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Department of Physics Colloquia - Spring 2017

Peter Johnson

Shining a light on High Tc Superconductivity

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 representing a transition from a doped Mott Insulator to more metallic like behavior. However the overdoped regime is also characterized by superconducting fluctuations resulting in a gap still evident in the spectra at temperatures above Tc. Careful investigation of the latter phenomena allow us to reveal a clear link between the pseudogap state at high temperatures and the superconducting state at low temperatures. 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.

Monday, March 20, 2017    4:00pm in 301 Riddick Hall Sayers Auditorium