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 distinct entities: normal electron and superconducting electrons. They sound similar—they involve electrons after all—but, how do they—normal electron and superconducting electrons—differ? At high temperature, it is those normal electrons that conduct the electricity. For instance, this is how the electricity flows in a metal wire such as copper wire or aluminum wire. However, 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 at the same time it expels any magnetic field (perfect dia-magnet). For instance, this happens when temperautre drops below 1.2 K for aluminum. According to the BCS theory, which is the theory of the superconductors that we have so far, the superconductivity occurs due to the formation, and the coherent (i.e., completely synchronized) movement of, pairs of electrons (Cooper pairs).

What the BCS theory taught us is that the charge carriers in the normal state (individual electrons) and the charge carriers in the superconducting state (Cooper pairs) are fundamentally different! Perhaps using a cheesy analogy, they are as different as the single life and the married life are different.

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 electricity with a little bit of charge carriers introduced 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 imagining a toilet bowl or a vase when you hear the word ceramic, you would be correct. Ceramics are usually insulators. Indeed, high temperature superconductor materials are basically insulators, and they acquire some metalicity through chemical doping of charge carriers: they are still not good metals at all. Thus, high temperature superconductors revealed the paradoxical truth about the superconductivity: certain bad metals become superconductors much more easily, i.e., have higher transition temperature, than good metals. Explaining exactly how this occurs is the key puzzle 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.