1,442 research outputs found
Magnetic phase diagram of the Kondo lattice model with quantum localized spins
The magnetic phase diagram of the ferromagnetic Kondo lattice model is
determined at T=0 in 1D, 2D, and 3D for various magnitudes of the quantum
mechanical localized spins ranging from S=1/2 to classical spins. We consider
the ferromagnetic phase, the paramagnetic phase, and the
ferromagnetic/antiferromagnetic phase separated regime. There is no significant
influence of the spin quantum number on the phase boundaries except for the
case S=1/2, where the model exhibits an instability of the ferromagnetic phase
with respect to spin disorder. Our results give support, at least as far as the
low temperature magnetic properties are concerned, to the classical treatment
of the S=3/2-spins in the intensively investigated manganites, for which the
ferromagnetic Kondo-lattice model is generally employed to account for
magnetism.Comment: 8 pages, 6 figure
Potential-energy (BCS) to kinetic-energy (BEC)-driven pairing in the attractive Hubbard model
The BCS-BEC crossover within the two-dimensional attractive Hubbard model is
studied by using the Cellular Dynamical Mean-Field Theory both in the normal
and superconducting ground states. Short-range spatial correlations
incorporated in this theory remove the normal-state quasiparticle peak and the
first-order transition found in the Dynamical Mean-Field Theory, rendering the
normal state crossover smooth. For smaller than the bandwidth, pairing is
driven by the potential energy, while in the opposite case it is driven by the
kinetic energy, resembling a recent optical conductivity experiment in
cuprates. Phase coherence leads to the appearance of a collective Bogoliubov
mode in the density-density correlation function and to the sharpening of the
spectral function.Comment: 5 pages, 4 figure
Asymmetry between the electron- and hole-doped Mott transition in the periodic Anderson model
We study the doping driven Mott metal-insulator transition (MIT) in the
periodic Anderson model set in the Mott-Hubbard regime. A striking asymmetry
for electron or hole driven transitions is found. The electron doped MIT at
larger U is similar to the one found in the single band Hubbard model, with a
first order character due to coexistence of solutions. The hole doped MIT, in
contrast, is second order and can be described as the delocalization of
Zhang-Rice singlets.Comment: 18 pages, 19 figure
Quasiparticle spectral weights of Gutzwiller-projected high T_c superconductors
We analyze the electronic Green's functions in the superconducting ground
state of the t-J model using Gutzwiller-projected wave functions, and compare
them to the conventional BCS form. Some of the properties of the BCS state are
preserved by the projection: the total spectral weight is continuous around the
quasiparticle node and approximately constant along the Fermi surface. On the
other hand, the overall spectral weight is reduced by the projection with a
momentum-dependent renormalization, and the projection produces electron-hole
asymmetry in renormalization of the electron and hole spectral weights. The
latter asymmetry leads to the bending of the effective Fermi surface which we
define as the locus of equal electron and hole spectral weight.Comment: 6 pages, 5 figures; x-labels on Figs. 1 and 2 corrected, footnote on
particle number corrected, references adde
New measure of electron correlation
We propose to quantify the "correlation" inherent in a many-electron (or
many-fermion) wavefunction by comparing it to the unique uncorrelated state
that has the same single-particle density operator as it does.Comment: Final version to appear in PR
Interaction Effect in the Kondo Energy of the Periodic Anderson-Hubbard Model
We extend the periodic Anderson model by switching on a Hubbard for the
conduction electrons. The nearly integral valent (Kondo) limit of the
Anderson--Hubbard model is studied with the Gutzwiller variational method. The
new formula for the Kondo energy contains the -dependent chemical
potential of the Hubbard subsystem in the exponent, and the correlation-induced
band narrowing in the prefactor. Both effects tend to suppress the Kondo scale,
which can be understood to result from the blocking of hybridization (this
behaviour is the opposite of that found for Kondo--Hubbard models). At
half-filling, we find a Brinkman--Rice-type transition which leads from a
small-gap Kondo insulator to a Mott insulator.Comment: 4 pages (ReVTeX), submitted for publicatio
Multi-Channel Kondo Necklace
A multi--channel generalization of Doniach's Kondo necklace model is
formulated, and its phase diagram studied in the mean--field approximation. Our
intention is to introduce the possible simplest model which displays some of
the features expected from the overscreened Kondo lattice. The conduction
electron channels are represented by sets of pseudospins \vt_{j}, , which are all antiferromagnetically coupled to a periodic array of
|\vs|=1/2 spins. Exploiting permutation symmetry in the channel index
allows us to write down the self--consistency equation for general . For
, we find that the critical temperature is rising with increasing Kondo
interaction; we interpret this effect by pointing out that the Kondo coupling
creates the composite pseudospin objects which undergo an ordering transition.
The relevance of our findings to the underlying fermionic multi--channel
problem is discussed.Comment: 29 pages (2 figures upon request from [email protected]), LATEX,
submitted for publicatio
Optical conductivity of a metal-insulator transition for the Anderson-Hubbard model in 3 dimensions away from 1/2 filling
We have completed a numerical investigation of the Anderson-Hubbard model for
three-dimensional simple cubic lattices using a real-space self-consistent
Hartree-Fock decoupling approximation for the Hubbard interaction. In this
formulation we treat the spatial disorder exactly, and therefore we account for
effects arising from localization physics. We have examined the model for
electronic densities well away 1/2 filling, thereby avoiding the physics of a
Mott insulator. Several recent studies have made clear that the combined
effects of electronic interactions and spatial disorder can give rise to a
suppression of the electronic density of states, and a subsequent
metal-insulator transition can occur. We augment such studies by calculating
the ac conductivity for such systems. Our numerical results show that weak
interactions enhance the density of states at the Fermi level and the
low-frequency conductivity, there are no local magnetic moments, and the ac
conductivity is Drude-like. However, with a large enough disorder strength and
larger interactions the density of states at the Fermi level and the
low-frequency conductivity are both suppressed, the conductivity becomes
non-Drude-like, and these phenomena are accompanied by the presence of local
magnetic moments. The low-frequency conductivity changes from a sigma-sigma_dc
omega^{1/2} behaviour in the metallic phase, to a sigma omega^2 behaviour in
the nonmetallic regime. Our numerical results show that the formation of
magnetic moments is essential to the suppression of the density of states at
the Fermi level, and therefore essential to the metal-insulator transition
Low-energy properties of two-dimensional quantum triangular antiferromagnets: Non-perturbative renormalization group approach
We explore low temperature properties of quantum triangular Heisenberg
antiferromagnets in two dimension in the vicinity of the quantum phase
transition at zero temperature. Using the effective field theory described by
the matrix Ginzburg-Landau-Wilson model and the
non-perturbative renormalization group method, we clarify how quantum and
thermal fluctuations affect long-wavelength behaviors in the parameter region
where the systems exhibit a fluctuation-driven first order transition to a
long-range ordered state. We show that at finite temperatures the crossover
from a quantum theory to a renormalized two-dimensional classical
nonlinear sigma model region appears, and in this crossover region, massless
fluctuation modes with linear dispersion a la spin waves govern low-energy
physics. Our results are in good agreement with the recent experimental
observations for the two-dimensional triangular Heisenberg spin system,
NiGaS.Comment: 14 pages,7 figures, version accepted for publication in Physical
Review
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