| The low temperature properties of polycrystalline samples of the filled skutterudites {dollar}rm CeFesb4Sbsb{lcub}12{rcub}{dollar}, {dollar}rm Cesb{lcub}0.9{rcub}Fesb3CoSbsb{lcub}l2{rcub}{dollar} and {dollar}rm Lasb{lcub}0.9{rcub}Fesb3CoSbsb{lcub}12{rcub}{dollar} as well as the unfilled skutterudites {dollar}rm Fesb{lcub}0.05{rcub}Cosb{lcub}0.95{rcub}Sbsb3{dollar} and CoSb{dollar}sb3{dollar} have been investigated by means of electrical resistivity {dollar}rho,{dollar} specific heat {dollar}rm Csb{lcub}p{rcub}{dollar} and magnetic susceptibility {dollar}chi{dollar} measurements. The resistivity of {dollar}rm CeFesb4Sbsb{lcub}12{rcub}{dollar} exhibits a rather abrupt drop-off with decreasing temperature near 100 K; this drop-off temperature increases with increasing applied hydrostatic pressure, which is reminiscent of the onset of coherence in so-called Kondo lattice materials. The compounds {dollar}rm CeFesb4Sbsb{lcub}12{rcub}{dollar} and {dollar}rm Cesb{lcub}0.9{rcub}Fesb3CoSbsb{lcub}12{rcub}{dollar} exhibit values of the electronic specific heat coefficient and Pauli susceptibility at low temperature which are enhanced over those of the lanthanum-filled and unfilled skutterudites, and are indicative of a moderately heavy fermion ground state.; Low temperature measurements of {dollar}rmrho, Csb{lcub}p{rcub},{dollar} and {dollar}chi{dollar} of high purity samples of {dollar}rm Msb{lcub}1-x{rcub}Usb{lcub}x{rcub}Pdsb3{dollar} (M = Sc (x {dollar}le{dollar} 0.3), Y (x {dollar}le{dollar} 0.2)) presented herein show that the temperature dependences of these properties appear to be at odds with the predictions of Fermi liquid theory: {dollar}rm Csb{lcub}p{rcub}/T propto -ln(T), chipropto 1{lcub}-{rcub}Tsp{lcub}1/2{rcub},{dollar} (or {dollar}rm Csb{lcub}p{rcub}T proptochipropto Tsp{lcub}lambda-1{rcub}(lambda < 1)){dollar}, and {dollar}rho propto -T.{dollar} Scanning electron microscopy (SEM) and x-ray diffraction (XRD) measurements are presented, which were performed in order to characterize the nature of the metallurgical disorder in this system. Measurements of the ac magnetization, magnetic relaxation, {dollar}rho{dollar} and {dollar}rm Csb{lcub}p{rcub}{dollar} of samples with {dollar}rm 0.25 le x le 0.55{dollar} reveal the onset of spin glass and antiferromagnetic ordering at low temperature. As a comparison, measurements of the physical properties of the related systems {dollar}rm Ysb{lcub}0.9-y{rcub}Thsb{lcub}y{rcub}Usb{lcub}0.1{rcub}Pdsb3, Ysb{lcub}0.9-y{rcub}Lasb{lcub}y{rcub}Usb{lcub}0.1{rcub}Pdsb3, Lasb{lcub}1-x{rcub}Usb{lcub}x{rcub}Pd3,{dollar} and {dollar}rm Ysb{lcub}0.55{rcub}Thsb{lcub}0.45-x{rcub}Usb{lcub}x{rcub}Pdsb3{dollar} are also presented. The results of all of these measurements are discussed as they relate to several existing microscopic models which have been put forth to explain the observed NFL behavior in this system. These models include: single-ion Kondo scaling; a recently developed theory based on Griffiths phase singularities; fluctuations of an order parameter above a second order phase transition at T = 0; and a Kondo disorder model. |
