Electronic States on a Twin Boundary of a d -Wave Superconductor

We show that an induced s -wave harmonic in the superconducting gap of an orthorhombic d{sub x{sup 2}{minus}y{sup 2}} superconductor strongly affects the excitation spectrum near a twinning plane. In particular, it yields bound states of zero energy with areal density proportional to the relative weight of the s -wave component. An unusual scattering process responsible for the thermal conductivity across the twin boundary at low temperatures is also identified. {copyright} {ital 1997} {ital The American Physical Society}