598 research outputs found
Correlation and surface effects in Vanadium Oxides
Recent photoemission experiments have shown strong surface modifications in
the spectra from vanadium oxides as (V,Cr)_2O_3 or
(Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent
part of the surface spectrum is narrowed as compared to the bulk. The
quasiparticle weight is more sensitive at the surface than in the bulk against
bandwidth variations. We investigate these effects theoretically considering
the single-band Hubbard model for a film geometry. A simplified dynamical
mean-field scheme is used to calculate the main features of the interacting
layer-dependent spectral function. It turns out that the experimentally
confirmed effects are inherent properties of a system of strongly correlated
electrons. The reduction of the weight and the variance of the coherent part of
the surface spectrum can be traced back to the reduced surface coordination
number. Surface correlation effects can be strongly amplified by changes of the
hopping integrals at the surface.Comment: to appear in PRB; 8 pages, 6 figure
Mott transition in one dimension: Benchmarking dynamical cluster approaches
The variational cluster approach (VCA) is applied to the one-dimensional
Hubbard model at zero temperature using clusters (chains) of up to ten sites
with full diagonalization and the Lanczos method as cluster solver. Within the
framework of the self-energy-functional theory (SFT), different cluster
reference systems with and without bath degrees of freedom, in different
topologies and with different sets of variational parameters are considered.
Static and one-particle dynamical quantities are calculated for half-filling as
a function of U as well as for fixed U as a function of the chemical potential
to study the interaction- and filling-dependent metal-insulator (Mott)
transition. The recently developed Q-matrix technique is used to compute the
SFT grand potential. For benchmarking purposes we compare the VCA results with
exact results available from the Bethe ansatz, with essentially exact dynamical
DMRG data, with (cellular) dynamical mean-field theory and full diagonalization
of isolated Hubbard chains. Several issues are discussed including convergence
of the results with cluster size, the ability of cluster approaches to access
the critical regime of the Mott transition, efficiency in the optimization of
correlated-site vs. bath-site parameters and of multi-dimensional parameter
optimization. We also study the role of bath sites for the description of
excitation properties and as charge reservoirs for the description of filling
dependencies. The VCA turns out to be a computationally cheap method which is
competitive with established cluster approaches.Comment: 19 pages, 19 figures, v3 with minor corrections, extended discussio
Surface metal-insulator transition in the Hubbard model
The correlation-driven metal-insulator (Mott) transition at a solid surface
is studied within the Hubbard model for a semi-infinite lattice by means of the
dynamical mean-field theory. The transition takes place at a unique critical
strength of the interaction. Depending on the surface geometry, the interaction
strength and the wave vector, we find one-electron excitations in the coherent
part of the surface-projected metallic spectrum which are confined to two
dimensions.Comment: LaTeX, 9 pages, 5 eps figures included, Phys. Rev. B (in press
Mott transitions in correlated electron systems with orbital degrees of freedom
Mott metal-insulator transitions in an M-fold orbitally degenerate Hubbard
model are studied by means of a generalization of the linearized dynamical
mean-field theory. The method allows for an efficient and reliable
determination of the critical interaction U_c for any integer filling n and
different M at zero temperature. For half-filling a linear dependence of U_c on
M is found. Inclusion of the (full) Hund's rule exchange J results in a strong
reduction of U_c. The transition turns out to change qualitatively from
continuous for J=0 to discontinuous for any finite J
Quantum Monte Carlo calculation of the finite temperature Mott-Hubbard transition
We present clear numerical evidence for the coexistence of metallic and
insulating dynamical mean field theory(DMFT) solutions in a half-filled
single-band Hubbard model with bare semicircular density of states at finite
temperatures. Quantum Monte Carlo(QMC) method is used to solve the DMFT
equations. We discuss important technical aspects of the DMFT-QMC which need to
be taken into account in order to obtain the reliable results near the
coexistence region. Among them are the critical slowing down of the iterative
solutions near phase boundaries, the convergence criteria for the DMFT
iterations, the interpolation of the discretized Green's function and the
reduction of QMC statistical and systematic errors. Comparison of our results
with those of other numerical methods is presented in a phase diagram.Comment: 4 pages, 5 figure
Influence of Spin Wave Excitations on the Ferromagnetic Phase Diagram in the Hubbard-Model
The subject of the present paper is the theoretical description of collective
electronic excitations, i.e. spin waves, in the Hubbard-model. Starting with
the widely used Random-Phase-Approximation, which combines Hartree-Fock theory
with the summation of the two-particle ladder, we extend the theory to a more
sophisticated single particle approximation, namely the
Spectral-Density-Ansatz. Doing so we have to introduce a `screened`
Coulomb-interaction rather than the bare Hubbard-interaction in order to obtain
physically reasonable spinwave dispersions. The discussion following the
technical procedure shows that comparison of standard RPA with our new
approximation reduces the occurrence of a ferromagnetic phase further with
respect to the phase-diagrams delivered by the single particle theories.Comment: 8 pages, 9 figures, RevTex4, accepted for publication in Phys. Rev.
Sliding wear investigation of suspension sprayed WC-Co nanocomposite coatings.
Sliding wear evaluation of nanostructured coatings deposited by Suspension High Velocity Oxy-Fuel (S-HVOF) and conventional HVOF (Jet Kote (HVOF-JK) and JP5000 (HVOF-JP)) spraying were evaluated. S-HVOF coatings were nanostructured and deposited via an aqueous based suspension of the WC-Co powder, using modified HVOF (TopGun) spraying. Microstructural evaluations of these hardmetal coatings included X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDX). Sliding wear tests on coatings were conducted using a ball-on-flat test rig against steel, silicon nitride (Si3N4) ceramic and WC-6Co balls. Results indicated that nanosized particles inherited from the starting powder in S-HVOF spraying were retained in the resulting coatings. Significant changes in the chemical and phase composition were observed in the S-HVOF coatings. Despite decarburization, the hardness and sliding wear resistance of the S-HVOF coatings was comparable to the HVOF-JK and HVOF-JP coatings. The sliding wear performance was dependent on the ball-coating test couple. In general a higher ball wear rate was observed with lower coating wear rate. Comparison of the total (ball and coating) wear rate indicated that for steel and ceramic balls, HVOF-JP coatings performed the best followed by the S-HVOF and HVOF-JK coatings. For the WC-Co ball tests, average performance of S-HVOF was better than that of HVOF-JK and HVOF-JP coatings. Changes in sliding wear behavior were attributed to the support of metal matrix due to relatively higher tungsten content, and uniform distribution of nanoparticles in the S-HVOF coating microstructure. The presence of tribofilm was also observed for all test couples
Fictive Impurity Models: an Alternative Formulation of the Cluster Dynamical Mean Field Method
"Cluster" extensions of the dynamical mean field method to include longer
range correlations are discussed. It is argued that the clusters arising in
these methods are naturally interpreted not as actual subunits of a physical
lattice but as algorithms for computing coefficients in an orthogonal function
expansion of the momentum dependence of the electronic self-energy. The
difficulties with causality which have been found to plague cluster dynamical
mean field methods are shown to be related to the "ringing" phenomenon familiar
from Fourier analysis. The analogy is used to motivate proposals for simple
filtering methods to circumvent them. The formalism is tested by comparison to
low order perturbative calculations and self consistent solutions
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.
Dynamical mean-field study of ferromagnetism in the periodic Anderson model
The ferromagnetic phase diagram of the periodic Anderson model is calculated
using dynamical mean-field theory in combination with the modified perturbation
theory. Concentrating on the intermediate valence regime, the phase boundaries
are established as function of the total electron density, the position of the
atomic level and the hybridization strength. The main contribution to the
magnetic moment stems from the f-electrons. The conduction band polarization
is, depending on the system parameters either parallel or antiparallel to the
f-magnetization. By investigating the densities of states, one observes that
the change of sign of the conduction band polarization is closely connected to
the hybridization gap, which is only apparent in the case of almost complete
polarization of the f-electrons. Finite-temperature calculations are also
performed, the Curie temperature as function of electron density and f-level
position are determined. In the intermediate-valence regime, the phase
transitions are found to be of second order.Comment: 12 pages, 11 figures, accepted by Phys. Rev.
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