AMORPHOUS SOLID WITHOUT LOW-ENERGY EXCITATIONS

We have measured the low temperature internal friction (Q(-1)) of amor phous silicon (a-Si) films. e-beam evaporation or Si-28(+) implantatio n leads to the temperature-independent Q(0)(-1) plateau common to all amorphous solids. For hydrogenated amorphous silicon with 1 at. %H pro duced by hot wire chemical vapor deposition, however, Q(0)(-1) is over 200 times smaller than for e-beam a-Si. This is the first observation of an amorphous solid without any significant low energy excitations. It offers the opportunity to study amorphous solids containing contro lled densities of tunneling defects, and thus to explore their nature.