Strain dependence of the acoustic properties of amorphous metals below   1K: Evidence for the interaction between tunneling states
  We have conducted a thorough study of the acoustic properties between 10^-4 and 1 Kelvin for the amorphous metal Zr_x Cu_1-x (x=0.3 and x=0.4), by measuring the relative change of sound velocity dv/v and internal friction Q^-1 as a function of temperature and also of the applied strain, in both superconducting and normal state. We have found that when plotted versus the ratio of strain energy to thermal energy, all measurements display the same behavior: a crossover from a linear regime of ``independent'' tunneling systems at very low strains and/or high enough temperatures to a nonlinear regime where dv/v and Q^-1 depend on applied strain and the tunneling systems cannot be considered as independent. 
R. Koenig, M. A. Ramos, I. Usherov-Marshak, J. Arcas-Guijarro, A.   Hernando-Maneru, P. Esquinazi