"Fragile superconductivity": a kinetic glass transition in the vortex   matter of the high-temperature superconductor YBa2Cu3O7-d
  Of the many different kinds of glassy states found in nature, spin glasses and structural glasses (e.g. normal window glass) have probably received the most attention. One of the central questions concerning the glassy state is whether the "glass" is a distinct thermodynamic phase from the "liquid" phase. It is believed that this may actually be the case for spin glasses, whereas the glass transition in structural glasses appears to be mostly kinetic in nature. Supercooled structural liquids, however, exhibit a wide range of behaviors, which can be classified using the "strong-fragile" scheme of Angell, where the terms "strong" and "fragile" refer to Arrhenius and highly non-Arrhenius relaxation behaviors, respectively. For the extremely fragile liquids, a thermodynamic origin of the glass transition cannot be ruled out. A microscopic understanding of the complexity leading to different degrees of fragility and affecting the kinetic glass transition is, however, lacking. Also the connection, if any, between the different kind of glass states (spin, structural, etc.) is unclear. Here we examine the glassy behavior of a novel type of matter, namely the vortex matter in the high-temperature superconductor (HTSC) YBa2Cu3Ox. 
Rolf Lortz, Christoph Meingast, Alexandre I. Rykov, Setsuko Tajima