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 on specific heat ͑C p ͒, transport, Hall probe, and penetration depth measurements performed on Fe͑Se 0.5 Te 0.5 ͒ single crystals ͑T c ϳ 14 K͒. The thermodynamic upper critical field H c2 lines has been deduced from C p measurements up to 28 T for both H ʈ c and H ʈ ab, and compared to the lines deduced from transport measurements ͑up to 55 T in pulsed magnetic fields͒. We show that this thermodynamic H c2 line presents a very strong downward curvature for T → T c which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that H c2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T / T c ϳ 0.99 ͑for H ʈ ab͒ which is the consequence of a very small value of the coherence length c ͑0͒ϳ4 Å ͓and ab ͑0͒ϳ15 Å͔, confirming the strong renormalization of the effective mass ͑as compared to DMFT calculations͒ previously observed in ARPES measurements ͓A. Phys. Rev. Lett. 104, 097002 ͑2010͔͒. H c1 measurements lead to ab ͑0͒ = 430Ϯ 50 nm and c ͑0͒ = 1600Ϯ 200 nm and the corresponding anisotropy is approximatively temperature independent ͑ϳ4͒, being close to the anisotropy of H c2 for T → T c . The temperature dependence of both ͑ϰT 2 ͒ and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system.