1,265 research outputs found
Spin susceptibility and magnetic short-range order in the Hubbard model
The uniform static spin susceptibility in the paraphase of the one-band
Hubbard model is calculated within a theory of magnetic short--range order
(SRO) which extends the four-field slave-boson functional-integral approach by
the trans- formation to an effective Ising model and the self-consistent
incorporation of SRO at the saddle point. This theory describes a transition
from the paraphase without SRO for hole dopings to a
paraphase with anti- ferromagnetic SRO for . In this region the susceptibility consists of interrelated
`itinerant' and `local' parts and increases upon doping. The zero--temperature
susceptibility exhibits a cusp at and reduces to the usual
slave-boson result for larger dopings. Using the realistic value of the
on--site Coulomb repulsion for LSCO, the peak position () as well as the doping dependence reasonably agree with low--temperature
susceptibility experiments showing a maximum at a hole doping of about 25\%.Comment: 4 pages, 1 Postscript figure, revtex-style, accepted for publishing:
Phys. Rev. B, 54, ... (1996
Theory of magnetic short-range order for itinerant electron systems
On the basis of the one--band t-t'-Hubbard model a self-consistent
renormalization theory of magnetic short--range order (SRO) in the paramagnetic
phase is presented combining the four-field slave-boson functional-integral
scheme with the cluster variational method. Contrary to previous SRO approaches
the SRO is incorporated at the saddle-point and pair-approximation levels. A
detailed numerical evaluation of the theory is performed at zero temperature,
where both the hole- and electron-doped cases as well as band-structure effects
are studied. The ground--state phase diagram shows the suppression of magnetic
long-range order in favour of a paramagnetic phase with antiferromagnetic SRO
in a wide doping region. In this phase the uniform static spin susceptibility
increases upon doping up to the transition to the Pauli paraphase. Comparing
the theory with experiments on high--T_c cuprates a good agreement is found.Comment: 33 pages, 4 Postscript figure
Quantum to classical crossover in the 2D easy-plane XXZ model
Ground-state and thermodynamical properties of the spin-1/2 two-dimensional
easy-plane XXZ model are investigated by both a Green's-function approach and
by Lanczos diagonalizations on lattices with up to 36 sites. We calculate the
spatial and temperature dependences of various spin correlation functions, as
well as the wave-vector dependence of the spin susceptibility for all
anisotropy parameters . In the easy--plane ferromagnetic region , the longitudinal correlators of spins at distance change sign
at a finite temperature . This transition, observed in
the 2D case for the first time, can be interpreted as a quantum to classical
crossover.Comment: 4 pages, 6 figures, Contribution to the Ising Centennial Colloquium,
ICM2000, Belo Horizonte, Brazil, August 200
Exciton formation in strongly correlated electron-hole systems near the semimetal-semiconductor transition
The region surrounding the excitonic insulator phase is a three-component
plasma composed of electrons, holes, and excitons. Due to the extended nature
of the excitons, their presence influences the surrounding electrons and holes.
We analyze this correlation. To this end, we calculate the density of bound
electrons, the density of electrons in the correlated state, the
momentum-resolved exciton density, and the momentum-resolved density of
electron-hole pairs that are correlated but unbound. We find qualitative
differences in the electron-hole correlations between the weak-coupling and the
strong-coupling regime.Comment: 10 pages, 5 figure
Mesoscopic model for the fluctuating hydrodynamics of binary and ternary mixtures
A recently introduced particle-based model for fluid dynamics with continuous
velocities is generalized to model immiscible binary mixtures. Excluded volume
interactions between the two components are modeled by stochastic multiparticle
collisions which depend on the local velocities and densities. Momentum and
energy are conserved locally, and entropically driven phase separation occurs
for high collision rates. An explicit expression for the equation of state is
derived, and the concentration dependence of the bulk free energy is shown to
be the same as that of the Widom-Rowlinson model. Analytic results for the
phase diagram are in excellent agreement with simulation data. Results for the
line tension obtained from the analysis of the capillary wave spectrum of a
droplet agree with measurements based on the Laplace's equation. The
introduction of "amphiphilic" dimers makes it possible to model the phase
behavior and dynamics of ternary surfactant mixtures.Comment: 7 pages including 6 figure
Electron-hole pair condensation at the semimetal-semiconductor transition: a BCS-BEC crossover scenario
We act on the suggestion that an excitonic insulator state might
separate---at very low temperatures---a semimetal from a semiconductor and ask
for the nature of these transitions. Based on the analysis of electron-hole
pairing in the extended Falicov-Kimball model, we show that tuning the Coulomb
attraction between both species, a continuous crossover between a BCS-like
transition of Cooper-type pairs and a Bose-Einstein condensation of preformed
tightly-bound excitons might be achieved in a solid-state system. The precursor
of this crossover in the normal state might cause the transport anomalies
observed in several strongly correlated mixed-valence compounds.Comment: 5 pages, 5 figures, substantially revised versio
Magnetic properties of the 2D t-t'-Hubbard model
The two-dimensional (2D) t-t'-Hubbard model is studied within the slave-boson
(SB) theory. At half-filling, a paramagnetic to antiferromagnetic phase
transition of first order at a finite critical interaction strength U_c(t'/t)
is found. The dependences on U/t and t'/t of the sublattice magnetization and
of the local magnetic moment are calculated. Our results reasonably agree with
recent (Projector) Quantum Monte Carlo data. The SB ground-state phase diagram
reveals a t'-induced electron-hole asymmetry, and, depending on the ratio t'/t,
the antiferromagnetic or ferromagnetic phases are stable down to U=0 at a
critical hole doping.Comment: 2 pages, 3 Postscript figure, submitted to Int. Conf. M2S-HTSC-V
Beijing 97, to appear in Physica
Slave-boson field fluctuation approach to the extended Falicov-Kimball model: charge, orbital, and excitonic susceptibilities
Based on the SO(2)-invariant slave-boson scheme, the static charge, orbital,
and excitonic susceptibilities in the extended Falicov-Kimball model are
calculated. Analyzing the phase without long-range order we find instabilities
towards charge order, orbital order, and the excitonic insulator (EI) phase.
The instability towards the EI is in agreement with the saddle-point phase
diagram. We also evaluate the dynamic excitonic susceptibility, which allows
the investigation of uncondensed excitons. We find qualitatively different
features of the exciton dispersion at the semimetal-EI and at the
semiconductor-EI transition supporting a crossover scenario between a BCS-type
electron-hole condensation and a Bose-Einstein condensation of preformed bound
electron-hole pairs.Comment: 8 pages, 9 figures, final versio
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