113 research outputs found
Insulating phases of the infinite-dimensional Hubbard model
A theory is developed for the T=0 Mott-Hubbard insulating phases of the
infinite-dimensional Hubbard model at half-filling, including both the
antiferromagnetic (AF) and paramagnetic (P) insulators. Local moments are
introduced explicitly from the outset, enabling ready identification of the
dominant low energy scales for insulating spin- flip excitations. Dynamical
coupling of single-particle processes to the spin-flip excitations leads to a
renormalized self-consistent description of the single-particle propagators
that is shown to be asymptotically exact in strong coupling, for both the AF
and P phases. For the AF case, the resultant theory is applicable over the
entire U-range, and is discussed in some detail. For the P phase, we consider
in particular the destruction of the Mott insulator, the resultant critical
behaviour of which is found to stem inherently from proper inclusion of the
spin-flip excitations.Comment: 13 pages Revtex, 12 postscript figure
A river model of space
Within the theory of general relativity gravitational phenomena are usually
attributed to the curvature of four-dimensional spacetime. In this context we
are often confronted with the question of how the concept of ordinary physical
three-dimensional space fits into this picture. In this work we present a
simple and intuitive model of space for both the Schwarzschild spacetime and
the de Sitter spacetime in which physical space is defined as a specified set
of freely moving reference particles. Using a combination of orthonormal basis
fields and the usual formalism in a coordinate basis we calculate the physical
velocity field of these reference particles. Thus we obtain a vivid description
of space in which space behaves like a river flowing radially toward the
singularity in the Schwarzschild spacetime and radially toward infinity in the
de Sitter spacetime. We also consider the effect of the river of space upon
light rays and material particles and show that the river model of space
provides an intuitive explanation for the behavior of light and particles at
and beyond the event horizons associated with these spacetimes.Comment: 22 pages, 5 figure
Magnetic Properties of the t-J Model in the Dynamical Mean-Field Theory
We present a theory for the spin correlation function of the t-J model in the
framework of the dynamical mean-field theory. Using this mapping between the
lattice and a local model we are able to obtain an intuitive expression for the
non-local spin susceptibility, with the corresponding local correlation
function as input. The latter is calculated by means of local Goldstone
diagrams following closely the procedures developed and successfully applied
for the (single impurity) Anderson model.We present a systematic study of the
magnetic susceptibility and compare our results with those of a Hubbard model
at large U. Similarities and differences are pointed out and the magnetic phase
diagram of the t-J model is discussed.Comment: 28 pages LaTeX, postscript figures as compressed and uuencoded file
included fil
Conductivity in a symmetry broken phase: Spinless fermions with corrections
The dynamic conductivity of strongly correlated electrons in
a symmetry broken phase is investigated in the present work. The model
considered consists of spinless fermions with repulsive interaction on a simple
cubic lattice. The investigated symmetry broken phase is the charge density
wave (CDW) with wave vector which occurs at
half-filling. The calculations are based on the high dimensional approach, i.e.
an expansion in the inverse dimension is used. The finite dimensionality
is accounted for by the inclusion of linear terms in and the true finite
dimensional DOS. Special care is paid to the setup of a conserving
approximation in the sense of Baym/Kadanoff without inconsistencies. The
resulting Bethe-Salpeter equation is solved for the dynamic conductivity in the
non symmetry broken and in the symmetry broken phase (AB-CDW). The
dc-conductivity is reduced drastically in the CDW. Yet it does not vanish in
the limit due to a subtle cancellation of diverging mobility and
vanishing DOS. In the dynamic conductivity the energy gap
induced by the symmetry breaking is clearly discernible. In addition, the
vertex corrections of order lead to an excitonic resonance lying within
the gap.Comment: 23 pages, 19 figures included with psfig, Revtex; Physical Review
B15, in press (October/November 1996) depending on the printer/screen driver,
it might be necessary to comment out figures 3,4,5,10,11,12,19 and have them
printed separatel
Are groups more rational than individuals? A review of interactive decision making in groups
Many decisions are interactive; the outcome of one party depends not only on its decisions or on acts of nature but also on the decisions of others. In the present article, we review the literature on decision making made by groups of the past 25 years. Researchers have compared the strategic behavior of groups and individuals in many games: prisoner's dilemma, dictator, ultimatum, trust, centipede and principal-agent games, among others. Our review suggests that results are quite consistent in revealing that groups behave closer to the game-theoretical assumption of rationality and selfishness than individuals. We conclude by discussing future research avenues in this area
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
Frustrated electron liquids in the Hubbard model
The ground state of the Hubbard model is studied within the constrained
Hilbert space where no order parameter exists. The self-energy of electrons is
decomposed into the single-site and multisite self-energies. The calculation of
the single-site self-energy is mapped to a problem of self-consistently
determining and solving the Anderson model. When an electron reservoir is
explicitly considered, it is proved that the single-site self-energy is that of
a normal Fermi liquid even if the multisite self-energy is anomalous. Thus, the
ground state is a normal Fermi liquid in the supreme single-site approximation
(S^3A). In the strong-coupling regime, the Fermi liquid is stabilized by the
Kondo effect in the S^3A and is further stabilized by the Fock-type term of the
superexchange interaction or the resonating-valence-bond (RVB) mechanism beyond
the S^3A. The stabilized Fermi liquid is frustrated as much as an RVB spin
liquid in the Heisenberg model. It is a relevant unperturbed state that can be
used to study a normal or anomalous Fermi liquid and an ordered state in the
whole Hilbert space by Kondo lattice theory. Even if higher-order multisite
terms than the Fock-type term are considered, the ground state cannot be a Mott
insulator. It can be merely a gapless semiconductor even if the multisite
self-energy is so anomalous that it is divergent at the chemical potential. A
Mott insulator is only possible as a high temperature phase.Comment: 11 pages, no figur
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