Fragile superconductivity at high magnetic fields.

Ever since the discovery of superconductivity by Kamerlingh Onnes’ group in 1911, this phenomenon has continued to intrigue successive generations of scientists. One of the most spectacular discoveries since 1911 was that of high-temperature superconductivity in a copper oxide (1). Tens of thousands of papers have been written on these cuprate materials, yet 35 years later, they continue to surprise us. In PNAS, Hsu et al. (2) demonstrate that a fragile form of superconductivity persists in a cuprate up to the highest magnetic field they can muster. The original superconductor discovered by Onnes’ group is of a class now known as type I superconductors. These superconductors expel a magnetic field, with superconductivity surviving up to a critical field where the energy gain due to superconductivity equals that associated with the magnetic field. However, in 1957, Abrikosov (3) proposed another class of superconductors, now known as type II superconductors. In these superconductors (as illustrated in Fig. 1, Left ), above a lower critical field, superconductivity survives in the form of a unique state of matter where the magnetic field penetrates as an array of quantized magnetic flux lines. Once the cores of these so-called vortices overlap, then superconductivity disappears at a higher field known as the upper critical field. Since superconductivity survives to much higher magnetic fields, most practical applications involve type II superconductors. This requires that the vortices be pinned, as their motion leads to dissipation and thus a finite resistance. Fig. 1. Left : The phases of a type II superconductor in the field–temperature plane (3). In the Meissner state, the magnetic field is expelled. Above this, the magnetic field penetrates as an array of quantized vortices (vortex lattice). Above the upper critical field, the vortex cores overlap and superconductivity disappears (normal state). Right : In cuprates and other low-dimensional … [↵][1]1Email: norman{at}anl.gov. [1]: #xref-corresp-1-1