Mechanical Resonance Dispersion in Single Crystals

Single crystals of sodium chloride, copper sulfate, and Rochelle salt exhibit sharp resonance dispersions in dynamic shear compliance in the range from 100 to 5000 cps. The resonances are similar to those previously reported for polycrystalline metals, crystalline polymers, fused quartz, and single crystals of quartz. The data can be analyzed and fitted closely by means of a generalized stress-strain relation involving a linear combination of the strain and its first and second time derivatives. Inclusion of the third derivative leads to an antidissipative term which can be used to account qualitatively for a negative absorption or induced emission observed over a narrow frequency range in sodium chloride. The negative absorption seems to be connected with static clamping stresses applied to the sample, decreases slowly with time, and can be reactivated by restressing. The results in general are consistent with a tentative explanation which assumes the resonances correspond to exchange frequencies for energy passing back and forth among only a few of the available modes of vibration of the crystal lattice.