Effect of melting of the metallic component on the anomalous far-infrared absorption of superconducting Sn particle composites

By means of a simple experimental heat-treatment procedure, the anomalous far-infrared absorption of superconducting Sn particle composites is shown to be associated with clustering. The structural insights thus obtained lead to a new theory in which the composite electric dipole absorption is dominated by poorly conducting clusters and is much larger than that of isolated metal particles. For superconducting particles, the theory predicts the absorption at frequencies above the gap frequency to be larger than in normal state, in agreement with experiment.