Inversion symmetric vs. asymmetric excitations and the low-temperature universal properties of Ar:N ₂ and Ar:N ₂ :CO glasses

Abstract

The bias energies of various two-level systems (TLSs) and their strengths of interactions with the strain are calculated for Ar:N2 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: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 the 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 the 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 is typically observed.

Inexpensive discrete atomistic model technique for studying excitations on infinite disordered media: The case of orientational glass ArN2

“…We applied the developed technique for an analysis of the coupling on an Ar:N 2 mixture, as it is a physically intriguing case of disordered lattice where ample experimental information is available. A discussion about the physical properties of this system, which employs the method presented here, is performed in .…”

“…We can see that we have over 700 possible TLSs for each case on this range and is enough to be considered as a statistically representative sample even for a low density study. The results for higher densities and their physical meaning, which are outside the scope of this paper, are further discussed in .…”

“…Displaying only weak van der Waals forces and being an intriguing system in the context of universality of low‐temperature properties of disordered systems, Ar:N 2 is particularly well‐suited for exemplifying our approach, which is extensible to many systems of different nature. The present work deals mainly with the new methodology, while the physical consequences of the results we obtain are presented elsewhere .…”

Anomalous low-energy properties in amorphous solids and the interplay of electric and elastic interactions of tunneling two-level systems

“…First considering only elastic interactions, the Two-TLS model divides TLSs into two groups, with bimodal distribution of their interaction strengths with the strain, denoted by γ w for the weakly interacting τ -TLSs, which correspond to the abundant TLSs at low energies, and by γ s for the other defects, where g ≡ γ w /γ s ≈ 0.02 [13,[44][45][46][47][48]. The Hamiltonian 2 is then derived as the low-energy effective Hamiltonian of the system (see Ref.…”

“…The Two-TLS model was microscopically derived [13] and thoroughly validated [44][45][46][47][48] for disordered lattices which share the same universal low temperature phenomena with amorphous solids [2]. Here we assume the generalized validity of the Two-TLS model in describing TLSs in amorphous solids.…”