"Optical Conductivity and the Correlation Strength in High-Tc Cuprates: Are the materials Mott insulators and what do we mean by a Mott insulator anyway? "

Andrew Millis

Columbia University

Recent advances in theoretical technique have made it possible to address properties of "correlated electron" materials in a (semi) quantitative way. This talk will present the new theoretical results and their application to spectroscopic data (mainly optics) on high-Tc cuprate superconductors. The analysis indicates that the materials are not appropriately described as Mott (or charge transfer) insulators, but rather that the problem is an intermediate coupling one in which intersite correlations play an essential role.

References: Phys. Rev. Lett. {\bf 97} 076405/1-4 (2006); "Optical conductivity and the correlation strength of high temperature copper-oxide superconductors", Nature Physics (2008).