The Role of Electrons in the Kapitza Resistance of Lead

Abstract

In a pure metal, thermal conduction is very largely by conduction electrons so that the Kapitza conductance is determined by the rate at which energy can be transferred to these electrons from the excitations in the helium. Two routes are possible in principle. In one, energy passes first to the phonon system in the metal presumably by processes similar to those for a dielectric. It is then transferred to the electrons. In the other a more direct interaction between the electrons and the helium is envisaged. The first process introduces an additional electron-phonon contact resistance which is not of course present in a dielectric while the second provides an additional conductance. The earlier experimental investigations of these effects were made by comparing the conductances in the normal and superconducting states of a metal. Much of this work is now held to be unreliable because it is thought that surface strain induced perhaps by differential contraction reduces the thermal conductivity in the surface region. The effect should be less in thin foils and measurements on these¹ show that the conductance is somewhat larger in the superconducting state for lead, tin and indium. This suggests that the electron-phonon contact resistance is important: an earlier interpretation of these data as a size effect¹ now seems less likely in view of recent theoretical work².