Sunday, March 05, 2006

HIgh Tc: where are we

As I said in my previous post, Nature Physics has run a fascinating piece surveying a number of theorists about the current state of the high Tc problem. I encourage you to read it, and I'll summarize very briefly for those without access to the journal. Things that everyone seems to agree on:
  • The symmetry of the superconducting pairing is d-wave.
  • The parent compounds of the high Tcs are "Mott Insulators". In the absence of strong electron-electron interactions, these materials would be metals; however, strong on-site repulsions on the coppers (so that no copper site d-orbital can be doubly occupied) lead to insulating behavior, and antiferromagnetic ordering at low temperatures.
  • The normal phase above Tc for the optimally doped compounds is really weird. It appears that the normal concept of quasiparticles fails there. When superconductivity is killed by whopping huge magnetic fields, the weirdness of the normal state persists down to T=0.
  • Understanding the normal phase is probably a good idea for understanding superconductivity.
  • There are signs, even within the superconducting phase, that there can be some kind of charge ordering ("stripe order" is a phrase that is used a lot).
  • In the underdoped compounds, there is a pseudogap in the density of states that exists to temperatures far higher than Tc.
Things that some people agree on:
  • The resonating valence bond picture accurately describes the superconducting phase; there is something called a spin liquid, and the pseudogap essentially corresponds to the formation of some kind of pair-like correlations without global phase coherence.
  • The pairing mechanism is purely electronic (as opposed to phonons in conventional superconductors).
  • The superconductivity is a general feature of doped Mott insulators.
  • There are quasi-2d Mott insulators that do not superconduct at all when doped.
  • There is no quantum phase transition (that is, at T=0 as a function of, say, doping) in these materials.
  • There is a quantum phase transition in these materials, and therefore there is a well-defined (if very hard to detect) breaking of symmetry when going from the strange metal phase to the pseudogap phase.
  • The stripe order is crucial, and competes with superconductivity.
  • The stripe order is incidental and unimportant.
What I think is interesting, as a bystander:
  • Everyone has their favorite handful of experiments that they treasure, and is appreciative that the materials growers have gotten so good at making clean samples of these nasty quaternary compounds.
  • Only Chandra Varma explicitly addresses the reason why copper is special, chemically, in his microscopic picture (which has almost no relation at all to simple concepts of pairing, as far as I can tell).
  • Very few people bother to address the existence of electron-doped superconductivity in these systems.
  • It is clear that the whole field is strongly hampered by the fact that chemical doping is a real bear at these levels - it introduces large amounts of disorder. Field-effect experiments would be great, if only they could really change the charge density by chemically interesting amounts.
The field continues to progress, though it's not for the faint of heart. The holy grail of room temperature superconductivity still beckons, though there are those who make reasonably persuasive arguments that the copper compounds are awfully special, and may be as good as it gets. For me, I'll stick to nanostructures for now.

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