ARPES Study of the Superconducting Gap Anisotropy in Bi_2Sr_2CaCu_2O_8+x
  We report measurements of the momentum dependence of the superconducting gap in Bi_2Sr_2CaCu_2O_{8+x} (Bi2212) with angle-resolved photoemission spectroscopy using a dense sampling of the Brillouin zone in the vicinity of the Fermi surface. In the Y quadrant of the zone, where there are no complications from ghost bands caused by the superlattice, we find a gap function consistent within error bars to the form cos(k_x)-cos(k_y) expected for a d-wave order parameter. Similar results are found in the X quadrant with the photon polarization chosen to enhance main band emission over that due to ghost bands. 
H. Ding, M. R. Norman, J. C. Campuzano, M. Randeria, A. F. Bellman, T.   Yokoya, T. Takahashi, T. Mochiku, and K. Kadowaki