In recent years, some physicists have found some cuprates in which the transition to zero resistance occurs at a lower temperature than the Meisner effect. So at low temperatures, the cuprate acts like a normal superconductor. As the temperature rises, it goes through a first transition and loses its zero resistance while maintaining the Meisner effect. Then as the temperature rises further, it goes through a second transition in which the Meisner effect disappears and the material becomes an ordinary conductor. In underdoped yttrium barium copper oxide (YBCO), the first transition occurs at 85K while the second at over 200K.
Vladimir Kresin at the Lawrence Berkeley National Laboratory and Stuart Wolf at the University of Virginia put forward a theory. They think that these cuprates consist of two components with different transition temperatures: the component with the higher transition temperature forms islands in a matrix with a lower transition temperature.
That explains why the material has two transition temperatures, they say. Below 85K, both components are superconductors. But as the temperature rises above 85K, the matrix becomes a conventional conductor introducing finite resistance. However, the island component maintains its superconductivity. Their idea is that the high temperature islands form where atomic isotopes subtly change the material properties.
Kresin and Wolf say that one experiment has shown that the substitution of O-18 for O-16 in another cuprate dramatically increases the second transition temperature. That’s potentially exciting. In effect, these guys say they’ve discovered a room temperature superconductor, albeit one that works inside a lower temperature superconductor. Whether this materials can be isolated so that the effect appears in a standalone bulk material will be an important question to investigate.
In 2006, Joe Eck at superconductors.org (a private experimenter) reported 150K superconductor (for islands of a material)
In 2007, Joe Eck reported 175K superconductor (for islands of a material)
In 2008, Joe reported 200K and then 212K superconductor
In 2009, Joe reported 233K, 242K and then 254K superconductor islands
In 2010, Joe reported 277K
In 2011, Joe reported islands of 18.5C (291.5 K)
Doped cuprates are inhomogeneous superconductors. The concept of an
intrinsic critical temperature whose value greatly exceeds that for the resistive is supported by a number of experimental studies, including those performed recently. These data are discussed in this review. The anomalous diamagnetism observed at high temperatures is a manifestation of the presence of superconducting clusters embedded into a normal metallic matrix. The value of intrinsic critical temperature in some cuprates reaches the value which is close to room temperature. The a.c. properties of such inhomogeneous systems are discussed.
Conclusion. In usual superconductors the resistive and Meissner transitions occur at the same temperature. The inhomogeneous nature of the cuprates leads to a different scenario. Namely, the Meissner effect which is the most fundamental manifestation of superconducting pairing occurs in some cuprates is close to room temperature and corresponds to the appearance of superconducting clusters inside of the normal metallic matrix. One can propose ta.c. experiments allowing the determination of the presence of superconductivity (the clusters) at such high temperatures with a potential for their applications.