Superconducting Gap and Order-Parameter in Bi2sr2cacu2o8+X

Using angle-resolved photoemission, we observed a nonzero superconducting gap and the presence of a superconducting condensate along all three major symmetry directions. We find that the gap is highly anisotropic, with a gap as small as 1-2 meV along GAMMA-Y, 4-8 meV along GAMMA-X, and 14-20 meV along GAMMA-M. We argue that our data imply either an anisotropic s-wave order parameter or an unconventional, two-component order parameter, and that measuring the size of the gap does not by itself distinguish between the two possibilities. We propose a phenomenological unconventional order parameter, and note the quantitative agreement between theory and experiment.