced:
logarithmic contributions to the self energy arise, Kondo-like peak at the
Fermi level appears, and the Friedel sum rule is fulfilled. Our approach
describes also renormalization of atomic resonances due to hybridization with a
conduction band. A generalization of the proposed scheme to a multi-orbital
case can be important for the realistic description of correlated solids.Comment: 6 pages, 5 figure
Electron energy spectrum of the spin-liquid state in a frustrated Hubbard model
Non-local correlation effects in the half-filled Hubbard model on an
isotropic triangular lattice are studied within a spin polarized extension of
the dual fermion approach. A competition between the antiferromagnetic
non-collinear and the spin liquid states is strongly enhanced by an
incorporation of a k-dependent self-energy beyond the local dynamical
mean-field theory. The dual fermion correc- tions drastically decrease the
energy of a spin liquid state while leaving the non-collinear magnetic states
almost non-affected. This makes the spin liquid to become a preferable state in
a certain interval of interaction strength of an order of the magnitude of a
bandwidth. The spectral function of the spin-liquid Mott insulator is
determined by a formation of local singlets which results in the energy gap of
about twice larger than that of the 120 degrees antiferromagnetic Neel state.Comment: 6 pages, 4 figure
Plasmons in strongly correlated systems: spectral weight transfer and renormalized dispersion
We study the charge-density dynamics within the two-dimensional extended
Hubbard model in the presence of long-range Coulomb interaction across the
metal-insulator transition point. To take into account strong correlations we
start from self-consistent extended dynamical mean-field theory and include
non-local dynamical vertex corrections through a ladder approximation to the
polarization operator. This is necessary to fulfill charge conservation and to
describe plasmons in the correlated state. The calculated plasmon spectra are
qualitatively different from those in the random-phase approximation: they
exhibit a spectral density transfer and a renormalized dispersion with enhanced
deviation from the canonical -behavior. Both features are reminiscent
of interaction induced changes found in single-electron spectra of strongly
correlated systems.Comment: 5 pages, 5 figures + appendix (3 pages, 1 figure
Topological defects, pattern evolution, and hysteresis in thin magnetic films
Nature of the magnetic hysteresis for thin films is studied by the
Monte-Carlo simulations. It is shown that a reconstruction of the magnetization
pattern with external field occurs via the creation of vortex-antivortex pairs
of a special kind at the boundaries of stripe domains. It is demonstrated that
the symmetry of order parameter is of primary importance for this problem, in
particular, the in-plane magnetic anisotropy is necessary for the hysteresis.Comment: Accepted to EPL; 7 pages, 3 color figure
Self-consistent Dual Boson approach to single-particle and collective excitations in correlated systems
We propose an efficient dual boson scheme, which extends the DMFT paradigm to
collective excitations in correlated systems. The theory is fully
self-consistent both on the one- and on the two-particle level, thus describing
the formation of collective modes as well as the renormalization of electronic
and bosonic spectra on equal footing. The method employs an effective impurity
model comprising both fermionic and bosonic hybridization functions. Only
single- and two-electron Green's functions of the reference problem enter the
theory, due to the optimal choice of the self-consistency condition for the
effective bosonic bath. We show that the theory is naturally described by a
dual Luttinger-Ward functional and obeys the relevant conservation laws.Comment: 17 pages, 12 figure
Expanding the scope of ligand substitution from [M(S2C2Ph2] (M = Ni2+, Pd2+, Pt2+) to afford new heteroleptic dithiolene complexes
The scope of direct substitution of the dithiolene ligand from [M(S2C2Ph2)2] [M = Ni2+ (1), Pd2+ (2), Pt2+ (3)] to produce heteroleptic species [M(S2C2Ph2)2Ln] (n = 1, 2) has been broadened to include isonitriles and dithiooxamides in addition to phosphines and diimines. Collective observations regarding ligands that cleanly produce [M(S2C2Ph2)Ln], do not react at all, or lead to ill-defined decomposition identify soft σ donors as the ligand type capable of dithiolene substitution. Substitution of MeNC from [Ni(S2C2Ph2)(CNMe)2] by L provides access to a variety of heteroleptic dithiolene complexes not accessible from 1. Substitution of a dithiolene ligand from 1 involves net redox disproportionation of the ligands from radical monoanions, –S•SC2Ph2, to enedithiolate and dithione, the latter of which is an enhanced leaving group that is subject to further irreversible reactions
Genetic engineering of peptide hormones : II. Possible polymorphism of preprolactin in cattle. Data of molecular cloning
Primary structure is determined of an insertion of a clone isolated from the library of hypophyseal cDNA of cattle by hybridization with a probe specific for prolactin. Analysis of nucleotide sequences showed that in the process of cloning, reorganization occurred in structure of preprolactin cDNA, including an inversion of the 5'-terminal and deletion of the central section of cDNA. Nevertheless, from structure of cDNA, nucleotide sequences can be deduced of extended 5'- and 3'-terminal sections of preprolactin mRNA in cattle with lengths of 257 and 551 nucleotide residues, respectively. When these sequences are compared to those established previously, some differences were found in primary structure. The most important of them is the presence of an additional codon which codes alanine at the position (-22) of the signal peptide. It is suggested that heterogeneity of preprolactin mRNA of cattle in the section coding the signal peptide is the result of alternative splicing, as was shown for preprolactin mRNA in rats
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