High temperature superconductivity?

The fundamental mystery of the high temperature superconductivity seems to run very deep. Here I will try to explain an important aspect of it, in a way that is, hopefully, understandable to any one with healthy curiosity.

The superconductivity

Let us discuss the superconductivity, first, before discussing the high-temperature superconductivity. It is a well-known fact that practically all heroes of physics in the early 20th century (Einstein, Bohr, Feynman, et al.) have tried to explain the superconductivity, and did not succeed. Why was it so difficult?

Normal electron and superconducting electrons

The triumph of the BCS (Bardeen-Cooper-Schrieffer) theory is based on the distinction between these two: normal electron and superconducting electrons. At high temperature, normal electrons conduct the electricity. For instance, this is how the electricity conducts in Copper wires. If a metal is cooled down to very low temperature, it is often found to super-conduct—the resistivity drops to zero, suddenly (perfect conductor), and it expels any magnetic field (perfect dia-magnet). According to the BCS theory, which is the theory of the superconductors that we have so far, the superconductivity occurs due to the movement of pairs of electrons (Cooper pairs). Recognizing this fundamental difference between the charge carriers of the normal conductor and the superconductor was the key to solving the superconductivity puzzle.

High temperature superconductivity

In 1987, Bednorz and Müller discovered the superconductivity in a very unlikely material—a Lanthanium Barium Copper oxide, which is essentially a ceramic material made to conduct with some charge carriers by chemical doping. Indeed, when they received the Nobel prize, the citation was “for their important breakthrough in the discovery of superconductivity in ceramic materials.” This was the beginning of the high temperature superconductivity puzzle!

Bad metal and good superconductor

If you are recalling a toilet bowl or a vase when you hear the word ceramic, you would be correct. High temperature superconductors revealed the paradoxical truth of the superconductivity: certain bad metals become superconductors much more easily, i.e., have higher transition temperature, than good metals. What this means exactly is the key question of the field today. This is why the Nobelist Phil Anderson suggests that understanding the bad/strange metal phase is the key to understanding the high Tc problem. What precursors of the superconductity lurk in the properties of the bad metal phase? In the Gweon group, we are discovering and interpreting illuminating essential features of the bad metal, or strange metal, phase.