For the next two weeks I am at the Aspen Center for Physics participating in a workshop on Superconductivity. A blog for the meeting captures its flavour, spanning a diverse range of systems and debates. I was not here for the first two weeks. Here are two related experimental results for the underdoped cuprates that have generated a lot of discussion.
1. A charge density wave (CDW) phase.
This has been observed directly with X-rays. The figure below is taken from this paper.
2. A jump in the charge carrier density versus doping.
Hall resistance measurements at high magnetic fields imply that for small doping the charge density scales with the doping p [p=0 corresponds to the Mott insulator that occurs at half filling] and at higher dopings, 1+p. This is summarised in the figure below from this paper.
A few comments.
1. Is the CDW phase relevant to understanding the pseudogap, superconductivity, and the strange metal phase?
There is debate about this. On the one hand it does compete with superconductivity. It could provide the much sought after quantum critical point below the superconducting dome in the phase diagram. CDW fluctuations could produce the pseudogap and/or strange metal properties. On the other hand, it may just be an "artefact" due to residual interactions that only become important when the magnetic field suppresses the interactions that determine the zero field phase diagram.
Why does it generate so much interest?
Well it is a concrete result and we are desperate for them and some clue to these long standing puzzles.
2. The large Fermi surface and carrier density equal to 1+ p at large doping is what one expects from a simple Fermi liquid and Luttinger's theorem. The fact that at small doping the charge density is proportional to p may have a boring explanation or an interesting one.
Boring: the antiferromagnetic (AFM) order that occurs for small p reconstructs the Fermi surface due to the periodicity associated with AFM.
Interesting: this is a strongly correlated effect associated with doping a Mott insulator.
3. In a strongly correlated metal deducing information about the charge carrier density and the Fermi surface from measurements of the Hall coefficient and/or the thermoelectric power is subtle, as emphasised by Shastry, and discussed here.