Nature of Decoupling in the Mixed Phase of Extremely Type-II Layered Superconductors

The uniformly frustrated layered XY model is analyzed in its Villain form. A decouple pancake vortex liquid phase is identified. It is bounded by both first-order and second-order decoupling lines in the magnetic field versus temperature plane. These transitions, respectively, can account for the flux-lattice melting and for the flux-lattice depinning observed in the mixed phase of clean high-temperature superconductors.Comment: 11 pages of PLAIN TeX, 1 postscript figure, published version, many change

Double fluctuations on the attractive Hubbard model: ladder approximation

We explore, for the first time, the effect of double fluctuations on both the diagonal and off-diagonal self-energy. We use the T-Matrix equations below $T_c$, developed recently by the Z\"urich group (M.H. Pedersen et al) for the local pair attraction Hamiltonian. Here, we include as well the effect of fluctuations on the order parameter (beyond the BCS solution) up to second order in $U/t$. This is equivalent to approximating the effective interaction by $U$ in the off-diagonal self-energy. For $U/t = -6.0$, $T/t = 0.05$, $\mu/t = - 5.5$ and $\Delta/t = 1.5$, we find four peaks both for the diagonal, $A(n(\pi/16,\pi/16),\omega)$, and off-diagonal, $B(n(\pi/16,\pi/16),\omega)$, spectral functions. These peaks are not symmetric in pairs as previously found. In addition: (a) in $A(n(\pi/16,\pi/16),\omega)$, the far left peak has a vanishing small weight; (b) in $B(n(\pi/16,\pi/16),\omega)$ the far left and far right peaks have very small weights. The physical picture is, then, that the pair physics in the normal phase ($T > T_c$) is still valid below $T_c$. However, the condensation of the e-h pairs produces an additional gap around the chemical potential as in BCS, in other words, superconductivity opens a gap in the lower branch of a Hubbard-type-I solution.Comment: LaTeX, 7 pages. 8 figures available on request. To appear in Z. Physik

Degradation of Phase Coherence by Defects in a Two-Dimensional Vortex Lattice

The thermodynamic nature of two-dimensional vortex matter is studied theoretically through a duality analysis of the XY model over the square lattice with low uniform frustration. A phase-coherent vortex lattice state is found at low temperature if rigid translations are prohibited. It shows a non-zero phase rigidity that is degraded exclusively by the creation of dislocation pairs. The unbinding of such pairs causes the vortex lattice to simultaneously lose phase coherence and to melt at a continuous (Kosterlitz-Thouless) phase transition. General phase auto-correlation functions are also computed, and these results are used to argue for the existence of a continuous melting transition of vortex matter in layered superconductors.Comment: 11 pgs. of PLAIN TeX, to appear in PRL, some improvement