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We have measured the heat conduction between 0.05 K and 1.0 K of high puritysilicon wafers carrying on their polished faces a variety of thin dielectricfilms and polycrystalline thin metallic films. Using a Monte Carlo simulationto analyze the conduction measurements, we have determined the phonon mean freepath within the films, and found all of them to be much shorter even than intypical bulk amorphous solids, with no exceptions. We have also measured theinternal friction of these films below 10 K and found, however, their internalfriction at low temperatures strikingly close to that of amorphous solids, bothin magnitude and in their temperature independence, with the exception of theMBE Si and alloy Al 5056, whose internal friction is even much smaller thanthat of amorphous solids. The internal friction results indicate the phononscattering in these thin films is the same as, or even much less stronger than,in other amorphous solids, according to the Tunneling Model. Thus, we concludethat the heat conduction measurements do not support the picture that thelattice vibrations of these films are glasslike, as had been surmised earlierfor thin metallic films, on the basis of low temperature internal frictionmeasurements alone [Phys. Rev. B 59, 11767 (1999)]. At the least, the filmsmust contain additional scattering centers which lead to the very small phononmean free path. Most remarkably, the MBE Si shows the same strong scattering ofthermal phonons as do the other films, while having the negligible internalfriction expected for a perfect film. The disorder causing the strongscattering of the thermal phonons in this film is completely unknown. Thenon-glasslike phonon scattering phenomena observed here in thin dielectric andmetallic films deserve further investigations.Comment: 11 pages, 12 figures in REVTeX. Submitted to PR

Phonon scattering and internal friction in dielectric and metallic films at low temperatures