Electron Mean Free Path Independence of the Glasslike Properties of Polycrystalline Metals

Abstract
We have measured the sound velocity and attenuation of polycrystalline Cu, Al, Nb, Nb₄₈Ti₅₂, Pt, Pt₇₀Rh₃₀, Pt(1%W), Pd, Ta, NiCr alloy and amorphous PdSiCu at frequencies 0.1 kHz<v<20kHz and at temperatures 40mK<T<10K. All polycrystalline samples show acoustic properties which resemble those of dielectric glasses. Their electrical resistivity is in the range 10^-10Ωm<ο<10^-5Ωm with an estimated electron mean free path of 10^-10m<l<;10^-5m. Our results indicate no correlation between the tunneling system density of states or coupling constant and the electron mean free path, supporting earlier work on the absence of electron assisted relaxation rate. For polycrystalline metals we found that the ratio T_co/v^1/3≈(0.006+/-0.002) K/Hz^1/3 in agreement with dielectric glasses (T_co is the temperature at the sound velocity maximum).