The Search for Unconventional Superconductivity - A Long Story with a Happy End

The onset of superconductivity in metals was first observed by Kamerlingh Onnes in 1911 when the resistivity on Hg dropped to zero as he cooled it below 4K.For many years the microscopic origin of this spectacular phenomenon remained mysterious.The discovery of quantum mechanics in the nineteen twenties led to great advances in the general understanding of metals but did not illuminate superconductivity. Einstein's proposal of boson condensation leading to macroscopic occupation of the lowest single boson state underlies superfluidity in liquid helium, but electrons are fermions and obey the Pauli principle forbiding multiple occupancy of single fermion states. A microscopic theory for superconductivity was finally developed by Bardeen, Cooper and Schrieffer in the mid-nineteen fifties.It was based on the pairing of elelectrons due to weak attraction between them mediated by lattice vibrations. It was an immediate success and explained the properties of all superconductors known at that time. Some time later, it was speculated that other unconventional forms of superconductivity could be possible,derived in some way from the Coulomb repulsion between electrons. The vicinity of magnetic order was proposed as best place to search for such unconventional superconductors. but for over two decades no examples could be found. In the early nineteen eighties superconductivity was found in heavy fermion metals with features consistent with an unconventional form. But these are complex metals difficult to describe microscopically. A big breakthrough came with the discovery of the high-Tc cuprate superconductors in the mid-eighties. The unconventional nature of their superconductivity has been established but the many unexpected and anomalous features of these special materials continue to pose a challenge. Finally ten years ago another unconventional superconducting transition metal oxide was found -a ruthenate, whose superconductivity is a realization of the original theoretical ideas.