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

Abstract

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.

‘Fragile Superconductivity’: A Kinetic Glass Transition in the Vortex Matter of the High-temperature Superconductor YBa2 Cu3O7-δ

“…The vicinity of such a critical point to T g could provide another explanation for the extreme heat capacity of the vortex glass former. Indeed, interesting features have been observed recently in the vortex matter phase diagram of hightemperature superconductors, such as inverse melting below a Kauzmann point 24,28 and evidence for a liquid-liquid transition in its vicinity 14 . These might be related to the features observed in structural glasses.…”

“…1 and the above discussion about T K in the vortex matter in HTSCs. With the introduction of random vortex pinning defects, however, the first-order melting transition vanishes and is replaced by a more continuous transition 4,[11][12][13][14] , commonly referred to as the 'vortex-glass' transition. There is evidence for different glassy phases (Bose glass, Bragg Glass and vortex glass) 4 , the detailed nature of which however remains controversial.…”

Grain boundaries in vortex matter

“…Experimental results suggest that an increase in temperature above a certain critical value T m determines the transition to a liquid phase, [5][6][7] while the effects of disorder associated with high magnetic fields are responsible for the insurgence of a glassy phase. [8][9][10] A deep theoretical understanding of such transition phenomena, accounting for their microscopic origin, has not been achieved yet.…”

“…As in conventional matter strong enough fluctuations destroy longrange order: when temperature is raised the vortex lattice melts into a vortex liquid. [5][6][7] Fluctuations are also provided by quenched disorder that is intrinsically present in these materials, leading to complex glassy phases. [8][9][10][11][12][13] While several experimental methods have been used to investigate vortex matter, a direct image of the geometrical and topological properties of the vortices can be obtained by the Bitter decoration technique.…”

Tearing transition and plastic flow in superconducting thin films

“…In particular, much experimental and theoretical effort has been devoted to characterizing the phase diagram of type II superconductors [4]. Depending on the value of the magnetic field H, temperature T , and sample preparation, vortices can either form a crystal [5], which at higher temperatures melts into a liquid [6,7,8,9], or, due to quenched disorder, they can be found in more complex phases, such as the vortex glass [10], the Bose glass [11] or the Bragg glass [12,13]. Of special importance is the non-equilibrium response of vortex matter to the flow of an external current [14,15], because the dissipative motion of the vortices induces an undesirable macroscopic resistance.…”