Unusual low-energy phonon dynamics in the negative thermal expansion compound ZrW2O8.

An infrared study of the phonon spectra of ZrW2O8 as a function of temperature which includes the low-energy (2-10 meV) region relevant to negative thermal expansion is reported and discussed in the context of specific heat and neutron density of state results. The prevalence of infrared active phonons at low energy and their observed temperature dependence are highly unusual and indicative of exotic low-energy lattice dynamics. Eigenvector calculations indicate a mixing of librational and translational motion within each low-frequency IR mode.

Kondo scaling in the optical response of YbIn1-xAgxCu4.

Theoretical work on Kondo systems predicts universality in the scaling of observable quantities with the Kondo temperature, T(K). Here we report infrared-frequency optical response measurements of the correlated system YbIn(1-x)AgxCu4. We observe that x-dependent variations in the frequency and strength of a low-energy excitation are related to the x-dependent Kondo temperature.