118 research outputs found
Correlated-Electron Theory of Strongly Anisotropic Metamagnets
We present the first correlated-electron theory of metamagnetism in strongly
anisotropic antiferromagnets. Quantum-Monte-Carlo techniques are used to
calculate the field vs. temperature phase diagram of the infinite-dimensional
Hubbard model with easy axis. A metamagnetic transition scenario with 1.~order
and 2.~order phase transitions is found. The apparent similarities to the phase
diagram of FeBr and to mean-field results for the Ising model with
competing interactions are discussed.Comment: 4 pages, RevTeX + one uuencoded ps-file including 3 figure
Inhomogeneous metallic phase upon disordering a two dimensional Mott insulator
We find that isoelectronic disorder destroys the spectral gap in a
Mott-Hubbard insulator in 2D leading, most unexpectedly, to a new metallic
phase. This phase is spatially inhomogeneous with metallic behavior coexisting
with antiferromagnetic long range order. Even though the Mott gap in the pure
system is much larger than antiferromagnetic exchange, the spectral gap is
destroyed locally in regions where the disorder potential is high enough to
overcome the inter-electron repulsion thereby generating puddles where charge
fluctuations are enhanced. With increasing disorder, these puddles expand and
concomitantly the states at the Fermi energy get extended leading to a metallic
phase. We discuss the implications of our results for experiments.Comment: (4 pages, 5 figures
Non-perturbative approaches to magnetism in strongly correlated electron systems
The microscopic basis for the stability of itinerant ferromagnetism in
correlated electron systems is examined. To this end several routes to
ferromagnetism are explored, using both rigorous methods valid in arbitrary
spatial dimensions, as well as Quantum Monte Carlo investigations in the limit
of infinite dimensions (dynamical mean-field theory). In particular we discuss
the qualitative and quantitative importance of (i) the direct Heisenberg
exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a
high spectral density near the band edges caused by an appropriate lattice
structure and/or kinetic energy of the electrons. We furnish evidence of the
stability of itinerant ferromagnetism in the pure Hubbard model for appropriate
lattices at electronic densities not too close to half-filling and large enough
. Already a weak direct exchange interaction, as well as band degeneracy, is
found to reduce the critical value of above which ferromagnetism becomes
stable considerably. Using similar numerical techniques the Hubbard model with
an easy axis is studied to explain metamagnetism in strongly anisotropic
antiferromagnets from a unifying microscopic point of view.Comment: 11 pages, Latex, and 6 postscript figures; Z. Phys. B, in pres
Ordered states in the disordered Hubbard model
The Hubbard model is studied in which disorder is introduced by putting the
on-site interaction to zero on a fraction f of (impurity) sites of a square
lattice. Using Quantum Monte Carlo methods and Dynamical Mean Field theory we
find that antiferromagnetic long-range order is initially enhanced at
half-filling and stabilized off half-filling by the disorder. The Mott-Hubbard
charge gap of the pure system is broken up into two pieces by the disorder: one
incompressible state remains at average density n=1 and another can be seen
slightly below n=1+f. Qualitative explanations are provided.Comment: 17 pages, including 8 figures. Paper for Festschrift in honor of Hans
van Leeuwen's 65th birthda
Two-particle propagator and magnetic susceptibility in the Hubbard model- An improved treatment
We treat the two-particle Green's function in the Hubbard model using the
recently developed tau-CPA, a hybrid treatment that applies the
coherent-potential approximation (CPA) up to a time tau related to the inverse
of the band width, after which the system is averaged using the virtual-crystal
approximation (VCA). This model, with suitable approximations, does predict
magnetism for a modified Stoner criterion. The evaluation of the two-particle
propagator in the tau-CPA requires the solution of the pure CPA, within whose
formalism the vertex correction and the weighted Green's functions are
obtained. The dynamical susceptibility, including the vertex correction and the
weighted scattering by the residual interaction, is calculated and shows a spin
wave spectrum in the ferromagnetic regime
Проектирование экспертной системы прогнозирования аварийных ситуаций на технологических объектах нефтегазодобычи
В данной работе проведен комплексный анализ отказов, методов их предотвращения, методов и моделей прогнозирования отказов оборудования нефтегазодобычи. А так же разработана система комплексного анализа данных промысла, представленных в виде временных рядов, которая предназначена для прогнозирования отказов оборудования нефтегазодобычи.A comprehensive analysis of failures, methods for their prevention, methods and models for predicting failures of oil and gas production equipment is carried out in this work. A system of comprehensive analysis of field data presented in the form of time series, which is designed to predict failures of oil and gas production equipment is developed
Metal--Insulator Transitions in the Falicov--Kimball Model with Disorder
The ground state phase diagrams of the Falicov--Kimball model with local
disorder is derived within the dynamical mean--field theory and using the
geometrically averaged (''typical'') local density of states. Correlated metal,
Mott insulator and Anderson insulator phases are identified. The
metal--insulator transitions are found to be continuous. The interaction and
disorder compete with each other stabilizing the metallic phase against
occurring one of the insulators. The Mott and Anderson insulators are found to
be continuously connected.Comment: 6 pages, 7 figure
Derivation of the Curie-Weiss Law in Dynamical Mean-Field Theory
We present an analytic derivation of the linear temperature dependence of the
inverse static susceptibility near the
transition from a paramagnetic to a ferromagnetic correlated metal within the
dynamical mean-field theory (DMFT) for the Hubbard model. The equations for the
critical temperature and interaction strength of the transition are also
determined.Comment: 5 pages, no figure
Electronic Structure of Paramagnetic V_2O_3: Strongly Correlated Metallic and Mott Insulating Phase
LDA+DMFT, the computation scheme merging the local density approximation and
the dynamical mean-field theory, is employed to calculate spectra both below
and above the Fermi energy and spin and orbital occupations in the correlated
paramagnetic metallic and Mott insulating phase of V_2O_3. The self-consistent
DMFT equations are solved by quantum Monte Carlo simulations. Room temperature
calculations provide direct comparison with experiment. They show a significant
increase of the quasiparticle height in comparison with the results at 1160 K.
We also obtain new insights into the nature of the Mott-Hubbard transition in
V_2O_3. Namely, it is found to be strikingly different from that in the
one-band Hubbard model due to the orbital degrees of freedom. Furthermore we
resolve the puzzle of the unexpectedly small Mott gap in Cr-doped V_2O_3.Comment: 14 pages, 22 figure
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