Inversion symmetric vs. asymmetric excitations and the low-temperature universal properties of Ar:N2 and Ar:N2: CO glasses

The bias energies of various two- level systems ( TLSs) and their strengths of interactions with the strain are calculated for Ar:N-2 glass. Unlike the case in KBr: CN, a distinct class of TLSs having weak interaction with the strain and untypically small bias energies is not found. The addition of CO molecules introduces CO flips which form such a class of weakly interacting TLSs, albeit at much lower coupling than that at which they are typically observed in solids. We conclude that because of the absence of a distinct class of weakly interacting TLSs, Ar:N-2 is a non- universal glass, the first such system in three dimensions and in ambient pressure. Our results further suggest that Ar:N-2:CO may show universal properties, but at temperatures lower than approximate to 0.1K, much smaller than the typical temperature approximate to 3K associated with universality, because of the untypical softness of this system. Our results thus shed light on two long- standing questions regarding the low- temperature properties of glasses: the necessary and sufficient conditions for quantitative universality of phonon attenuation, and what dictates the energy scale of approximate to 3 K below which universality is typically observed. Copyright (C) EPLA, 2015