54 research outputs found
The origin of the 90 degree magneto-optical Kerr rotation in CeSb
We calculate the linear magneto-optical Kerr rotation for CeSb in the
near-infrared spectral range. Using an exact formula for large Kerr rotation
angles and a simplified electronic structure of CeSb we find at \hbar \omega =
0.46 eV a Kerr rotation of 90 degree which then for decreasing \omega jumps to
-90 degree as recently observed. We identify the general origin of possible 180
degree polarization rotations as resulting from mainly nonmagnetic optical
properties, in particular from the ratio of the dominant interband resonance
frequency to the plasma frequency. The dependence of the Kerr rotation on
moments and magnetization is discussed.Comment: 6 pages, REVTEX, 5 eps figure
A new approach for perovskites in large dimensions
Using the Hubbard Hamiltonian for transition metal-3d and oxygen-2p states
with perovskite geometry, we propose a new scaling procedure for a nontrivial
extension of these systems to large spatial dimensions . The scaling
procedure is based on a selective treatment of different hopping processes for
large and can not be generated by a unique scaling of the hopping element.
The model is solved in the limit by the iterated
perturbation theory and using an extended non-crossing approximation. We
discuss the evolution of quasi particles at the Fermi-level upon doping,
leading to interesting insight into the dynamical character of the charge
carriers near the metal insulator instability of transition metal oxide
systems, three dimensional perovskites and other strongly correlated transition
metal oxides.Comment: 5 pages (TeX) with 2 figures (Postscript
Theory for the Doping Dependence of Spin Fluctuation Induced Shadow States in High-T Superconductors
We analyze the doping dependence of the intensity and energetical position of
shadow states in high -T superconductors within the 2D Hubbard model and
using our recently developed numerical method for the self consistent summation
of bubble and ladder diagrams. It is shown that shadow states resulting from
short range antiferromagnetic correlations occur for small but finite
excitation energies which decrease for decreasing doping, reflecting a
dynamically broken symmetry with increasing lifetime. Simultanously, the
intensity of these new states increases, the quasiparticle dispersion is
strongly flattened, and a pseudogap in the density of states occurs. Finally,
we discuss the importance of flat bands at the Fermi level and nesting of the
Fermi surface as general prerequisites for the observability of shadow states.Comment: 9 pages (TeX) with 3 figures (Postscript
Electronic Theory for Bilayer-Effects in High-T_c Superconductors
The normal and the superconducting state of two coupled CuO_2 layers in the
High-T_c superconductors are investigated by using the bilayer Hubbard model,
the FLEX approximation on the real frequency axis and the Eliashberg theory. We
find that the planes are antiferromagnetically correlated which leads to a
strongly enhanced shadow band formation. Furthermore, the inter-layer hopping
is renormalized which causes a blocking of the quasi particle inter-plane
transfer for low doping concentrations. Finally, the superconducting order
parameter is found to have a d_{x^2-y^2} symmetry with significant additional
inter-layer contributions.Comment: 5 pages, Revtex, 4 postscript figure
Theory for the Interdependence of High-T Superconductivity and Dynamical Spin Fluctuations
The doping dependence of the superconducting state for the 2D one-band
Hubbard Hamiltonian is determined. By using an Eliashberg-type theory, we find
that the gap function has a symmetry in momentum
space and T becomes maximal for doping. Since we determine the
dynamical excitations directly from real frequency axis calculations, we obtain
new structures in the angular resolved density of states related to the
occurrence of {\it shadow states} below T. Explaining the anomalous
behavior of photoemission and tunneling experiments in the cuprates, we find a
strong interplay between -wave superconductivity and dynamical spin
fluctuations.Comment: 4 pages (REVTeX) with 4 figures (Postscript
Magnetocrystalline Anisotropy Energy of a Transition Metal Monolayer: A Non-perturbative Theory
The magnetocrystalline anisotropy energy for a monolayer of Fe and
Ni is determined using a fully convergent tight-binding calculation including
- hybridization. The spin-orbit interaction is treated
non-perturbatively. Remarkably, we find and
important contributions to due to the lifting of degeneracies near
the Fermi-level. This is supported by the calculated decrease of the anisotropy
energy with increasing temperature on a scale of several hundred K. Our results
clarify the present debate on the origin of .Comment: 11 pages (RevTeX) with 2 figures, appended as Postscript file
Electronic Theory for the Transition from Fermi-Liquid to Non-Fermi-Liquid Behavior in High-T Superconductors
We analyze the breakdown of Fermi-liquid behavior within the 2D Hubbard model
as function of doping using our recently developed numerical method for the
self consistent summation of bubble and ladder diagrams. For larger doping
concentrations the system behaves like a conventional Fermi-liquid and for
intermediate doping similar to a marginal Fermi-liquid. However, for smaller
doping pronounced deviations from both pictures occur which are due to the
increasing importance of the short range antiferromagnetic spin fluctuations.
This is closely related to the experimental observed shadow states in the
normal state of high- superconductors. Furthermore, we discuss the
implications of our results for transport experiments.Comment: 11 pages (REVTeX) with 4 figures (Postscript
Model study for the nonequlibrium magnetic domain structure during the growth of nanostructured ultrathin films
The nonequilibrium magnetic domain structure of growing ultrathin
ferromagnetic films with a realistic atomic structure is studied as a function
of coverage and temperature. We apply a kinetic Monte Carlo method to a
micromagnetic model describing the transition from superparamagnetic islands at
low coverages to a closed ferromagnetic film. The magnetic relaxation and the
island growth happen simultaneously. Near the percolation threshold a
metastable magnetic domain structure is obtained with an average domain area
ranging between the area of individual magnetic islands and the area of the
large domains observed for thicker ferromagnetic films. We conclude that this
micro-domain structure is controlled and stabilized by the nonuniform atomic
nanostructure of the ultrathin film, causing a random interaction between
magnetic islands with varying sizes and shapes. The average domain area and
domain roughness are determined. A maximum of the domain area and a minimum of
the domain roughness are obtained as a function of the temperature.Comment: 19 pages, 4 Postscript figures; to be published in J. Magn. Magn.
Mater., accepted (2001); completely revised manuscrip
Phonon anomalies at the valence transition of SmS : An inelasticX-ray scattering study under pressure
The phonon dispersion curve of SmS under pressure was studied by inelastic
x-ray scattering around the pressure-induced valence transition. A significant
softening of the longitudinal acoustic modes propagating along the [111]
direction was observed spanning a wide region from
() up to the zone boundary as
SmS becomes metallic. The largest softening occurs at the zone boundary and
stays stable up to the highest measured pressure of 80 kbar while a gradual
hardening of the low modes simultaneously appears. This phonon spectrum
indicates favorable conditions for the emergence of pressure-induced
superconductivity in SmS.Comment: 4 pages, 3 figure
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