We show that an induced $s$-wave harmonic in the superconducting gap of an orthorhombic $d_{x^2{-y}^2}$ 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.

• Received 9 May 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.1734