Superconducting gap and strong in-plane anisotropy in untwinned YBa2Cu3O7-delta

With significantly improved sample quality and instrumental resolution, we clearly identify in the (pi, 0) photoemission spectra from YBa2Cu3O6.993, i n the superconducting state, the long-sought "peak- dip-hump" structure. Th is advance allows us to investigate the large a-b anisotropy of the in-plan e electronic structure including. in particular, a 50% difference in the ma gnitude of the superconducting gap that scales with the energy position of the hump feature. This anisotropy, likely induced by the presence of the Cu O chains, raises serious questions about attempts to quantitatively explain the YBa2Cu3O7-delta data from various experiments using models based on a perfectly square lattice.