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 ${\cal D}$ 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 $t/U$ 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 $\epsilon$, optimal domain wall filling $\nu$ and chemical potential $\mu$ for increasing doping by a systematic slave-boson study of an array of vertical stripes separated by up to $d=11$ lattice constants. Our findings obtained in the Hubbard model with the next-nearest neighbor hopping $t'=-0.15t$ agree qualitatively with the experimental data for the cuprates in the doping regime $x\lesssim 1/8$. It is found that $t'$ modifies the optimal filling $\nu$ and triggers the crossover to the diagonal (1,1) spiral phase at increasing doping, stabilized already at $x\simeq 0.09$ for $t'=-0.3t$.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 Ca3_3Co4_4O9_9 thin films

Transport properties of the good thermoelectric misfit oxide Ca$_3$Co$_4$O$_9$ 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 ($\sim$ 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.