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We present scanning superconducting quantum interference device microscopy data on the superconductors Sr 2 RuO 4 ͑T c = 1.5 K͒ and PrOs 4 Sb 12 ͑T c = 1.8 K͒. In both of these materials, superconductivity-related timereversal symmetry-breaking fields have been observed by muon spin rotation; our aim was to visualize the structure of these fields. However, in neither Sr 2 RuO 4 nor PrOs 4 Sb 12 do we observe spontaneous superconductivity-related magnetization. In Sr 2 RuO 4 , many experimental results have been interpreted on the basis of a p x Ϯ ip y superconducting order parameter. This order parameter is expected to give spontaneous magnetic induction at sample edges and order parameter domain walls. Supposing large domains, our data restrict domain wall and edge fields to no more than ϳ0.1% and ϳ0.2% of the expected magnitude, respectively. Alternatively, if the magnetization is of the expected order, the typical domain size is limited to ϳ30 nm for random domains or ϳ500 nm for periodic domains.

The results of inelastic neutron scattering provide a solution for the crystal field level scheme in PrOs4Sb12, in which the ground state in the cubic crystal field potential of T h symmetry is a Γ1 singlet. The conduction electron mass enhancement is consistent with inelastic exchange scattering, and we propose that inelastic quadrupolar, or aspherical Coulomb, scattering is responsible for enhancing the superconducting transition temperature. PrOs4Sb12 appears to be the first compound in which aspherical Coulomb scattering is strong enough to overcome magnetic pair-breaking and increase Tc.PACS numbers: 74.70. Tx,71.27.+a,75.10.Dg,78.70.Nx Praseodymium filled skutterudite compounds, with general formula PrT 4 M 12 , where T is one of the transition metals Fe, Ru, or Os, and M is a pnictogen (P, As, or Sb), show a remarkable variety of interesting physical phenomena, including metal-insulator transitions [1], quadrupolar heavy fermion behavior [2,3], and superconductivity [4]. In particular, PrOs 4 Sb 12 has attracted attention as a heavy fermion superconductor, in which quadrupolar fluctuations may play an important role in the pairing mechanism [5]. This proposal is based on two key observations; (i) there is a significant f-electroninduced mass enhancement of the conduction electrons, observed in specific heat, upper critical field [5,6], and de Haas-van Alphen measurements [7], and (ii) the magnetic susceptibility indicates that the crystal field ground state is non-magnetic [5].A knowledge of the crystal field ground state is essential to understanding the role of the f-electrons in the superconductivity. In analyzing the magnetic susceptibility and specific heat, Bauer et al considered two possible crystal field models [5]. In cubic symmetry, the Pr 3+ ion splits into a singlet (Γ 1 ), a non-magnetic doublet (Γ 3 ), and two magnetic triplets (Γ 4 and Γ 5 ). Crystal field models with either the Γ 1 singlet or Γ 3 doublet as ground state were both broadly consistent with the data; in both cases, the Γ 5 triplet was the lowest excited level estimated to be at less than 1 meV in energy. A Γ 3 non-Kramers doublet ground state is of particular interest as it provides the necessary conditions for quadrupolar Kondo fluctuations to be responsible for the heavy fermion behavior [8], and was favored by analyses of the entropy [9,10]. However, the alternative Γ 1 singlet ground state has also been proposed following experiments that have explored the crossover to a field-induced ordered phase [11,12,13], so this important question remains unresolved.Inelastic neutron scattering is the most direct method of determining the crystal field potential and level scheme of metallic rare earth systems. In this report, we present the results of a comprehensive set of measurements of crystal field transitions in PrOs 4 Sb 12 as a function of temperature. From a simultaneous profile refinement of all the spectra, normalized on an absolute intensity scale, we have concluded that the Γ 1 singlet is the ground state level. Discr...