Optical Properties of MFe_4P_12 filled skutterudites

Infrared reflectance spectroscopy measurements were made on four members of the MFe_4P_12 family of filled skutterudites, with M=La, Th, Ce and U. In progressing from M=La to U the system undergoes a metal-insulator transition. It is shown that, although the filling atom induces such dramatic changes in the transport properties of the system, it has only a small effect on lattice dynamics. We discuss this property of the compounds in the context of their possible thermoelectric applications.Comment: Manuscript in ReVTeX format, 7 figures in PostScirpt forma

Evidence for possible interaction between CDW and magnetic vortices in 2H-NbSe2 : comparison with 2H-NbS2

We have performed vibrating reed (f~1 kHz) and ac susceptibility (f~800 kHz, i.e. RF penetration depth) measurements on superconducting 2H-NbSe2 and 2H-NbS2 in magnetic fields. For NbSe2, a low temperature dissipation peak is correlated with a dip in vibrating reed resonant frequency and peak in ac susceptibility, signifying softening of the flux line lattice ; these features are not observed in NbS2. We suggest that the softening of the FLL is due to an interaction with the CDW, which does not occur in NbS2

/sup 57/Fe Mossbauer spectroscopy of U/sub 6/Fe

Samples of U/sub 6/Fe were prepared by arc melting U and enriched /sup 57/Fe. The samples display superconducting transitions which are sensitive to long-term exposure to air. Room temperature spectra of U/sub 6/Fe typically display a majority site A (quadrupole splitting ..delta.. = 0.749(3) mm/s, isomer shift delta = 0.482(6) mm/s relative to Cu) and a minority site B (..delta.. = 0.55(3) mm/s, sigma = 0.12(3) mm/s) with the site intensity ratio (I/sub B//I/sub A/) typically 0.05 to 0.10. The I/sub B//I/sub A/ intensity ratio increases upon air exposure of the samples. We have studied Moessbauer spectra as a function of temperature T in the range 15/sup 0/K less than or equal to T less than or equal to 310/sup 0/K paying particular attention to the T-variation of the isomer-shift and recoil-free-fraction of the majority site A. Results indicate a consistent softening of lattice vibrations of U/sub 6/Fe at T approx. = 170/sup 0/K