An unusual phase transition to a second liquid vortex phase in the superconductor YBa2Cu3O7

A magnetic field penetrates a superconductor through an array of 'vortices' , each of which carries one quantum of flux that is surrounded by a circula ting supercurrent. In this vortex state, the resistivity is determined by t he dynamical properties of the vortex 'matter'. For the high-temperature co pper 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 ca n 'melt' into a 'liquid' phase, in which their mobility gives rise to a fin ite resistance. (This melting phenomenon is also believed to occur in conve ntional superconductors, but in an experimentally inaccessible part of the phase diagram(2).) For the case of YBa2Cu3O7, there are indications of the existence of a critical point, at which the character of the melting change s(3-10). 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 determi ne the phase diagram in this material to 26 T, well above the critical poin t. Our results reveal the presence of a reversible second-order transition above the critical point. An unusual feature of this transition-namely, tha t the high-temperature phase is the less symmetric in the sense of the Land au theory(11)-is in accord with theoretical predictions(12-14) of a transit ion to a second vortex-liquid phase.