Twenty-five years on from its discovery, high-temperature superconductivity remains without a satisfactory explanation. The latest studies on the electronic phase diagram of copper oxide compounds reveal why this is so. See Letter p.73
Some years ago, I lectured at a National Science Foundation summer workshop for high-school physics teachers. My subject was superconductivity. One of my co-instructors was Robert Laughlin. Scrawled across the top of Bob's first projector slide was the phrase, 'The Theory of Everything', and I thought, “Oh, boy, here we go, the standard model of particle physics — again”. But underneath the title, he had written instead the many-body Schrödinger equation, summed over all the interactions between electrons and nuclei, and thus containing, once electron spin is included, the complete chemistry and physics of ordinary, terrestrial matter.
Of course, the devil is always in the details, in this case the enormous summation over particle coordinates that is required to achieve a scale of, say, Avogadro's number. From this summation emerge life, the climate, smartphones ... and high-temperature (high-TC) superconductivity. And it is on this last that Jin et al.1 (page 73 of this issue) and He et al.2 (in an earlier study published in Science) make the latest effort to illuminate qualitatively the microscopic origins. They do this by attempting to unravel the enigmas of the electronic phase diagram of materials known as copper oxide perovskites. Within this phase diagram (Fig. 1) reside several quantum states, characterized by one or more 'quantum critical points', in rough analogy to the classical critical points characterizing the separation of the gas, liquid and solid states of macroscopic matter. How the various phases in the electronic phase diagram compete or cooperate in generating the emergent superconducting state constitutes what I term the great quantum conundrum.