Condensed-Matter Physics & Materials Science Seminar
"Approaching the Holographic Viscosity Bound in High Temperature Superconductors"
Presented by Jonathon Rameau, BNL
Friday, November 8, 2013, 3:15 pm
Small Seminar Room, Bldg. 510
Hosted by: by Raju Venugopalan/Peter Johnson
Measurements of the ratio of shear viscosity to entropy density, /s, for the Quark-Gluon Plasma (QGP) created in heavy ion collisions at RHIC and the LHC (T~1013 K), as well as in condensates of ultracold Fermionic atoms (T~10-8 K) have indicated these strongly correlated quantum fluids may approach the lower bound of /s≥ħ/4kB conjectured by Kovtun, Son and Starinets (KSS) using AdS/CFT. The near saturation of the bound in the QGP around its phase transition temperature, as well as the apparently generic appearance of minima in /s for a variety of systems, has indicated the possibility that the viscosity bound may play a role in quantum phase transitions of strongly correlated matter more generally. However, detailed T-dependent measurements of /s in these systems are currently in a state of flux and clear examples of both the saturation of the bound as well as its role in phase transitions have not been forthcoming. Presently, we have developed a general method of measuring /s as a function of T (for T~1 to 10^2 K) for solid state electronic systems using Angle Resolved Photoemission Spectroscopy (ARPES). We have applied this method to the sine qua non of strongly correlated electron systems, the cuprate high transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+x (BSCCO). We find that /s for BSCCO does indeed have a minimum at Tc very near the KSS bound and that /s asymptotically approaches a constant value on the order of twice the KSS bound at high T. Additionally, we find that several other strongly correlated electron systems displaying a range of interesting phase transitions appear to have /s at or very near the KSS bound. The implications of this analysis for phase transitions in strongly correlated electron systems as well as comparisons to the /s of the QGP will be discussed.