A slave boson representation for the degenerate Hubbard model is introduced. The location of the metal to insulator transition that occurs at commensurate densities is shown to depend weakly on the band degeneracy M . The relative weights of the Hubbard sub-bands depend strongly on M , as well as the magnetic properties. It is also shown that a sizable Hund's rule coupling is required in order to have a ferromagnetic instability appearing. The metal to insulator transition driven by an increase in temperature is a strong function of it.

Motivated by the recently observed pattern of unidirectional domains in high-Tc superconductors [Y. Kohsaka et al., Science 315, 1380], we investigate the emergence of spontaneous modulations in the d-wave superconducting resonating valence bond phase using the t-J model at x = 1/8 doping. Half-filled charge domains separated by four lattice spacings are found to form along one of the crystal axis leading to modulated superconductivity with out-of-phase d-wave order parameters in neighboring domains. Both renormalized mean-field theory and variational Monte Carlo calculations yield that the energies of modulated and uniform phases are very close to each other. 74.20.Mn, 75.40.Mg Puzzling properties of the high-T c superconductors have often been attributed to competing instabilities. 1Indeed, it is believed that doping an antiferromagnetic (AF) Mott insulator (which could be described e.g. by the so-called t-J model 2 ) leads to quantum disordered states with short-ranged magnetic correlations between S = 1/2 spins and exotic properties.3 Among them, the resonating valence bond (RVB) state was the first theoretical proposal supposed to capture the essence of high-T c superconductivity.4 Remarkably, this approach based on the Gutzwiller-projected BCS trial wavefunction, the parameters of which are usually determined either by using renormalized mean-field theory 5 (RMFT) or by variational Monte Carlo (VMC) method, 6 not only correctly predicted the d-wave symmetry of the superconducting (SC) order parameter, 7 but reproduced in addition the experimentally observed doping dependence of a variety of physical observables in the SC regime. 8In fact, exactly at half-filling where a particle-hole SU(2) symmetry applies, the RVB phase is equivalent to the staggered flux (SF) state, 9 a projected Slater determinant build from a tight binding model under a staggered magnetic flux. Remarkably, short-ranged staggered orbital current correlations have been seen in the Gutzwiller-projected d-wave RVB phase 10 and in the exact ground state of a small t-J cluster.11 Upon doping the SU(2) symmetry is broken leading to two distinct phases, a d-wave RVB superconductor and a doped SF phase, a candidate for the pseudogap phase, 12,13 characterized by the opening of an antinodal gap in the excitation spectrum. Indeed, coexistence of sharp nodal quasiparticles and broad antinodal excitations have been found in angle-resolved photoemission spectroscopy (ARPES) studies on an array of underdoped cuprates such as La 2−x Sr x CuO 4 (LSCO), 14 Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212), 15 and Ca 2−x Na x CuO 2 Cl 2 (Na-CCOC). 16However, there are also some low-T properties of the SC state which cannot be simply explained within the original RVB framework. For example, following its theoretical prediction, 17 static charge and spin stripe order has been detected in neutron scattering experiments and resonant soft x-ray scattering in some cuprate compounds as Nd-LSCO 18 and La 2−x Ba x CuO 4 (LBCO). 19More microscopic evidences of inhomogeneities h...

We consider a slave boson representation of lattice electrons in terms of bosons for the empty, singly and doubly occupied site. The approach generalizes an earlier formulation by Kotliar and Ruckenstein. As examples the Hubbard model and the t-J model are considered. The emphasis is on a detailed derivation and discussion of the formal aspects. Simple meanfield solutions are discussed for illustrative purposes.

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AbstracL We determine the ground state phase diagram of the Hubbard model on the square lattice allowing for homogeneous spiral, antiferromagnetic, ferromagnetic and paramagnetic phases. 'lhis is oblained from a saddie-point approximation of a spin-rotation-invariant form of the slave bson representation introduced by Kolliar and Ruckenstein. W obtain vely g d agreement in energy with exact diagonalization maleulations and, mnceming the magnetic structure, qualitative agreement with experimental data.

Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic (T_N = 24 K) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t_2g orbitals of Cr^3+ and that spin polarization is predicted with 3 mu_B per Cr^3+. The structural characterizations of CuCr1-xMgxO2 reveal a very limited range of Mg^2+ substitution for Cr^3+ in this series. As soon as x = 0.02, a maximum of 1% Cr ions substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01. This explains the saturation of the Mg^2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01. Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr^4+ "holes", created by the Mg^2+ substitution, in the matrix of high spin Cr^3+ (S = 3/2) are responsible for the transport properties of these compounds.Comment: 9 pages, 11 figures, more information at http://www.physik.uni-augsburg.de/~eyert

The origin of the quasi two-dimensional behavior of PdCoO2 and PtCoO2 is investigated by means of electronic structure calculations. They are performed using density functional theory in the generalized gradient approximation as well as the new full-potential augmented spherical wave method. We show that the electric conductivity is carried almost exclusively by the in-plane Pd (Pt) d orbitals. In contrast, the insulating CoO2 sandwich layers of octahedrally coordinated Co atoms may be regarded as charge carrier reservoirs. This leads to a weak electronic coupling of the Pd (Pt) layers. The obtained nearly cylindrical Fermi surface causes the strong anisotropy of the electric conductivity.

We investigate the electron density distribution and the stability of stripe phases in the realistic two-band model with hopping elements between eg orbitals at Ni sites on the square lattice, and compare these results with those obtained for the doubly degenerate Hubbard model with two equivalent orbitals and diagonal hopping. For both models we determine the stability regions of filled and half-filled stripe phases for increasing hole doping x = 2 − n in the range of x < 0.4, using Hartree-Fock approximation for large clusters. In the parameter range relevant to the nickelates, we obtain the most stable diagonal stripe structures with filling of nearly one hole per atom, as observed experimentally. In contrast, for the doubly degenerate Hubbard model the most stable stripes are somewhat reminiscent of the cuprates, with half-filled atoms at the domain wall sites. This difference elucidates the crucial role of the off-diagonal eg hopping terms for the stripe formation in La2−xSrxNiO4. The influence of crystal field is discussed as well. I. STRIPE PHASES IN NICKELATESStripe phases are one of the most exciting phenomena of modern condensed matter physics. They have been observed in a variety of systems, including nickelates, 1,2,3,4,5,6,7,8 cuprates, 9,10,11,12,13,14,15 and manganites. 16,17,18 Among them, layered La 2−x Sr x CuO 4 (LSCO), La 2−x−y Nd y Sr x CuO 4 (Nd-LSCO), La 2−x Sr x NiO 4 (LSNO), and La 2 NiO 4+δ (LNO) compounds play plausibly the most prominent role. However the similarity between them is superficial only, and the stripes in the cuprates differ from the stripes in the nickelates in many respects. For instance, they are dynamical in the former, and static in the latter. In addition, in Nd-LSCO 9,10,11,12,13,14 and LSCO, 15 one finds the so-called half-filled stripes, with the density of one doped hole per two atoms along the domain wall (DW). In contrast, it is clear from a variety of experiments that magnetic states within doped NiO 2 planes of the nickelates are filled stripes with density of one doped hole per one atom in a DW. 1,2,3,4,5,6,7,8 The question of filling is not the only difference between the nickelate and cuprate stripes, however. Neutron diffraction measurements performed on Nd-LSCO revealed that magnetic peaks are displaced from the antiferromagnetic (AF) maximum at Q AF = π(1, 1) to the points Q s = π(1 ± 2ǫ, 1) and Q s = π(1, 1 ± 2ǫ) and the shift ǫ depends linearly on hole doping x for x < 1/8, while it is almost constant at higher doping. These values correspond to a superposition of vertical (01) and horizontal (10) DWs. The essentially identical modulation and doping dependence of ǫ was observed in superconducting crystals of LSCO with x > 0.05. Conversely, experiments on LSNO established that spin order is characterized by the wave vectors Q s = π(1 ± ǫ, 1 ± ǫ) with ǫ ≃ x for x < 1/3, corresponding to a constant charge of one hole/Ni ion along a diagonal DW, in agreement with the predictions made in the pioneering works by Zaanen and Gunnarsson 19 and others, 20,21,...