1,908 research outputs found
Single Impurity Anderson Model with Coulomb Repulsion between Conduction Electrons on the Nearest-Neighbour Ligand Orbital
We study how the Kondo effect is affected by the Coulomb interaction between
conduction electrons on the basis of a simplified model. The single impurity
Anderson model is extended to include the Coulomb interaction on the
nearest-neighbour ligand orbital. The excitation spectra are calculated using
the numerical renormalization group method. The effective bandwidth on the
ligand orbital, , is defined to classify the state. This quantity
decreases as the Coulomb interaction increases. In the
region, the low energy properties are described by the Kondo state, where
is the hybridization width. As decreases in this region, the
Kondo temperature is enhanced, and its magnitude becomes comparable to
for . In the region, the local
singlet state between the electrons on the and ligand orbitals is formed.Comment: 5 pages, 3 figures, LaTeX, to be published in J. Phys. Soc. Jpn Vol.
67 No.
Magnetic Impurity in the two-dimensional Heisenberg Antiferromagnet
We analyze the ground state properties of the two-dimensional quantum
antiferromagnet with a S=1/2 Kondo impurity. Perturbation theory around the
strong Kondo coupling limit is developed and the results compared with studies,
based on exact diagonalization of small clusters. We find that at intermediate
coupling the impurity is partially screened and the magnetization locally
suppressed. A local singlet between the impurity and the host spin is formed
asymptotically.Comment: 12 REVTex pages, 4 Postscript figure
On-site correlation in valence and core states of ferromagnetic nickel
We present a method which allows to include narrow-band correlation effects
into the description of both valence and core states and we apply it to the
prototypical case of nickel. The results of an ab-initio band calculation are
used as input mean-field eigenstates for the calculation of self-energy
corrections and spectral functions according to a three-body scattering
solution of a multi-orbital Hubbard hamiltonian. The calculated quasi-particle
spectra show a remarkable agreement with photoemission data in terms of band
width, exchange splitting, satellite energy position of valence states, spin
polarization of both the main line and the satellite of the 3p core level.Comment: 14 pages, 10 PostScript figures, RevTeX, submitted to PR
Long-range dynamics of magnetic impurities coupled to a two-dimensional Heisenberg antiferromagnet
We consider a two-dimensional Heisenberg antiferromagnet on a square lattice
with weakly coupled impurities, i.e. additional spins interacting with the host
magnet by a small dimensionless coupling constant g<<1. Using linear spin-wave
theory, we find that the magnetization disturbance at distance r from a single
impurity behaves as g/r for 1>1/g. Surprisingly
the disturbance is inversely proportional to the coupling constant! The
interaction between two impurities separated by a distance r is proportional to
g^2/r for 1>1/g. Hence at large distances, the
interaction is universal and independent of the coupling constant. We also find
that the frequency of Rabi oscillations between two impurities is proportional
to g^2 ln(gr) at 1<<r<<1/g, logarithmically enhanced compared to the spin-wave
width. This leads to a new mechanism for NMR, NQR and EPR line broadening. All
these astonishing results are due to the gapless spectrum of the magnetic
excitations in the quantum antiferromagnet.Comment: 6 pages, 5 figure
Spectral properties of the t-J model in the presence of hole-phonon interaction
We examine the effects of electron-phonon interaction on the dynamics of the
charge carriers doped in two-dimensional (2D) Heisenberg antiferromagnet. The
- model Hamiltonian with a Fr\"ohlich term which couples the holes to a
dispersionless (optical) phonon mode is considered for low doping
concentration. The evolution of the spectral density function, the density of
states, and the momentum distribution function of the holes with an increase of
the hole-phonon coupling constant is studied numerically. As the coupling
to a phonon mode increases the quasiparticle spectral weight decreases and a
``phonon satellite'' feature close to the quasi-particle peak becomes more
pronounced. Furthermore, strong electron-phonon coupling smears the
multi-magnon resonances (``string states'') in the incoherent part of the
spectral function. The jump in the momentum distribution function at the Fermi
surface is reduced without changing the hole pocket volume, thereby providing a
numerical verification of Luttinger theorem for this strongly interacting
system. The vertex corrections due to electron- phonon interaction are
negligible in spite of the fact that the ratio of the phonon frequency to the
effective bandwidth is not small.Comment: REVTeX, 20 pages, 9 figures, to be published in Phys. Rev. B (Nov. 1,
1996
Invisible Events with Radiative Photons at LEP
A study of the radiative neutrino counting reaction at LEP1 and LEP2 energies is presented. An approximate expression
for the spectrum of the observed photon is derived within the framework of the
-dependent structure function approach. This is compared with an exact
expression and found in agreement within the foreseen experimental accuracy.
This model describing single-photon radiation can be applied to the more
general case of initial-state single-photon emission accompanying invisible
final-state events. Higher-order QED corrections due to undetected
initial-state radiation are also included. The implementation in a Monte Carlo
event generator is briefly described.Comment: 10 pages, LaTeX, 7 figures available via anonymous ftp at:
ftp://cobra1.pv.infn.it/pub/phot/, files fig#n.ps with #=1,...,
Superconducting Spiral Phase in the two-dimensional t-J model
We analyse the t-t'-t''-J model, relevant to the superconducting cuprates. By
using chiral perturbation theory we have determined the ground state to be a
spiral for small doping \delta << 1 near half filling. In this limit the
solution does not contain any uncontrolled approximations. We evaluate the
spin-wave Green's functions and address the issue of stability of the spiral
state, leading to the phase diagram of the model. At t'=t''=0 the spiral state
is unstable towards a local enhancement of the spiral pitch, and the nature of
the true ground state remains unclear. However, for values of t' and t''
corresponding to real cuprates the (1,0) spiral state is stabilized by quantum
fluctuations (``order from disorder'' effect). We show that at \delta = 0.119
the spiral is commensurate with the lattice with a period of 8 lattice
spacings. It is also demonstrated that spin-wave mediated superconductivity
develops in the spiral state and a lower limit for the superconducting gap is
derived. Even though one cannot classify the gap symmetry according to the
lattice representations (s,p,d,...) since the symmetry of the lattice is
spontaneously broken by the spiral, the gap always has lines of nodes along the
(1,\pm 1) directions.Comment: 17 pages, 11 figure
Rare earth contributions to the X-ray magnetic circular dichroism at the Co K edge in rare earth-cobalt compounds investigated by multiple-scattering calculations
The X-ray magnetic circular dichroism (XMCD) has been measured at the Co K
edge in Co-hcp and R-Co compounds (R=La, Tb, Dy). The structure of the
experimental XMCD spectra in the near-edge region has been observed to be
highly sensitive to the magnetic environment of the absorbing site.
Calculations of the XMCD have been carried out at the Co K edge in Co metal,
LaCo and TbCo within the multiple-scattering framework including the
spin-orbit coupling. In the three systems, the XMCD spectra in the near-edge
region are well reproduced. The possibility to separate and quantitatively
estimate the local effects from those due to the neighboring atoms in the XMCD
cross section makes possible a more physical understanding of the spectra. The
present results emphasize the major role played by the states of the Tb
ions in the XMCD spectrum at the Co K edge in the TbCo compound.Comment: 34 pages, revtex, 10 eps figures included with epsf, after referee
revie
Modification of the standard model for the lanthanides
We show that incorporation of strong electron correlations into the Kohn-Sham
scheme of band structure calculations leads to a modification of the standard
model of the lanthanides and that this procedure removes the existing
discrepancy between theory and experiment concerning the ground state
properties. Within the picture suggested, part of the upper Hubbard -band is
occupied due to conduction band--mixing interaction (that is renormalized
due to correlations) and this contributes to the cohesive energy of the
crystal. The lower Hubbard band has zero width and describes fermionic
excitations in the shell of localized -s. Fully self-consistent calculations
(with respect to both charge density and many-electron population numbers of
the -shell) of the equilibrium volume and the bulk modulus of selected
lanthanides have been performed and a good agreement is obtained.Comment: 1 fi
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