V. S. Zapf

Superconductivity and crystalline electric field effects in the filled skutterudite series Pr ( Os 1 − x Ru x ) 4 Sb 12

X-ray powder-diffraction, magnetic-susceptibility Χ (T), and electrical resistivity ρ(T) measurements were made on single crystals of the filled skutterudite series Pr(Os 1 - x Ru x ) 4 Sb 1 2 . One end of the series (x=0) isa heavy fermion superconductor with a superconducting critical temperature T c =1.85 K, while the other end (x= I ) is a conventional superconductor with T c 1 K. The lattice constant a decreases approximately linearly with increasing Ru concentration x. As Ru (Os) is substituted for Os (Ru), T c decreases nearly linearly with substituent concentration and exhibits a minimum with a value of T c = 0.75 K at x=0.6, suggesting that the two types of superconductivity compete with one another. Crystalline electric field effects in Χ d c (T) and ρ(T) due to the splitting of the Pr 3 + ninefold degenerate Hunds rule J=4 multiplet are observed throughout the series, with the splitting between the ground state and the first excited state increasing monotonically as x increases. The fits to the Χ d c (T) and ρ(T) data are consistent with a Γ 3 doublet ground state for all values of x, although reasonable fits can be obtained for a Γ 1 ground state for x values near the end member compounds (x = 0 or x = 1).

Strongly correlated electron phenomena in filled skutterudite compounds

We report on a search for intermediate valence and heavy fermion behavior in filled skutterudite compounds. The compound YbFe4Sb12 was synthesized and, to our knowledge, is the first example of a filled skutterudite of a heavy lanthanide. Transport, magnetic, and thermal measurements on CeFe4Sb12 and YbFe4Sb12 indicate that these materials have a moderately heavy fermion ground state.

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.

Superconductivity and the high-field ordered phase in the heavy-fermion compound PrOs4Sb12

Superconductivity is observed in the filled skutterudite compound PrOs4Sb12 below a critical temperature Tc = 1.85 K and appears to develop out of a nonmagnetic heavy Fermi liquid with an effective mass m ≈ 50 m e, where me is the free electron mass. Features associated with a cubic crystalline electric field are present in magnetic susceptibility, specific heat, electrical resistivity, and inelastic neutron scattering measurements, yielding a Pr3+ energy level scheme consisting of a Γ3 nonmagnetic doublet ground state, a low-lying Γ5 triplet excited state at ~10 K, and Γ4 triplet and Γ1 singlet excited states at much higher temperatures. Measurements also indicate that the superconducting state is unconventional and consists of two distinct superconducting phases. At high fields and low temperatures, an ordered phase of magnetic or quadrupolar origin is observed, suggesting that the superconductivity may occur in the vicinity of a magnetic or quadrupolar quantum critical point.

The high field ordered phase and upper critical field of the filled skutterudite system Pr(Os1-xRux)4Sb12

To study the possible competition between unconventional and Bardeen–Cooper–Schrieffer superconductivity in the filled skutterudites Pr(Os1−xRux)4Sb12, the evolution of superconductivity and the high field ordered phase in single-crystal specimens has been investigated by means of electrical resistivity measurements in magnetic fields up to 18 T. Whereas the upper critical field Hc2(T) curves have conventional shapes for x<0.4, the Hc2(T) curves are nearly linear for . For all x, Hc2(0) matches the calculated value of the orbital critical field. Features in the electrical resistivity associated with the high field ordered phase, observed clearly for PrOs4Sb12, weaken with increasing x and vanish for .

Non-Fermi-liquid behaviour in CeRhSn

Measurements of the temperature T dependence of the electrical resistivity ρ, specific heat C and magnetic susceptibility χ have been made on the compound CeRhSn from about 10 K to temperatures as low as 0.1 K. Over certain temperature intervals, these properties are observed to have non-Fermi-liquid (NFL) temperature T dependences: with and . While the NFL behaviour can be described by the Griffiths–McCoy model, it may also be related to a weak magnetic transition at 6.2 K that depends on atomic disorder. The resistively determined upper critical field versus temperature curve of the isostructural reference compound without f electrons, LaRhSn, is also reported.

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.

f-Electron materials: a reservoir of novel electronic states and phenomena

Abstract f-Electron materials exhibit a rich variety of strongly correlated electron states and phenomena. Two examples, non-Fermi liquid (NFL) behavior at low temperatures and superconductivity in the vicinity of quantum critical points, are discussed in this article. Non-Fermi liquids are characterized by weak power law or logarithmic temperature dependences of the electrical resistivity, specific heat, and magnetic susceptibility at low temperatures, as well as a dynamical susceptibility whose imaginary part scales with frequency ω divided by T. These NFL characteristics were first established in chemically substituted f-electron materials and later observed in stoichiometric f-electron compounds, suggesting that the NFL behavior is a general phenomenon whose underlying mechanism(s) is (are) not primarily associated with atomic disorder. In certain antiferromagnetic and ferromagnetic f-electron intermetallic compounds, superconductivity has been observed in the vicinity of the critical pressure Pc where the Neel or Curie temperature vanishes. We discuss recent experiments on the ferromagnetic compound UGe2 and suggest that superconductivity, which is found to occur within the ferromagnetic phase below Pc in this compound, may have s-wave character and coexist in a spatially inhomogeneous manner with the ferromagnetism.

Heavy fermion superconductivity in the filled skutterudite compound PrOs4Sb12

Abstract The filled skutterudite compound PrOs 4 Sb 12 has been found to exhibit superconductivity with a critical temperature T c =1.85 K that develops out of a heavy Fermi liquid with an effective mass m ∗ ≈50m e . The current experimental situation regarding the heavy fermion state, the superconducting state, and a high field, low temperature phase that is apparently associated with magnetic or quadrupolar order in PrOs 4 Sb 12 is briefly reviewed herein.

Non-Fermi liquid behavior and quantum criticality in Sc1-xUxPd3Sc1-xUxPd3 and URu2-xRexSi2URu2-xRexSi2

The compounds Sc1-xUxPd3Sc1-xUxPd3 and URu2-xRexSi2URu2-xRexSi2 exhibit non-Fermi liquid behavior in the vicinity of zero-temperature magnetic phase transitions accessed by chemical substitution. Notably, the quantum phase transitions in these pseudobinary systems are associated with spin glass and ferromagnetic ordering, respectively. Recent studies have uncovered new details of the unconventional physical phenomena occurring in these materials.