The anisotropic glass-like properties of disordered crystals
The low temperature acoustic and thermal properties of amorphous, glassy materials are remarkably similar. All these properties are described theoretically with reasonable quantitative accuracy by assuming that the amorphous solid contains dynamical defects that can be described at low temperatures as an ensemble of two-level systems (TLS), but the deep nature of these TLSs is not clarified yet. Moreover, glassy properties were found also in disordered crystals, quasicrystals, and even perfect crystals with a large number of atoms in the unit cell. In crystals, the glassy properties are not universal, like in amorphous materials, and also exhibit anisotropy. Recently it was proposed a model for the interaction of two-level systems with arbitrary strain fields (Phys. Rev. B 75, 64202, 2007), which was used to calculate the thermal properties of nanoscopic membranes at low temperatures. The model is also suitable for the description of anisotropic crystals. We describe here the results of the calculation of anisotropic glass-like properties in crystals of various lattice symmetries.
Anisotropic interaction of two-level systems with acoustic waves in disordered cubic crystals
We apply the model introduced in Phys. Rev. B 75, 064202 (2007), cond-mat/0610469, to calculate the anisotropy effect in the interaction of two level systems with phonons in disordered crystals. We particularize our calculations to cubic crystals and compare them with the available experimental data to extract the parameters of the model. With these parameters we calculate the interaction of the dynamical defects in the disordered crystal with phonons (or sound waves) propagating along other crystalographic directions, providing in this way a method to investigate if the anisotropy comes from the two-level systems being preferably oriented in a certain direction or solely from the lattice anisotropy with the two-level systems being isotropically oriented.
GENERATION OF LOW-ENERGY EXCITATIONS IN SILICON
In order to understand the low-energy vibrational excitations common to amorphous solids, we have studied their evolution in ion-implanted crystalline silicon by measuring internal friction and heat conduction. The spectral density of these low-energy excitations evolves with increasing dose exactly towards that observed in the amorphous phase. More importantly, this evolution is unrelated to that of the amorphicity. We conclude that the defects in the crystal should be used to model the excitations in the amorphous silicon, rather than the amorphous structure itself. [S0031-9007(98)07337-2]