Scaling Laws in High-Temperature Superconductors as Revealed through Infrared Spectroscopy

Superconductivity refers to a fascinating state of matter where the electrical resistivity is precisely zero. Originally discovered in elemental metals such as mercury and tin in the early part of the last century, the mechanism of superconductivity was elusive and nearly 50 years passed before a comprehensive theory for superconductivity in metals was proposed by Bardeen, Cooper and Schrieffer (the 'BCS' theory). In a normal metal, the resistivity is determined by the elastic scattering of carriers. However, when a metal becomes a superconductor, the charge carriers are no longer single electrons, but rather pairs of electrons ('Cooper pairs'), which are bound together by a phonon interaction (phonons are the vibrations of the atomic lattice), and flow without resistance. The BCS theory of superconductivity has been tremendously successful at describing this phenomenon in metals and alloys. However, an aspect of the phonon-mediated pairing mechanism is that the superconducting critical temperature (Tc) will be restricted to values below ∼30 K. This prediction, as well as the observation for a number of years of a record Tc = 23 K in Nb3Ge, implied that the superconductivity would remain a curiosity restricted to low-temperature physics labs, removed from mainstream applications