The strengths of the interactions of various two-level systems (TLSs) with the strain are calculated for Ar:N2 glass. Unlike the case in KBr:CN, a continuous --rather than bimodal-- spectrum is obtained. The addition of CO molecules introduces CO flips as a separate class of weakly interacting TLSs, albeit at much lower coupling than are typically observed in solids. We conclude that because of the absence of the weakly interacting excitations, Ar:N2 is a non-universal glass, the first such system in three dimensions and in ambient pressure. Our results further suggest that Ar:N2:CO may show universal properties, but at temperatures lower than ≈ 0.1$ K, much smaller than typical temperature ≈ 3 K associated with universality, because of the untypical softness of this system. Our results thus shed light on two long standing questions regarding low temperature properties of glasses: the necessary and sufficient conditions for quantitative universality of phonon attenuation, and what dictates the energy scale of ≈3 K below which universality it typically observed.
|Journal||arXiv preprint arXiv:1405.2217|
|State||Published - 2014|