Eric James Freeman

Kondo insulating behaviour in the filled skutterudite compound CeOs4Sb12

The filled skutterudite CeOs4Sb12 and its nonmagnetic analogue LaOs4Sb12 have been synthesized in single-crystal form using the molten-metal-flux growth technique with Sb flux. Electrical resistivity measurements on CeOs4Sb12 indicate that it is a Kondo insulator with an energy gap of ΔE/kB~10 K. Lattice parameter and magnetic susceptibility measurements suggest that the Ce ions are trivalent in CeOs4Sb12. Specific heat and magnetization measurements reveal an enhanced value of the electronic specific heat coefficient γ~92 mJ mol-1 K-2 and Pauli susceptibility, reminiscent of a moderately heavy-fermion material.

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}

Single-ion scaling of the low-temperature properties of f-electron materials with non-Fermi-liquid groundstates

Certain chemically substituted Ce and U compounds have low-temperature physical properties that exhibit non-Fermi-liquid (NFL) characteristics and apparently constitute a new class of strongly correlated f-electron materials. The NFL behaviour takes the form of weak power law or logarithmic divergences in the temperature dependence of the physical properties that scale with a characteristic temperature T 0 , which, in some systems, can be identified with the Kondo temperature T K . These systems have complex temperature T-chemical substituent composition x phase diagrams, which contain regions displaying the Kondo effect, NFL behaviour, spin glass freezing, magnetic order, quadrupolar order, and, sometimes, even superconductivity. Possible origins of the NFL behaviour include a multichannel Kondo effect and fluctuations of an order parameter in the vicinity of a second-order phase transition at T = 0 K. Recent experiments on the systems Y 1-x U x Pd 3 and U 1-x M x Pd 2 Al 3 (M = Th, Y) are reviewed. In the Y 1-x U x Pd 3 and U 1-x Th x Pd 2 Al 3 systems, the low-temperature physical properties in the NFL regime scale with the U concentration and T K , suggesting that single-ion effects are responsible for the NFL behaviour.

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.

High-pressure study of ferromagnetic UxM1-x (M = Pt, Ir) compounds

Measurements of the electrical resistivity under pressure to 20 kbar and the magnetization and specific heat at zero pressure were performed on the itinerant ferromagnetic compounds UxPt1-x (0.50≤x≤0.54) and UIr. The UxPt1-x materials order magnetically at TC1~18 K and TC2~27 K and the ordering temperatures exhibit a small pressure dependence. Above 10 kbar, only the lower-temperature phase is present. The Curie temperature of UIr, on the other hand, decreases from 47 K at ambient pressure to 27 K by 19 kbar, and the critical pressure PC for the complete suppression of ferromagnetism is estimated to be ~40 kbar. UPt and UIr obey the Kadowaki-Woods relation and have Wilson ratios of order unity and could be classified as moderately heavy-fermion ferromagnetic materials.

Strongly correlated electron behavior in ROs4Sb12 (R=Ce, Pr) filled skutterudites

The physical properties of single crystals of the filled skutterudites CeOs 4 Sb 12 and PrOs 4 Sb 12 have been investigated. Transport and thermodynamic measurements on CeOs 4 Sb 12 indicate that it is a small band-gap semiconductor with an energy gap of ∼10 K, whereas PrOs 4 Sb 12 is a superconductor with a T C of 1.85 K.

Non-Fermi liquid regimes in the low-temperature phase diagrams of the systems U1−xMxPd2Al3 (M=Y,Th)

Abstract Temperature–composition (T–x) phase diagrams and non-Fermi liquid (NFL) characteristics in the electrical resistivity, specific heat, and magnetic susceptibility at low temperatures for the U1−xMxPd2Al3 (M=Y,Th) systems are described. The T–x phase diagrams, the NFL characteristics, and the underlying mechanisms for the NFL behavior exhibit distinct differences for M=Y3+ and Th4+, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.

Comparison of magnetic and NFL behavior in U1−xMxPd2Al3 (M=La, Y, Th)☆

Abstract The heavy fermion antiferromagnetic superconductor UPd 2 Al 3 displays a number of interesting magnetic and non-Fermi liquid (NFL) ground states when the uranium site is partially occupied by Y, La, or Th. Whereas the magnetic behavior in these systems is determined by valence of the substituent atom, the temperature dependence of the electrical resistivity at low temperatures in the NFL regime correlates with the size of the substituent atom. The phase diagrams, electrical resistivity data, and the lattice parameters of the three systems are compared and contrasted.

Strongly correlated electron behaviour in the compound CeRhSn

Abstract Electrical resistivity ρ, magnetic susceptibility χ, specific heat C, X-ray photoelectron spectroscopy (XPS) and X-ray and neutron diffraction measurements were made on the compound CeRhSn. The measurements reveal an enhancement of the electronic specific heat coefficient γ = C(T)/T ≈ 160 mJ mol−1 K−2 at about 1 K and non-Fermi-liquid-like temperature dependences ρ(T) α T 0.75 and χ(T) α T −0.5 at low temperatures. At high temperatures, the physical properties indicate Kondo-intermediate-valence behaviour. The XPS measurements are consistent with a Ce valence of 3.07. The neutron diffraction measurements reveal anomalies in the planar Ce-Rh and Ce-Ce bond distances in the vicinity of 100 K which are apparently associated with the Ce valence instability in CeRhSn.