October 25, 2006
Interschool Lab, 7th floor, Schapiro/CEPSR
Hosted by: Yasutomo Uemura - CISE
Speaker: Tonica Valla, Brookhaven National Laboratory, New York
In conventional superconductors, the appearance of an energy gap in the electronic spectrum indicates pairing of electrons into Cooper pairs and a simultaneous transition into a macroscopic superconducting state. In contrast, in the underdoped high temperature superconductors, an energy gap is already present in the normal state. An understanding of this normal state gap or ‘pseudogap’ has proven elusive, because its ground state electronic structure was unknown. Here, we present the first studies of electronic structure in La2-xBaxCuO4, a unique system where the superconductivity is strongly suppressed and static spin and charge orders or ‘stripes’ develop near a doping level of x=1/8. Using angle resolved photoemission we detect an energy gap at the Fermi surface that vanishes only at four nodal points and has a momentum dependence consistent with d-wave symmetry. And in tunneling spectroscopy, we find that the density of states DOS(E)|E|, with zero-DOS falling exactly at the Fermi energy. In theory, an energy gap due to d-wave electron pairing interactions has such a structure. Remarkably, this gap persists and has the maximal magnitude at x=1/8, precisely where superconductivity vanishes. Thus, the non-superconducting La1.875Ba0.125CuO4 exhibits an electronic ‘pseudogap’ consistent with a phase incoherent d-wave superconductor whose Cooper pairs are localized into spin/charge ordered structures - ‘stripes’.