15 March 2016
Venue: GO Jones Room 610
In recent years a ubiquitous new phenomenon has been discovered in High-Tc superconductors. This is a “Charge Density Wave” (CDW), which competes with superconductivity and suppresses both the critical temperature ant the upper critical field when the materials are “underdoped”. The CDW is associated with a “Fermi Surface Reconstruction” which is revealed by a change from a “large” hole-like Fermi surface, containing all the carriers in the overdoped regime, to a “small” electron-like Fermi surface below optimal doping. The CDW gives rise to X-ray diffraction satellites which have intensities ~ 10-6 of the lattice Bragg peaks, which explains why they remained unobserved for so long; however the energies associated with the lattice distortions are comparable with those associated with superconductivity, and show that the lattice cannot be ignored in High-Tc materials, even though the mechanism of superconductivity is not the conventional phonon one.
Using synchrotron x-rays, we have succeeded in measuring hundreds of the CDW satellites in YBa2Cu3O6.5, and established that the CDWs in this material have a most unexpected symmetry, which remains at high magnetic fields where the Fermi Surface is measured. We have used this symmetry to demonstrate how the CDW may lead to Fermi Surface reconstruction in this material.
http://www.nature.com/ncomms/2015/151209/ncomms10064/abs/ncomms10064.html (Dec 2015)
http://link.aps.org/doi/10.1103/PhysRevB.93.094502 (March 2016)