A magnetic field penetrates a superconductor through an array of 'vortices', each of which carries one quantum of flux that is surrounded by a circulating supercurrent. In this vortex state, the resistivity is determined by the dynamical properties of the vortex 'matter'. For the high-temperature copper oxide superconductors (see ref.1 for a theoretical review), the vortex phase can be a 'solid', in which the vortices are pinned, but the solid can 'melt' into a 'liquid' phase, in which their mobility gives rise to a finite resistance. (This melting phenomenon is also believed to occur in conventional superconductors, but in an experimentally inaccessible part of the phase diagram.) For the case of YBa2Cu3O7, there are indications of the existence of a critical point, at which the character of the melting changes. But neither the thermodynamic nature of the melting, nor the phase diagram in the vicinity of the critical point, has been well established. Here we report measurements of specific heat and magnetization that determine the phase diagram in this material to 26 T, well above the critical point. Our results reveal the presence of a reversible second-order transition above the critical point. An unusual feature of this transition-namely, that the high-temperature phase is the less symmetric in the sense of the Landau theory-is in accord with theoretical predictions of a transition to a second vortex-liquid phase.

We report measurements of the de Haas-van Alphen (dHvA) effect in the quasi-two-dimensional organic metal κ-(BEDT-TTF)2Cu(NCS)2 by means of a capacitive torquemeter. The experiments were carried out in magnetic fields up to 29.5 T, at temperatures down to 0.4 K and for tilt angles θ between -35° and 80°. In the angular dependence of the fundamental frequency we observed five spin-zeros, the absolute amplitude at θ = 0° amounts to μ0M = 4·10-6 T (B = 13.8 T, T = 1.4 K). Above 15 T a series of high-frequency oscillations due to magnetic breakdown is observed. The main breakdown orbit could be detected up to θ = 55°, the angular dependence is also characterized by spin-splitting effects. We determined the effective cyclotron mass of three different breakdown orbits at θ = 5.6°. Our results are in good agreement with a simple model of magnetic breakdown. The occurrence of weak oscillations forbidden within this model is discussed.

We report resistivity measurements performed on a high quality single crystal of CePd 2 Si 2 under hydrostatic pressure. At ambient pressure the de Haas van Alphen effect has been also studied. Two different frequencies with weak angular dependence were detected with magnetic field lying in the basal plane, while another frequency was found with magnetic field parallel to the tetragonal c axis. Near the critical pressure, P c t27 kbar, where the antiferromagnetic transition vanishes, the normal state resistivity does not follow the usual Fermi-liquid (FL) behavior and is described by a T 1.3 law, while just below P c , the resistivity shows clearly separated spin wave and electron electron contributions. At P c , the FL form of \(T) is not restored even at magnetic field up to 6 T. The first appearance of superconductivity is observed at P=19 kbar, and the critical temperature increases with pressure up to 27 kbar. The analysis of the upper critical field at P c shows that the superconducting state is well described by a weak coupling, clean limit model with a slightly anisotropic orbital limit and a strongly anisotropic paramagnetic one.