Gap renormalization in dirty anisotropic superconductors: Implications for the order parameter of the cuprates

We contrast the effects of nonmagnetic impurities on the properties of superconductors having a d_x²-y² order parameter, and a highly anisotropic s-wave (ASW) gap with the same nodal structure. The nonvanishing, impurity-induced, off-diagonal self-energy in the ASW state is shown to inhibit the low-energy excitations present in the clean system, leading to a qualitatively different impurity response of the single-particle density of states compared to the d_x²-y² state. We discuss how this behavior can be employed to distinguish one state from the other by an analysis of high-resolution angle-resolved photoemission spectra.