1. W.A. Phillips, Amorphous Solids: Low-Temperature Properties (Springer, Berlin, 1981)
2. R.O. Pohl, X. Liu, E. Thompson, Rev. Mod. Phys. 74, 991 (2002)
3. U. Buchenau, G. D’Angelo, G. Carini, X. Liu, M.A. Ramos (2020). arXiv:2012.10139
4. P. Esquinazi, R. König, Influence of Tunneling Systems on the Acoustic Properties of Disordered Solids (Springer, Berlin, 1998), pp. 145–222
5. W. Schirmacher, G. Ruocco, T. Scopigno, Phys. Rev. Lett. 98, 025501 (2007)