Working in his cryogenics laboratory at Leiden University in April 1911, Heike Kammerlingh Onnes noted that the resistivity of the metal mercury appeared to drop to zero as the temperature in the cryostat was decreased to 4.2 K. He recognized that the effect was real and not due to a faulty electrical contact or some other experimental artifact, and found that other metals placed in the cryostat exhibited similar behavior. Superconductivity research was born.
It took nearly half a century before theorists formulated a comprehensive microscopic description of superconductivity. The picture involved electrons teaming up in pairs and colluding with distortions in the ionic lattice of the crystal so that the electrons could propagate through the lattice without any resistance.
This Bardeen-Cooper-Schrieffer, or BCS, mechanism (named after its inventors) gives a fine description of “classical” superconductors. After the mechanism was formulated, however, researchers began discovering exotic superconductors—including the heavy-fermion systems that came on the scene in the 1970s and the high-temperature superconducting cuprates that appeared in the 1980s—that defied such a simple explanation. The microscopic mechanisms of such unconventional superconductors are still hotly debated today.
Two Reviews in this special issue look at exotic superconductors—one new, the other not so new. On p. 196, Norman discusses the theoretical work that has developed over the past four decades to try to describe how unconventional superconductivity arises in the cuprates and heavy-fermion systems. Illustrating that such a mature field can still generate surprises and exotica, Wang and Lee (p. 200) discuss superconductivity in iron-based materials, a finding that has opened up an “iron age of superconductivity.” Since they were found to be superconductors less than half a decade ago, these materials have generated much interest in trying to fathom how magnetism and superconductivity, usually each other's arch-nemeses, actually collude to form a superconducting state.
Meanwhile, as a pair of News stories makes clear, superconductivity and the theory behind it have enriched basic science in ways that nobody could have foreseen. Adrian Cho (p. 190) surveys how, over the past half-century, physicists have applied the BCS model of particle pairing to puzzles as diverse as the unexpected stability of certain nuclei, hiccups in the spin of neutron stars, and the bizarre behavior of liquid helium and supercold atomic gases. It has even helped inspire the discovery of new subatomic particles. Robert F. Service (p. 193) describes how condensed-matter physicists may be about to beat particle physicists at their own game. In experiments with superconductors, several teams of researchers are searching for so-called Majorana fermions: theoretically predicted particles that are their own antiparticles. Some now think they are on the brink of a discovery that could open a new window into quantum mechanics.
A century on, for more reasons than ever before, physicists and materials scientists remain enthralled with the observation of disappearing electrical resistance.