726 research outputs found
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
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
Nonperturbative Scaling Theory of Free Magnetic Moment Phases in Disordered Metals
The crossover between a free magnetic moment phase and a Kondo phase in low
dimensional disordered metals with dilute magnetic impurities is studied.
We perform a finite size scaling analysis of the distribution of the Kondo
temperature as obtained from a numerical renormalization group calculation of
the local magnetic susceptibility and from the solution of the self-consistent
Nagaoka-Suhl equation. We find a sizable fraction of free (unscreened) magnetic
moments when the exchange coupling falls below a disorder-dependent critical
value . Our numerical results show that between the free moment
phase due to Anderson localization and the Kondo screened phase there is a
phase where free moments occur due to the appearance of random local pseudogaps
at the Fermi energy whose width and power scale with the elastic scattering
rate .Comment: 4 pages, 6 figure
On the Bethe Ansatz for the Jaynes-Cummings-Gaudin model
We investigate the quantum Jaynes-Cummings model - a particular case of the
Gaudin model with one of the spins being infinite. Starting from the Bethe
equations we derive Baxter's equation and from it a closed set of equations for
the eigenvalues of the commuting Hamiltonians. A scalar product in the
separated variables representation is found for which the commuting
Hamiltonians are Hermitian. In the semi classical limit the Bethe roots
accumulate on very specific curves in the complex plane. We give the equation
of these curves. They build up a system of cuts modeling the spectral curve as
a two sheeted cover of the complex plane. Finally, we extend some of these
results to the XXX Heisenberg spin chain.Comment: 16 page
Sensitivity of cosmic-ray experiments to ultra-high-energy photons: reconstruction of the spectrum and limits on the superheavy dark matter
We estimate the sensitivity of various experiments detecting
ultra-high-energy cosmic rays to primary photons with energies above 10^19 eV.
We demonstrate that the energy of a primary photon may be significantly (up to
a factor of ~ 10) under- or overestimated for particular primary energies and
arrival directions. We consider distortion of the reconstructed cosmic-ray
spectrum for the photonic component. As an example, we use these results to
constrain the parameter space of models of superheavy dark matter by means of
both the observed spectra and available limits on the photon content. We find
that a significant contribution of ultra-high-energy particles (photons and
protons) from decays of superheavy dark matter is allowed by all these
constraints.Comment: 18 pages, 7 figure
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