N. R. Dilley

Heavy fermion behaviour of the cerium-filled skutterudites and

The low temperature properties of polycrystalline samples of the filled skutterudites and , as well as the unfilled skutterudites and , have been investigated by means of electrical resistivity, specific heat and magnetic susceptibility measurements. The resistivity of 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 and 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. These quantities yield a Wilson ratio of order unity, which indicates that they both correspond to the properties of itinerant electrons. These transport, magnetic and thermodynamic properties suggest a moderately heavy fermion ground state in and .

Evidence for a Common Physical Description of Non-Fermi-Liquid Behavior in Chemically Substituted f -Electron Systems

The non-Fermi-liquid (NFL) behavior observed in the low temperature specific heat C(T) and magnetic susceptibility {chi}(T) of many chemically substituted f -electron systems is analyzed within the context of a recently developed theory based on Griffiths{close_quote} singularities. Measurements of C(T) and {chi}(T) in the systems Th{sub 1{minus}x}U {sub x}Pd{sub 2}Al{sub 3} , Y{sub 1{minus}x}U {sub x}Pd{sub 3} , and UCu{sub 5{minus}x }M{sub x} (M=Pd,thinspPt ) are found to be consistent with C(T)/T{proportional_to}{chi}(T){proportional_to}T{sup {minus}1+{lambda}} predicted by this model with {lambda}{lt}1 in the NFL regime. These results suggest that the NFL properties observed in a wide variety of disordered f -electron systems can be described within the context of a common physical picture. {copyright} {ital 1998} {ital The American Physical Society}

Thermoelectric properties of chemically substituted skutterudites YbyCo4SnxSb12−x

We report the results of a study of thermoelectric properties of chemically substituted quasiternary materials related to the recently discovered filled skutterudite compound YbFe4Sb12. The study explored partial filling at the Yb site as well as chemical doping with Sn at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system YbyCo4SnxSb12−x. Our measurements of these physical quantities from room temperature down to T=10 K indicate that, in our study, only the alloy Yb0.44Co4Sb12 possessed thermoelectric properties that are improved over the parent compound YbFe4Sb12, attaining a value of ZT=0.1 at T=300 K.

Electronic and magnetic investigation of the filled skutterudite compound CeRu4Sb12

We synthesized the filled skutterudite CeRu4Sb12 in single-crystal form using a molten-metal-flux technique with Sb flux. The specific heat and magnetic susceptibility of CeRu4Sb12 are well described by a logarithmic divergence or a power law in temperature indicating non-Fermi-liquid behaviour at low temperatures. The electrical resistivity is sample dependent with some specimens exhibiting non-Fermi-liquid behaviour below T~5 K in which ρ∝Tn with n~1.4. The application of magnetic fields up to H = 80 kOe does not significantly change the non-Fermi-liquid ground state.

Optical properties of MFe4P12 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.

Pressure dependence of the electrical resistivity of the filled skutterudites LnFe4Sb12 (Ln = Ce, Yb)

We report the pressure dependence of the electrical resistivity of two filled skutterudite compounds YbFe4 Sb12 and CeFe4 Sb12 from 1.2 K up to room temperature and under applied pressures up to 17 kbar. Both YbFe4 Sb12 and CeFe4 Sb12 show a rapid decrease of at ~50 K and ~100 K, respectively, which is caused by the onset of coherent scattering of electrons from the rare-earth sublattice. The resistivity curves can be collapsed onto the ambient pressure curve using a single scaling temperature T 0 , indicating that a single characteristic energy dominates the transport properties. For CeFe4 Sb12 , T 0 is found to increase with pressure, while the opposite behaviour is observed for YbFe4 Sb12 . The pressure dependence of as well as the change of T 0 are similar to the findings for a number of other intermediate-valence and heavy-fermion systems.

Atomic Displacement Parameters: A Useful Tool in the Search for New Thermoelectric Materials?

The atomic displacement parameters (ADPs) measure the mean-square displacement amplitude of an atom about its equilibrium position in a crystal. It is demonstrated that the ADPs can be used to identify crystalline solids with unusually low lattice thermal conductivties. A low lattice thermal conductivity is essential in the design of thermoelectric materials with improved efficiencies.The atomic displacement parameters (ADPs) have been measured using powder neutron diffraction as a function of temperature for several clathrate-like compounds (R x Co 4-y Fe y Sb 12 , where R= La, Ce, Yb or TI, x=0.22, 0.8, 1, y=0, 1;Tl 2 SnTe 5 and Tl 2 GeTe 5 ). The ADP data show that in each of the compounds one of the atoms is weakly bound and “rattles” within its atomic cage. This atomic “rattling” severely reduces the ability of these crystals to conduct heat and in some cases the lattice thermal conductivity approaches the theoretical minimum value. In many clathrate-like compounds, the ADP can also be used to estimate the Einstein frequency of the “rattler”, and to predict the existence of localized vibrational modes.

The origin of the second relaxation process in the [Mn12O12(O2CR)16(H2O)4] single-molecule magnets: ‘Jahn–Teller isomerism’ in the [Mn12O12] core

The origin of the second, faster relaxation process in Mn12 molecules has been identified as a different relative orientation of the Jahn–Teller elongation axes of the MnIII ions, which we have termed ‘Jahn–Teller isomerism’.

Effect of stoichiometry and alloying on the peak effect in CeRu2

Abstract We report the effects of variation of the CeRu stoichiometry and substitutions of Co for Ru and Nd for Ce in polycrystalline samples of the type II superconductor CeRu 2 . A magnetic study of the normal and superconducting state properties of these compounds was made, including the ‘peak effect’ (PE) enhancement of the critical current density J c that occurs between a field H i and the upper critical field H c2 in the superconducting mixed state. The width of the PE region ( H c2 − H i )/ H c2 as well as the magnitude of the pinning are correlated with the supeconduting critical temperature in the substituted alloys, but are not sensitive to material parameters such as the electronic mean free path. These results suggest that the PE in CeRu 2 is of conventional origin, instead of arising from an exotic mixed state such as the spatially nonuniform Fulde-Ferrell-Larkin-Ovchinnikov state. The superconducting H  T phase diagrams for the alloys in the substitution study are also presented.

Thermoelectric properties of chemically substituted skutterudites Yb{sub y}Co{sub 4}Sn{sub x}Sb{sub 12-x}

We report the results of a study of thermoelectric properties of chemically substituted quasiternary materials related to the recently discovered filled skutterudite compound YbFe4Sb12. The study explored partial filling at the Yb site as well as chemical doping with Sn at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system YbyCo4SnxSb12−x. Our measurements of these physical quantities from room temperature down to T=10 K indicate that, in our study, only the alloy Yb0.44Co4Sb12 possessed thermoelectric properties that are improved over the parent compound YbFe4Sb12, attaining a value of ZT=0.1 at T=300 K.