Search results

Sort By:

Limit search:

Indirect RKKY-Type Interaction by Direct Oxygen Hopping

100%

Rodriguez-Nuńez J., Coqblin B., Beck H., Konior J.

Acta Physica Polonica A

1994

vol. 85

issue 2

317-322

The three-band Emery model, describing the holes in the CuO_{2} planes of the high-temperature superconducting oxides, is considered. The model includes the direct oxygen-oxygen hopping integral t_{pp}. The exact Bogolyubov transformation is used to exclude one oxygen band and obtain a two-dimensional Anderson model. Afterward, the effective Hamiltonian is obtained by eliminating the second oxygen band with the use of two approximate canonical transformations. The effective Hamiltonian describes the spins residing on the copper sites and interacting through an indirect interaction J_{SX}(R), where R is the distance between two copper ions. J_{SX}(R) depends on the doping rate δ and is a decaying function of R. Numerical results for J_{SX}(R) are given for different doping rates δ for the case of parabolic bands. The obtained interaction J_{SX}(R), when added to the original antiferromagnetic interaction (present in oxides at δ = 0), might lead to a frustration of the long-range antiferromagnetic ordering upon doping.

Crossover from BCS to Bose-Einstein in Hubbard Model

100%

Pedersen M. H., Schneider T., Beck H.

Acta Physica Polonica A

1997

vol. 91

issue 2

419-422

We generalize the standard attractive Hubbard, having an on-site pair creation operator Q_{i} = a_{i↑}a_{i↓}, to one with n components Q_{iα} = a_{iα↑}a_{iα↓}, α=1,2,...,n. In the limit n → ∞ we obtain the Ginzburg-Landau functional. On this basis we explore the crossover from weak (BCS) to strong coupling (Bose-Einstein condensation) superconductivity. The associated self-consistent equations for the Ginzburg-Landau parameters are similar to those of the T-matrix approach. The evolution of the band structure with increasing interaction strength is studied and correlated with the behavior of the pair propagator and the transition temperature. We find that the pairing interaction creates a new band which moves downwards in energy as the interaction strength increases and separates into a lower Hubbard band when the interaction strength becomes comparable to the band width. In the strong coupling regime, a third band with small spectral weight is also found in between the lower and upper Hubbard bands.

Refine search results

2 Acta Physica Polonica A

1 1997

1 1994

2 Beck H.

1 Coqblin B.

1 Konior J.

1 Pedersen M. H.

1 Rodriguez-Nuńez J.

1 Schneider T.

Search results

Indirect RKKY-Type Interaction by Direct Oxygen Hopping

Crossover from BCS to Bose-Einstein in Hubbard Model