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The Pseudogap Phenomenon in High-T_{c} Superconductors

100%
Ranninger J., Romano A.
Acta Physica Polonica A
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2000
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vol. 97
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issue 1
157-164
EN
We review the possible mechanisms leading up to the phenomenon of a pseudogap which characterizes the normal-state properties of high-T_{c} superconductors. We suggest that this pseudogap is not due to superconducting phase fluctuations and hence is not related to a crossover between a BCS state of Cooper pairs and a Bose-Einstein condensation of local pairs. We rather argue that it is due to uncorrelated pairing which is already manifest in the local electronic structure and accessible by photoemission and tunneling experiments.
2
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Ferromagnetism in Hubbard Chains

81%
Cuoco M., Noce C., Romano A.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
387-390
EN
We analyze the possible occurrence of ferromagnetism in the Hubbard model, by means of an exact diagonalization study performed on 3- to 8-site chains with periodic boundary conditions. In the case of one hole in the half-filled configuration, we find that the Nagaoka state is reached only in the 3- and in the 4-site case. Ground states characterized by unsaturated ferromagnetism are found when the case of more than one hole is considered.
3
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Superconductivity in Anderson Lattice Model

81%
Romano A., Noce C., Micnas R.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
381-385
EN
We study the superconducting instabilities generated by the inclusion in the Anderson lattice model of a density-density attractive potential between correlated electrons on nearest-neighbouring sites. Using a description of the normal phase based on a perturbative expansion around the atomic limit, we treat the attractive potential in the broken-symmetry Hartree-Fock scheme and analyze which of the possible symmetries of the superconducting order parameter leads to the highest possible transition temperature in the case of a two-dimensional square lattice. For values of the on-site f-repulsion large compared to the hopping amplitude, a suppression of any possible superconducting phase occurs, regardless of the symmetry of the order parameter.
4
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Interplay Between Spin-Orbit Coupling and Structural Deformations in Heavy Transition-Metal Oxides with Tetrahedral Coordination

61%
Forte F., Guerra D., Avella A., Autieri C., Romano A., Noce C.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
394-397
EN
We analyze the effects of a large spin-orbit coupling on the magnetic state of a d¹ transition-metal ion located in a tetrahedral environment. While in the ideal tetrahedral symmetry the spin-orbit coupling acts only as a perturbation on the atomic energy levels set by the crystal-field splitting, we demonstrate that its effects are strongly enhanced in the case of distorted geometries. In particular, we consider the specific case in which the tetrahedron is compressed along the z direction, and show that, by increasing the degree of flattening, a large spin-orbit interaction (i) can induce a substantial anisotropic, unquenched orbital momentum and (ii) can affect the hierarchy of the lowest energy levels that are involved in the magnetic exchange.
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