436 research outputs found
Theory of Spin-Resolved Auger-Electron Spectroscopy from Ferromagnetic 3d-Transition Metals
CVV Auger electron spectra are calculated for a multi-band Hubbard model
including correlations among the valence electrons as well as correlations
between core and valence electrons. The interest is focused on the
ferromagnetic 3d-transition metals. The Auger line shape is calculated from a
three-particle Green function. A realistic one-particle input is taken from
tight-binding band-structure calculations. Within a diagrammatic approach we
can distinguish between the \textit{direct} correlations among those electrons
participating in the Auger process and the \textit{indirect} correlations in
the rest system. The indirect correlations are treated within second-order
perturbation theory for the self-energy. The direct correlations are treated
using the valence-valence ladder approximation and the first-order perturbation
theory with respect to valence-valence and core-valence interactions. The
theory is evaluated numerically for ferromagnetic Ni. We discuss the
spin-resolved quasi-particle band structure and the Auger spectra and
investigate the influence of the core hole.Comment: LaTeX, 12 pages, 8 eps figures included, Phys. Rev. B (in press
Pseudogap and high-temperature superconductivity from weak to strong coupling. Towards quantitative theory
This is a short review of the theoretical work on the two-dimensional Hubbard
model performed in Sherbrooke in the last few years. It is written on the
occasion of the twentieth anniversary of the discovery of high-temperature
superconductivity. We discuss several approaches, how they were benchmarked and
how they agree sufficiently with each other that we can trust that the results
are accurate solutions of the Hubbard model. Then comparisons are made with
experiment. We show that the Hubbard model does exhibit d-wave
superconductivity and antiferromagnetism essentially where they are observed
for both hole and electron-doped cuprates. We also show that the pseudogap
phenomenon comes out of these calculations. In the case of electron-doped high
temperature superconductors, comparisons with angle-resolved photoemission
experiments are nearly quantitative. The value of the pseudogap temperature
observed for these compounds in recent photoemission experiments has been
predicted by theory before it was observed experimentally. Additional
experimental confirmation would be useful. The theoretical methods that are
surveyed include mostly the Two-Particle Self-Consistent Approach, Variational
Cluster Perturbation Theory (or variational cluster approximation), and
Cellular Dynamical Mean-Field Theory.Comment: 32 pages, 51 figures. Slight modifications to text, figures and
references. A PDF file with higher-resolution figures is available at
http://www.physique.usherbrooke.ca/senechal/LTP-toc.pd
Phase diagram of orbital-selective Mott transitions at finite temperatures
Mott transitions in the two-orbital Hubbard model with different bandwidths
are investigated at finite temperatures. By means of the self-energy functional
approach, we discuss the stability of the intermediate phase with one orbital
localized and the other itinerant, which is caused by the orbital-selective
Mott transition (OSMT). It is shown that the OSMT realizes two different
coexistence regions at finite temperatures in accordance with the recent
results of Liebsch. We further find that the particularly interesting behavior
emerges around the special condition and J=0, which includes a new type
of the coexistence region with three distinct states. By systematically
changing the Hund coupling, we establish the global phase diagram to elucidate
the key role played by the Hund coupling on the Mott transitions.Comment: 4 pages, 6 figure
Influence of uncorrelated overlayers on the magnetism in thin itinerant-electron films
The influence of uncorrelated (nonmagnetic) overlayers on the magnetic
properties of thin itinerant-electron films is investigated within the
single-band Hubbard model. The Coulomb correlation between the electrons in the
ferromagnetic layers is treated by using the spectral density approach (SDA).
It is found that the presence of nonmagnetic layers has a strong effect on the
magnetic properties of thin films. The Curie temperatures of very thin films
are modified by the uncorrelated overlayers. The quasiparticle density of
states is used to analyze the results. In addition, the coupling between the
ferromagnetic layers and the nonmagnetic layers is discussed in detail. The
coupling depends on the band occupation of the nonmagnetic layers, while it is
almost independent of the number of the nonmagnetic layers. The induced
polarization in the nonmagnetic layers shows a long-range decreasing
oscillatory behavior and it depends on the coupling between ferromagnetic and
nonmagnetic layers.Comment: 9 pages, RevTex, 6 figures, for related work see:
http://orion.physik.hu-berlin.d
Electron-correlation effects in appearance-potential spectra of Ni
Spin-resolved and temperature-dependent appearance-potential spectra of
ferromagnetic Nickel are measured and analyzed theoretically. The Lander
self-convolution model which relates the line shape to the unoccupied part of
the local density of states turns out to be insufficient. Electron correlations
and orbitally resolved transition-matrix elements are shown to be essential for
a quantitative agreement between experiment and theory.Comment: LaTeX, 6 pages, 2 eps figures included, Phys. Rev. B (in press
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
Thermoelectric properties of the degenerate Hubbard model
We investigate the thermoelectric properties of a system near a pressure
driven Mott-Hubbard transition. The dependence of the thermopower and the
figure of merit on pressure and temperature within a degenerate Hubbard model
for integer filling n=1 is calculated using dynamical mean field theory.
Quantum Monte Carlo method is used to solve the impurity model. Obtained
results can qualitatively explain thermoelectric properties of various strongly
correlated materials.Comment: RevTex, 7 pages, 6 figure
Kondo screening and exhaustion in the periodic Anderson model
We investigate the paramagnetic periodic Anderson model using the dynamical
mean-field theory in combination with the modified perturbation theory which
interpolates between the weak and strong coupling limits. For the symmetric
PAM, the ground state is always a singlet state. However, as function of the
hybridization strength, a crossover from collective to local Kondo screening is
found. Reducing the number of conduction electrons, the local Kondo singlets
remain stable. The unpaired f-electrons dominate the physics of the system. For
very low conduction electron densities, a large increase of the effective mass
of the quasiparticles is visible, which is interpreted as the approach of the
Mott-Hubbard transition.Comment: 10 pages, 8 figures, accepted by Phys. Rev.
Well-Posedness and Symmetries of Strongly Coupled Network Equations
We consider a diffusion process on the edges of a finite network and allow
for feedback effects between different, possibly non-adjacent edges. This
generalizes the setting that is common in the literature, where the only
considered interactions take place at the boundary, i. e., in the nodes of the
network. We discuss well-posedness of the associated initial value problem as
well as contractivity and positivity properties of its solutions. Finally, we
discuss qualitative properties that can be formulated in terms of invariance of
linear subspaces of the state space, i. e., of symmetries of the associated
physical system. Applications to a neurobiological model as well as to a system
of linear Schroedinger equations on a quantum graph are discussed.Comment: 25 pages. Corrected typos and minor change
Strong-coupling approach for strongly correlated electron systems
A perturbation theory scheme in terms of electron hopping, which is based on
the Wick theorem for Hubbard operators, is developed. Diagrammatic series
contain single-site vertices connected by hopping lines and it is shown that
for each vertex the problem splits into the subspaces with ``vacuum states''
determined by the diagonal Hubbard operators and only excitations around these
vacuum states are allowed. The rules to construct diagrams are proposed. In the
limit of infinite spatial dimensions the total auxiliary single-site problem
exactly splits into subspaces that allows to build an analytical
thermodynamically consistent approach for a Hubbard model. Some analytical
results are given for the simple approximations when the two-pole
(alloy-analogy solution) and four-pole (Hartree-Fock approximation) structure
for Green's function is obtained. Two poles describe contribution from the
Fermi-liquid component, which is dominant for small electron and hole
concentrations (``overdoped case'' of high-'s), whereas other two describe
contribution from the non-Fermi liquid and are dominant close to half-filling
(``underdoped case'').Comment: 14 pages, revtex, feynmf, 5 EPS figures, two-column PRB style,
published in PR
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