Superconducting and antiferroquadrupolar (AFQ) transitions in a Pr-based compoundPrRh 2 Zn 20 have been found to occur simultaneously at T c =T Q =0.06 K. The superconducting transition manifests itself by zero resistance and large diamagnetic susceptibility. The specific heat exhibits a Schottky anomaly peaking at 14 K and magnetization curves measured at 2 K show anisotropic behaviors. The analysis of these data indicates that the crystalline electric field (CEF) ground state of the trivalent Pr ion is the non-Kramers Γ 3 doublet with the quadrupolar degrees of freedom. A sharp peak in the specific heat at 0.06 K has been attributed not to the superconducting transition but to the AFQ transition because the ordering temperature T Q decreases in B || [100] but increases in B|| [110] and B || [111] with increasing B up to 6 T. This anisotropic behavior of T Q (B) can be well explained by a two-sublattice mean-field calculation, which corroborates the AFQ ordered state below T Q . The entropy release at T Q is only 10% of Rln2 expected for the Γ 3 doublet, suggesting possible interplay between the quadrupolar degrees of freedom and the superconductivity.

Orbital degrees of freedom in condensed matters could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f -electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focussed on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at TQ = 0.11 K. We observed non-Fermi liquid (NFL) behaviors emerging in the electrical resistivity ρ and the 4f contribution to the specific heat, C 4f , in the paramagnetic state at T > TQ. Moreover, in magnetic fields B ≤ 6 T, all data set of ρ(T ) and C 4f (T ) are well scaled with characteristic temperatures T0's. This is the first observation of the NFL state in the nonmagnetic quadrupole-active system, whose origin is intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons with an energy scale of kBT0. At T ≤0.13 K, ρ(T ) and C 4f (T ) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.PACS numbers:

Electrical resistivity ρ(T) and specific heat C(T) measurements have been made on the diluted 4f^{2} system Y(Pr)Ir_{2}Zn_{20}. Both data of ρ and magnetic specific heat C_{m} per Pr ion are well scaled as a function of T/T_{0}, where T_{0} is a characteristic temperature of non-Fermi-liquid (NFL) behaviors. Furthermore, the temperature dependences of ρ and C_{m}/T agree with the NFL behaviors predicted by the two-channel Kondo model for the strong coupling limit. Therefore, we infer that the observed NFL behaviors result from the single-site quadrupole Kondo effect due to the hybridization of the 4f^{2} states with multichannel conduction electrons.

The caged compound LaRu 2 Zn 20 exhibits a structural transition at T S =150 K, whose driving mechanism remains elusive. We have investigated atomic dynamics by the measurements of specific heat C and inelastic X-ray scattering (IXS). The lattice part of the specific heat C lat divided by

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