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