Thermal-Conductivity Measurements of Gapless Behavior Produced by the Proximity Effect

The thermal conductivities $K_s$ and $K_n$ in the superconducting and normal states have been measured for six In-Bi films of varying thickness with Mn layers on each side. The graphs of $\frac{K_s}{K_n}$ against $T$ show the characteristic linear variation of gapless systems near the transition temperature. In contrast to the expected behavior, $\frac{K_s}{K_n}$ approaches a finite value at low temperatures. This is interpreted in terms of localized states near the $n-s$ boundary which cause gapless behavior at all temperatures.