Far infrared properties of disordered solids

Far-infrared (far-ir) spectroscopy is used to probe the low energy excitations that occur when impurities are added to a pure crystal. The fluorite mixed crystals, C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} doped with H{dollar}sb2{dollar}O, and ZnS doped with diamond, are considered. The first two systems are examined by means of temperature dependent far-ir absorption measurements while the reststrahlen spectra of ZnS-diamond composites are analyzed using room temperature reflectivity measurements.; Low-temperature absorption spectra obtained for a number of high concentration fluorite mixed crystal systems of the form (MF{dollar}sb2)sb{lcub}rm 1-x{rcub}{dollar}:(RF{dollar}sb3)sb{lcub}rm x{rcub}{dollar}, where M is an alkaline earth and R is a rare earth, reveal the presence of a broad glass-like distribution of two-level-systems (TLS's) for frequencies below 10 cm{dollar}sp{lcub}-1{rcub}{dollar}. Results at higher frequencies for both mixed crystals and glasses indicate the existence of previously unknown excited state transitions to levels above the ground-state TLS manifold.; The low temperature far-ir absorption spectra of lightly doped fluorite mixed crystals are shown to contain a number of sharp ir-active absorption bands. Almost all of these bands are highly temperature dependent and in most cases the absorption strength is greatest at 1.4 K. The behavior of some of the bands is consistent with a tunneling excitation that is shifted to high frequency by the presence of large internal electric fields.; The systems of C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} doped with H{dollar}sb2{dollar}O contain numerous low-temperature impurity bands. A large isotope shift is observed for some of the bands when H{dollar}sb2{dollar}O is replaced by D{dollar}sb2{dollar}O, consistent with rotational or librational behavior. Both C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} doped samples show an unusual magnetic field behavior in the presence of O{dollar}sb2{dollar} which could be indicative of a low-temperature field induced change from ortho to para H{dollar}sb2{dollar}O.; A Kramers-Kronig analysis of the room temperature reflectivity spectra of ZnS-diamond composites provides the first experimental verification of the generalized Lyddane-Sachs-Teller relation. The results are also shown to be compatible with those obtained within the Bruggerman effective-medium approximation.