Modulated pairs in superconducting cuprates

High-temperature cuprate superconductors are well known to the general science community for the simple reason that their transition temperature, T _c, is much higher than that of other superconductors. Perhaps less well known is the fact that T _c varies dramatically from one family of cuprates to the next. Understanding this dependence of T _c on crystalline structure would obviously be key to designing even higher temperature superconductors, but the origin of this variation is still not well understood, even after two decades of study. In this issue of PNAS, Slezak et al. (1) take an important step toward such understanding by using scanning tunneling microscopy (STM) to find a direct correlation between the size of the energy gap characterizing the superconducting state and a modulation of the atomic positions in the material. In doing so, they have also made the first definitive observation at zero magnetic field of a modulated superconducting state known as a “pair density wave” (2).

Investigation of the relationship between superconductivity and atomic structure has a long history. When the now-accepted microscopic theory of superconductivity was proposed in 1957 by Bardeen, Cooper, and Schrieffer (3), it was criticized by Bernd Matthias (4) because of its inability to correlate the superconducting T _c with known structural properties. Eventually, the theorists did catch up, to an extent, with the experimentalists; the strong coupling generalization of the Bardeen–Cooper–Schrieffer (BCS) theory is now known to give a reasonable estimate of the T _c of classical superconductors (5).

The layered cuprates, however, are a different story. Because there is no accepted microscopic theory related to these materials, it has been difficult to understand the dramatic variation in the maximum T _c from one family of cuprates to another. At ambient pressure, this maximum ranges from 26 K …

*E-mail: norman{at}anl.gov