Mottness

Phillip Phillips has a very good review of the normal state of the cuprate superconductors. One may wonder on why we care about the normal state behavior when what we want is what is causing it to be superconducting. Well, as with the conventional superconductor, the normal state tends to give us a lot of clues on what exactly is going on as one approach the superconducting regime. By looking at various parameters in the normal states, we can see how they evolve and maybe give us hints on the cause of the onset of superconductivity in these materials.

One of the interesting aspect of the normal state is the resistivity behavior. The linear resistivity as a function of temperature is seen almost over the whole doping range. While such a thing is predicted within the phenomenological model of a Marginal Fermi Liquid, it actually provides a strong argument for quantum criticality in the phase diagram of this material. This is illustrated in Fig. 1 of the article.

However, I don't believe that there has been a clear evidence of, say a highly overdoped cuprate that goes from the superconducting state into a Fermi Liquid State, and then into a Strange Metal state as one raises its temperature. I think there have been a few evidence that there might be 2 different gap scales in the underdoped cuprates, so this certainly matches that part of the phase diagram. But the overdoped part isn't clear. In angle-resolved photoemission on the overdoped cuprates, the coherent peak, representing the presence of well-defined quasiparticle states, survives till very high temperatures (150K or so). I don't think there's any evidence yet that the peak disappears abruptly as it crosses from the Fermi Liquid regime into the Strange Metal regime. We expect this to occur because for optimally doped cuprates, its normal state is the Strange Metal. As soon as the material goes above Tc, the quasiparticle peak disappears promptly.

The mystery continues...

Zz.