Relative variation in the sound velocity in polymethyl methacrylate at intermediate low temperatures

Abstract

We have measured the relative variation in the sound velocity in amorphous polymethyl methacrylate (PMMA) from 4 to 77 K in the frequency range from 3 to 30 MHz. Our experimental results on the temperature and frequency dependence of the velocity shift at intermediate low temperatures (4–30 K) cannot be described by thermally activated motions of defect states. We also find that the temperature coefficient β= ∂( Δν/ ν 0) ∂T of the velocity shift in PMMA appears to be frequency independent in a broad frequency range.