Thursday, June 11, 2009

Mind the gaps

Here is a rough summary of a few things I think I learnt this week. More to come later...
Please post corrections and clarifications.

A lot of attention in both STM and ARPES studies on the cuprates is being given to questions of particle-hole symmetry. This is because in a superconducting d-wave gap, the Bogoliubov quasi-particles have perfect particle-hole symmetry. In contrast, other possible nodal states such as the staggered flux phase (d-density wave, DDW) do not have this property.

The evidence from both STM and ARPES is that the physical origin of the gap near the nodes is quite different from the gap at the anti-nodes. They have different temperature and doping dependence. The consensus also seems to be that the gap near the nodes is from fluctuating superconductivity.
Some of the issues are nicely summarised in a Science Perspective by Andy Millis.

Electronic Raman Scattering (ERS) is a sensitive probe of the d-wave gap and pseudogap. The B1g polarisation ERS is dominated by quasi-particles near the anti-nodal part of the Brillouin zone. In contrast, the B2g polarisation response is dominated by the nodal part.

In the cuprates Tc tends to increase (decrease) with pressure in the underdoped (overdoped) region. Is this simply because pressure decreases correlations and so increasing pressure is equivalent to a small increase in doping?

No comments:

Post a Comment