148 research outputs found
Slave-boson based configuration-interaction approach for the Hubbard model
Based on the Kotliar-Ruckenstein slave-boson scheme we develop a
configuration-interaction (CI) approach which is suitable to improve the energy
of symmetry-broken saddle-point solutions. The theory is applied to
spin-polaron states in the Hubbard model and compared with analogous results
obtained within the Hartree-Fock approximation. In addition we show that within
the infinite prescription of the Gutzwiller method a CI approach
does not improve the variational result since in the thermodynamic limit matrix
elements between different inhomogeneous states vanish due to an 'orthogonality
catastrophe'.Comment: 8 pages, 2 figure
Exotic spin, charge and pairing correlations of the two-dimensional doped Hubbard model: a symmetry entangled mean-field approach
Intertwining of spin, charge and pairing correlations in the repulsive
two-dimensional Hubbard model is shown through unrestricted variational
calculations, with projected wavefunctions free of symmetry breaking. A
crossover from incommensurate antiferromagnetism to stripe order naturally
emerges in the hole-doped region when increasing the on-site coupling. Although
effective pairing interactions are identified, they are strongly fragmented in
several modes including d-wave pairing and more exotic channels related to an
underlying stripe. We demonstrate that the entanglement of a mean-field
wavefunction by symmetry restoration can largely account for interaction
effects.Comment: Minor corrections, one reference adde
Intertwined orders from symmetry projected wavefunctions of repulsively interacting Fermi gases in optical lattices
Unconventional strongly correlated phases of the repulsive Fermi-Hubbard
model, which could be emulated by ultracold vapors loaded in optical lattices,
are investigated by means of energy minimizations with quantum number
projection before variation and without any assumed order parameter. In a
tube-like geometry of optical plaquettes to realize the four-leg ladder Hubbard
Hamiltonian, we highlight the intertwining of spin-, charge-, and pair-density
waves embedded in a uniform d-wave superfluid background. As the lattice
filling increases, this phase emerges from homogenous states exhibiting spiral
magnetism and evolves towards a doped antiferromagnet. A concomitant
enhancement of long-ranged d-wave pairing correlations is also found. Numerical
tests of the approach for two-dimensional clusters are carried out, too.Comment: 26 pages, 15 figures ; replaced with the published manuscript ;
substantial changes from previous versio
Helicity Modulus and Effective Hopping in the Two-Dimensional Hubbard Model Using Slave-Boson Methods
The slave-boson mean-field method is used to study the two-dimensional
Hubbard model. A magnetic phase diagram allowing for paramagnetism, weak- and
strong ferromagnetism and antiferromagnetism, including all continuous and
first-order transitions, is constructed and compared to the corresponding phase
diagram using the Hartree-Fock approximation (HFA). Magnetically ordered
regions are reduced by a factor of about 3 along both the and density
axes compared to the HFA. Using the spin-rotation invariant formulation of the
slave-boson method the helicity modulus is computed and for half-filling is
found to practically coincide with that found using variational Monte Carlo
calculations using the Gutzwiller wave function. Off half-filling the results
can be used to compare with Quantum Monte Carlo calculations of the effective
hopping parameter. Contrary to the case of half-filling, the slave-boson
approach is seen to greatly improve the results of the HFA when off
half-filling. (Submitted to: Journal of Physics: Condensed Matter)Comment: 27 pages, LaTeX2e, 7 figures available upon request, INLO-PUB-10/9
Rhodium Doped Manganites : Ferromagnetism and Metallicity
The possibility to induce ferromagnetism and insulator to metal transitions
in small A site cation manganites Ln_{1-x}Ca_xMnO_3 by rhodium doping is shown
for the first time. Colossal magnetoresistance (CMR) properties are evidenced
for a large compositional range (0.35 \leq x < 0.60). The ability of rhodium to
induce such properties is compared to the results obtained by chromium and
ruthenium doping. Models are proposed to explain this behavior.Comment: 11 pages, 8 figure
Interplay between incommensurate phases in the cuprates
We establish the qualitative behavior of the incommensurability ,
optimal domain wall filling and chemical potential for increasing
doping by a systematic slave-boson study of an array of vertical stripes
separated by up to lattice constants. Our findings obtained in the
Hubbard model with the next-nearest neighbor hopping agree
qualitatively with the experimental data for the cuprates in the doping regime
. It is found that modifies the optimal filling and
triggers the crossover to the diagonal (1,1) spiral phase at increasing doping,
stabilized already at for .Comment: 7 pages, 4 figures, EPL styl
Spin and Charge Structure Factor of the 2-d Hubbard Model
The spin and charge structure factors are calculated for the Hubbard model on
the square lattice near half-filling using a spin-rotation invariant six-slave
boson representation. The charge structure factor shows a broad maximum at the
zone corner and is found to decrease monotonically with increasing interaction
strength and electron density and increasing temperature. The spin structure
factor develops with increasing interaction two incommensurate peaks at the
zone boundary and along the zone diagonal. Comparison with results of Quantum
Monte Carlo and variational calculations is carried out and the agreement is
found to be good. The limitations of an RPA-type approach are pointed out.Comment: 18 pages, revtex, 13 postscript figures, submitted to Phys. Rev.
Magnetic ordering in the striped nickelate La5/3Sr1/3NiO4: A band structure point of view
We report on a comprehensive study of the electronic and magnetic structure
of the striped nickelate La5/3Sr1/3NiO4. The investigation is carried out using
band structure calculations based on density functional theory. A magnetic
structure compatible with experiment is obtained from spin-polarized
calculations within the generalized gradient approximation (GGA), whereas
inclusion of a local Coulomb interaction in the LDA+U framework results in a
different ground state. The influence of the various interaction parameters is
discussed in detail.Comment: 5 pages, 4 figures, 2 tables, accepted by Europhys. Let
Unconventional Hall effect in oriented CaCoO thin films
Transport properties of the good thermoelectric misfit oxide
CaCoO are examined. In-plane resistivity and Hall resistance
measurements were made on epitaxial thin films which were grown on {\it c}-cut
sapphire substrates using the pulsed laser deposition technique. Interpretation
of the in-plane transport experiments relates the substrate-induced strain in
the resulting film to single crystals under very high pressure ( 5.5 GPa)
consistent with a key role of strong electronic correlation. They are confirmed
by the measured high temperature maxima in both resistivity and Hall
resistance. While hole-like charge carriers are inferred from the Hall effect
measurements over the whole investigated temperature range, the Hall resistance
reveals a non monotonic behavior at low temperatures that could be interpreted
with an anomalous contribution. The resulting unconventional temperature
dependence of the Hall resistance seems thus to combine high temperature
strongly correlated features above 340 K and anomalous Hall effect at low
temperature, below 100 K.Comment: Submitted to Physical Review B (2005
Vortex, skyrmion and elliptical domain wall textures in the two-dimensional Hubbard model
The spin and charge texture around doped holes in the two-dimensional Hubbard
model is calculated within an unrestricted spin rotational invariant
slave-boson approach. In the first part we examine in detail the spin structure
around two holes doped in the half-filled system where we have studied cluster
sizes up to 10 x 10. It turns out that the most stable configuration
corresponds to a vortex-antivortex pair which has lower energy than the
Neel-type bipolaron even when one takes the far field contribution into
account. We also obtain skyrmions as local minima of the energy functional but
with higher total energy than the vortex solutions. Additionally we have
investigated the stability of elliptical domain walls for commensurate hole
concentrations. We find that (i) these phases correspond to local minima of the
energy functional only in case of partially filled walls, (ii) elliptical
domain walls are only stable in the low doping regime.Comment: 7 pages, 6 figures, accepted for Phys. Rev.
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