Low-energy electronic structure of optimally doped YBa 2 Cu 3 O 7−␦ is investigated using laser-excited angleresolved photoemission spectroscopy. The surface state and the CuO chain band that usually overlap the CuO 2 plane derived bands are not detected, thus enabling a clear observation of the bulk superconducting state. The observed bilayer splitting of the Fermi surface is ϳ0.08 Å −1 along ͑0,0͒ − ͑ , ͒ direction, significantly larger than Bi 2 Sr 2 CaCu 2 O 8+␦ . The kink structure of the band dispersion reflecting the renormalization effect at ϳ60 meV shows up similarly as in other hole-doped cuprates. The momentum dependence of the superconducting gap shows d x 2 −y 2-wave-like amplitude but exhibits a nonzero minimum of ϳ12 meV along the ͑0,0͒ − ͑ , ͒ direction. Possible origins of such an unexpected "nodeless" gap behavior are discussed.

In thin films of anisotropic superconductors, the intervortex interaction may be strongly modified because of the interplay between the long-ranged repulsion caused by an extremely slow decay of the supercurrent induced by a single vortex line ͑Pearl's effect͒ and the attraction caused by the tilt of the vortex lines with respect to the anisotropy axes. We present a theoretical analysis and Lorentz microscopy experimental data for high-temperature superconducting cuprates which provide evidence of such interplay. Moreover we report on a theoretical prediction of a very special type of the vortex arrangement-the formation of the vortex molecules.

We report temperature dependences of B 1g electronic Raman spectra in slightly underdoped YBa 2 Cu 3 O 7−␦ samples. Above T c , we found no signature associated with the pseudogap while below T c we can acquire superconductivity-induced peaks which shift to higher energy with decreasing hole concentration ͑p͒ and disappear at p = 0.11. These behaviors are similar to those reported for other cuprates. The unusual properties of the antinodal electrons are discussed in comparison with the angle-resolved photoemission spectra.

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Temperature and magnetic field dependence of the vortex penetration into a superconductor and the resulting trapped vortex field (the vortex remanent state) were investigated for Bi 2 Sr 2 CaCu 2 O 8+x (BSCCO) and YBa 2 Cu 3 O 6+x (YBCO) single crystals and BSCCO thin films. The experiments revealed changes in the pinning regime (the magnitude and magnetic relaxation) of the trapped vortex field with an increasing temperature. The trapped vortex field, obtained by applying a constant magnetic field, exhibits a maximum at a certain temperature, that separates the partial vortex penetration regime at low temperatures from the complete vortex penetration state at higher temperatures. The corresponding vortex remanent states in these two regimes are characterized by two distinctly different relaxations, the logarithmic and the nonlogarithmic ones at temperatures below and above the maximum, respectively, for both BSCCO and YBCO. At temperatures close to T c surface/geometric barrier affect the relaxation rates. PACS number(s): 74.25. Op, 74.25.Uv, 74.25.Wx According to the Bean's model, 1 when an external magnetic field is applied to a superconductor, the internal magnetic field is not uniform and its local value depends on the position inside a superconductor. 2 When the external magnetic field is removed, a nonuniform vortex field is trapped in a superconductor. Trapping of the internal field inside a superconductor can be realized by using the following procedure: 3 apply an external magnetic field, H a , to a superconductor at different temperatures, which results in the penetration of vortices into the bulk of the superconductor. H a is subsequently reduced to zero and the vortex lines are trapped inside the sample. At a fixed temperature, the trapped magnetic field increases with an increasing H a and finally reaches a saturated (remanent) value. 4 The remanent value of the trapped internal field is proportional to the critical current.However, applying the same constant H a at different temperatures leads to a different situation. In this case, the magnitude of the trapped internal field is determined by the dependence of the penetration and pinning of the vortices on temperature. At low temperatures, the sample is partly penetrated by the field and the internal field is trapped at the sample's edges. It is expected that the trapped internal field increases with an increasing temperature and at a certain temperature, fully penetrates the sample, i.e., reaches a maximum value at the sample's center. The question is, however, what is the magnitude and the temperature dependence of this field at higher temperatures? Does it follow the temperature dependence of the remanent critical state where the current density acquires the critical value J c , or does it have a different temperature dependence? What are the relaxation rates of this field at temperatures corresponding to the partial and complete vortex penetration states? Are they affected by the surface/geometrical barriers? What is their dependence on the ...