Experiments about TLSs' interactions
1973 Nonlinear Phonon Propagation in Fused Silica below 1 K
Brage Golding, J. E. Graebner, B. I. Halperin, and R. J. Schutz
differences between dielectric and acoustic properties - electric versus elastic dipoles
Hunklinger 1977 + nonlinearity
paradox ## 1987 Spectral diffusion, phonon echoes, and saturation recovery in glasses at low temperatures J. L. Black and B.I. Halperin