Effect of nonmagnetic impurities on the gap of a dx2-y2 superconductor as seen by angle-resolved photoemission

An analysis of angle-resolved photoemission (ARPES) experiments in the superconducting state of the high-Tc copper oxides is presented. It is based on a phenomenological weak-coupling BCS model which incorporates the experimental normal-state dispersion extracted from ARPES, and nonmagnetic impurity scattering in the presence of a dx2-y2 order parameter (OP). It is shown that already in the pure case, the broadening by finite momentum resolution of the analyzer leads to a finite region of apparent open-quote open-quote gaplessness close-quote close-quote around the true node of the OP. Nonmagnetic impurities further amplify this effect by introducing additional spectral weight around zero frequency. At sufficiently large impurity concentrations ni≅0.02 endash 0.05, this results in an extended region of gaplessness up to δφ=±7 (φ the angle on the Fermi surface) around the true node for a large range of moderate to strong impurity potential strengths. Different ways to identify the presence of impurity scattering in the ARPES spectra are proposed. copyright 1996 The American Physical Society