University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Colloquium

Wednesday, March 4th 2009
3:35 pm:
Speaker: Juan-Carlos Campuzano (Univesity of Illinois in Chicago)
Subject: Pairing above $T_c$ in the high temperature superconductors

In the BCS paradigm for the superconducting state, electrons close to the Fermi level EF- form Cooper pairs which condense into a zero center of mass momentum state. This results in a gap in the electronic excitation spectrum that is symmetrically
centered about EF- . Above Tc- where the condensate is lost, the pairs dissociate, the energy gap collapses, and the normal state Fermi surface appears. On the other hand,in the underdoped high temperature superconductors, instead of a complete Fermi
surface above Tc- , only disconnected regions of the Fermi surface appear, separated by regions that still exhibit an energy (pseudo)gap. After introducing the technique of angle resolved photoemission, used to study the electronic excitations in materials, we show that in this pseudogap phase, the energy-momentum relation of electronic excitations near EF- behave as they do in a normal metal in regions with a Fermi surface, but like that of a superconductor in the gapped regions. We discuss these results in terms of competing order parameters, and the relationship between pairing and condensation

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