THERMAL CONDUCTIVITY ACROSS A TWIN BOUNDARY IN A D-WAVE SUPERCONDUCTOR

We consider excitation spectrum near a twin boundary in an orthorhombic $d+s$ superconductor. The low-energy spectrum is highly sensitive to the presence of the small amount of s-wave component. Robustness of the bound states at the Fermi level with respect to impurities and an extra boundary potential is investigated. The role of Andreev transmission process for the low-temperature thermal conductivity across twin boundaries is studied for an impure superconductors. At very low temperatures the bulk part and the twin boundaries part of the thermal conductivity have similar linear-$T$ dependences, whereas at intermediate temperatures the two contributions behave like $T^3$ and $T^2$, respectively.