Shining a light on High Tc Superconductivity.

Peter D. Johnson

Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory

Photoemission has developed into a powerful probe of condensed matter. Modern technical developments enable the study of not only the single particle spectra but also the interaction with collective excitations. In this talk we present an overview of the modern photoemission experiment followed by demonstrations of its application to the high Tc cuprate superconductors. We discuss insights into the complex phase diagram of the latter materials offered from photoemission studies. We show that in the underdoped pseudogap regime the Fermi surface is characterized by pockets and that these pockets evolve into a large Fermi surface beyond a critical doping of 0.2. The latter transition represents a transition from a doped Mott Insulator to Fermi liquid behavior. However the overdoped regime is also characterized by superconducting fluctuations resulting in a gap still evident in the spectra at temperatures above Tc. A refinement of the photoemission technique, pump-probe or two photon photoemission, is used to further investigate the character of the Fermi surface and the dynamics of these materials. By using ultrafast photo-doping and examining the system in non-equilibrium we are able to confirm the presence of hole pockets in the underdoped region of the phase diagram. The pump-probe technique also allows us to explore the unoccupied states above the Fermi level where we identify scattering from a mode, reminiscent of a well-studied kink and associated mass renormalization, observed below the Fermi level.

Host: R. A. Bartynski