1,608 research outputs found
Interplay of superconductivity and magnetism in strong coupling
A model is introduced describing the interplay between superconductivity and
spin-ordering. It is characterized by on-site repulsive electron-electron
interactions, causing antiferromagnetism, and nearest-neighbor attractive
interactions, giving rise to d-wave superconductivity. Due to a special choice
for the lattice, this model has a strong-coupling limit where the
superconductivity can be described by a bosonic theory, similar to the strongly
coupled negative U Hubbard model. This limit is analyzed in the present paper.
A rich mean-field phase diagram is found and the leading quantum corrections to
the mean-field results are calculated. The first-order line between the
antiferromagnetic- and the superconducting phase is found to terminate at a
tricritical point, where two second-order lines originate. At these lines, the
system undergoes a transition to- and from a phase exhibiting both
antiferromagnetic order and superconductivity. At finite temperatures above the
spin-disordering line, quantum-critical behavior is found. For specific values
of the model parameters, it is possible to obtain SO(5) symmetry involving the
spin- and the phase-sector at the tricritical point. Although this symmetry is
explicitly broken by the projection to the lower Hubbard band, it survives on
the mean-field level, and modes related to a spontaneously broken SO(5)
symmetry are present on the level of the random phase approximation in the
superconducting phase.Comment: 16 pages Revtex, 5 figure
Theory of site-disordered magnets
In realistic spinglasses, such as CuMn, AuFe and EuSrS, magnetic atoms are
located at random positions. Their couplings are determined by their relative
positions. For such systems a field theory is formulated. In certain limits it
reduces to the Hopfield model, the Sherrington-Kirkpatrick model, and the
Viana-Bray model. The model has a percolation transition, while for RKKY
couplings the ``concentration scaling'' T_g proportional to c occurs. Within
the Gaussian approximation the Ginzburg-Landau expansion is considered in the
clusterglass phase, that is to say, for not too small concentrations. Near
special points, the prefactor of the cubic term, or the one of the
replica-symmetry- breaking quartic term, may go through zero. Around such
points new spin glass phases are found.Comment: 26 pages Revtex, 6 figure
Metamorphosis? The Role of Article 47 of the EU Charter of Fundamental Rights in Cases Concerning National Remedies and Procedures under Directive 93/13
Art. 47 EU Charter of Fundamental Rights and civil courts:the case of arbitration clauses in consumer contracts (the Netherlands vs Spain)
Justice in Times of Crisis:The Signalling Function of Article 47 of the Charter of Fundamental Rights in Consumer Debt Collection Cases
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