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Single crystals of the perovskite-type 3d 1 metallic alloy system Ca 1Ϫx Sr x VO 3 were synthesized in order to investigate metallic properties near the Mott transition. The substitution of a Ca 2ϩ ion for a Sr 2ϩ ion reduces the bandwidth W due to a buckling of the V-O-V bond angle from ϳ180°for SrVO 3 to ϳ160°for CaVO 3 . Thus, the value of W can be systematically controlled without changing the number of electrons making Ca 1Ϫx Sr x VO 3 : one of the most ideal systems for studying bandwidth effects. The Sommerfeld-Wilson ratio (Ӎ2), the Kadowaki-Woods ratio ͑in the same region as heavy fermion systems͒, and a large T 2 term in the electric resistivity, even at 300 K, substantiate a large electron correlation in this system, though the effective mass, obtained by thermodynamic and magnetic measurements, shows only a systematic but moderate increase in going from SrVO 3 to CaVO 3 , in contrast to the critical enhancement expected from the Brinkmann-Rice picture. It is proposed that the metallic properties observed in this system near the Mott transition can be explained by considering the effect of a nonlocal electron correlation. ͓S0163-1829͑98͒03232-9͔ PHYSICAL REVIEW B 15 AUGUST 1998-II VOLUME 58, NUMBER 8 PRB 58 0163-1829/98/58͑8͒/4372͑12͒/$15.00 4372

We have studied superconducting and magnetic properties of RERu 4 Sb 12 prepared by an Sbflux method and found that LaRu 4 Sb 12 is superconducting below 3.58 K and PrRu 4 Sb 12 is a Van Vleck paramagnet which becomes superconducting below 1.04 K. CeRu 4 Sb 12 is a unique substance which shows intermediate-valence behavior with a characteristic broad peak around 100 K in the magnetic susceptibility and Kondo-like behavior in the electrical resistivity at high temperatures. Furthermore, non-Fermi liquid behavior is observed in the specific heat and electrical resistivity at low temperatures. NdRu 4 Sb 12 and EuRu 4 Sb 12 show a magnetic transition at 1.3 K and 3.3 K, respectively.The filled skutterudite with a general formula RET 4 Pn 12 (RE=light Rare Earth, Th and U, T=Fe, Ru and Os, Pn=P, As and Sb) crystallizes in a unique body-centered cubic structure of a space group Im3 (No. 204) of which 100 axes are not four-fold but two-fold. In this structure, the lanthanide atoms fill the voids of CoAs 3 (skutterudite) structure which consists of distorted octahedron of pnicogen atoms centered by a transition metal atom. 1-3) It was revealed that these rare earth skutterudites RET 4 Pn 12 show various interesting physical properties at low temperatures, such as superconductivity, magnetic order and semiconductivity. 4, 5) For instance, the ground state of the isostructural Th-, U-, La-, Ce-and Pr-iron phosphides is metallic, ferromagnetic semiconducting, superconducting, nonmagnetic semiconducting and antiferromagetic metallic, respectively. It is believed that the interaction between f-electrons and conduction electrons plays an important role in determining the electronic structure and the ground state. Furthermore it was recently reported that PrRu 4 P 12 exhibits a metal-insulator (MI) transition at 60 K and PrRu 4 As 12 a superconducting transition at 2.4 K. 6, 7) The of pnictogen and the relation etween MI transition and superconductivity would be interesting problems.

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