Phonon elastic anomalies

Elastic anomalies in actinide-based heavy-fermion systems are most pronounced for UPt3. The temperature dependences of two representative elastic modes are shown in flg. 36 (Yoshizawa et al. 1985). Unlike the case of UPdj, CF effects as a possible source do not seem very likely. The phonon dispersions of UBOjj, as measured by neutron scattering (Robinson et al. 1986), do not show strong anomalies. The elastic constant Cj2, however, is negative at low temperatures, similar to some IV compounds. Remarkably, UBejj possesses a low Debye... 

It has been emphasized (Mock et al. 1986) that besides a systematic understanding of the occurrence or absence of elastic and phonon anomalies in different IV compounds the concept introduced above allows for a first experimental estimate of charge fluctuation rates. The direct experimental investigation of these has not been feasible, unlike the magnetic relaxation rates, which have been investigated intensively by quasielastic neutron scattering (Holland-Moritz et al. 

Therefore in this review we shall focus on the electron-phonon coupling in metallic R compounds. They have shifted to the center of interest in the last ten years because of new phenomena such as valence fluctuations, Kondo lattice behavior and the formation of heavy-electron bands. The varying degree of hybridization between 4f states and conduction electron states leads to low lying electronic states whose character is very different from the localized states due to many-body effects. Consequently they lead to a rich variety of electronic and elastic or vibrational anomalies at low temperatures. 

Anomalies in the phonon spectra of UTe have also been found (Buyers and Holden 1985) and attempts were made by the authors to use the same theories described above, which essentially relate to an intermediate-valence picture. Unfortunately, as Buyers and Holden describe (see their p. 304) this does not lead to a convincing conclusion. There is, of course, no direct electronic evidence (e.g., from photoemission) that materials such as UTe exhibit valence fluctuations. We must conclude that the electron-phonon interaction is different in detail between the 4f intermediate-valence matmals and tiie actinide compounds. In both cases, however, the result is a negative Poisson ratio (which is directly related to the elastic constant C12). 

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