multiple theories of low-T universalities - Burin, Yu, etc.

Quasi-Classical theories

Turlakov 2003 Universal sound absorption in low-temperature glasses

Phenomenologically assuming a sharp decrease of shear relaxation time for large wavevector k > kξ
density modes (where kξ is of order of inverse of several interatomic distances a), I develop a general
elasto-hydrodynamic theory describing the low-energy excitations of glassy and amorphous solids

2021 Universality of Ultrasonic attenuation in amorphous systems at low temperatures - Pragya Shukla

Based on experimental evidence of ordered structure in glasses below MRO (10 → 30˚A) and its lack above, we describe a macroscopic size glass material as elastically coupled, spherical shape, generic blocks, with homogeneous dispersion interaction within each such block.

"Justification of T-TLSs"

Burin-Kagan - universal nature - broad distribution of parameters 1996 main paper
SOME COMMENTS ON THE NATURE OF UNIVERSAL PROPERTIES IN LOW–TEMPERATURE GLASSES
DAVID R. REICHMAN, PETER NEU, AND ROBERT J. SILBEY

more parameters - two types, etc

1986 Fleurov Tunnel modes and kinetic properties of glasses at low and high temperatures

Origins of double-well potentials (DWP) are discussed and two types of DWPS-Soft and rigid-are emphasised. Soft DWPS
are mainly responsible for low-temperature properties whereas rigid DWPS determine the
properties of glasses at higher temperatures

Low temperature universality in disordered solids 2009 M. Schechter, P. C. E. Stamp

We introduce a model that includes two types of two level systems (TLSs), which, based on their local symmetry, interact weakly or strongly with the phonon field

Yu-Carruzzo also explain ....

We present a novel view of the standard model of tunneling two level systems (TLSs) to explain the
puzzling universal value of a quantity, C ∼ 3 × 10−4, that characterizes phonon scattering in glasses below
1 K as reflected in thermal conductivity, ultrasonic attenuation, internal friction, and the change in sound

we have elucidated aspects implicit in the standard TLS model that include strongly interacting TLSs
[6,7,13], exponentially renormalized tunneling matrix elements [13], and a heretofore unrecognized broad distribution of TLS-phonon couplings

Frustrated Interactions and Tunneling: Two-Level Systems in Glasses 1991 S. N. Coppersmith

Typical strongly interacting defects have their tunneling suppressed, and observable tunneling occurs
only for TLS which appear to be weakly coupled because their interactions are frustrated