Enhanced far-infrared absorption in CePd3and YbCuSi2. Experiment

Abstract

Experimental results are presented for far-infrared optical measurements on the valence fluctuation compounds Ce${\mathrm{Pd}}_3$, Yb${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$, Ce${\mathrm{Al}}_3$, Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$, and YbCuAl at photon energies between 3 and 40 meV and temperatures between 4.2 and 150 K. The optical absorptivities of the valence-fluctuation compounds were measured using a dual-cavity technique. An improved analysis of the dual-cavity technique was developed so that an absolute measurement of the sample absorptivity could be extracted from the data. The absorptivities of Ce${\mathrm{Pd}}_3$ and Yb${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ at helium temperature show a broad anomaly near 20 meV. No such feature is seen in the isostructural integral valent-analog compounds Y${\mathrm{Pd}}_3$, Dy${\mathrm{Pd}}_3$, or Lu${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$. The absorptivity is also strongly temperature dependent, decreasing in magnitude with increasing temperature up to 150 K, where the absorptivity has a Drude-type frequency dependence. The anomaly in Ce${\mathrm{Pd}}_3$ is virtually independent of whether the optical surface is prepared by mechanical polishing or by chemical etching. Ce${\mathrm{Pd}}_3$ also has sharp absorption features at 14.7 and 21.6 meV. Their positions and widths are independent of temperature and applied magnetic field although the magnitude of the feature decreases with increasing temperature, vanishing by 150 K. Both of these features have been identified with $q=0\mathrm{}$ optic phonons. No anomalies have been seen in the absorptivities of Ce${\mathrm{Al}}_3$, Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$, or YbCuAl between 3 and 40 meV. The characteristic energies of anomalies in other physical properties of these compounds, however, are an order of magnitude smaller than those in Ce${\mathrm{Pd}}_3$ and Yb${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$, so that the absence of a resonance above 3 meV in the former compounds is to be expected.