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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...

Co 3 V 2 O 8 ͑CVO͒ has a different type of geometrically frustrated magnetic lattice, a kagomé staircase, where the full frustration of a conventional kagomé lattice is partially relieved. The crystal structure consists of two inequivalent ͑magnetic͒ Co sites, one-dimensional chains of Co͑2͒ spine sites, linked by Co͑1͒ cross-tie sites. Neutron powder diffraction has been used to solve the basic magnetic and crystal structures of this system, while polarized and unpolarized single crystal diffraction measurements have been used to reveal a rich variety of incommensurate phases, interspersed with lock-in transitions to commensurate phases. CVO initially orders magnetically at 11.3 K into an incommensurate, transversely polarized, spin density wave state, with wave vector k = ͑0,␦ ,0͒ with ␦ = 0.55 and the spin direction along the a axis. ␦ is found to decrease monotonically with decreasing temperature and then locks into a commensurate antiferromagnetic structure with ␦ = 1 2 for 6.9Ͻ T Ͻ 8.6 K. In this phase, there is a ferromagnetic layer where the spine site and cross-tie sites have ordered moments of 1.39 B and 1.17 B , respectively, and an antiferromagnetic layer where the spine-site has an ordered moment of 2.55 B , while the cross-tie sites are fully frustrated and have no observable ordered moment. Below 6.9 K, the magnetic structure becomes incommensurate again, and the presence of higherorder satellite peaks indicates that the magnetic structure deviates from a simple sinusoid. ␦ continues to decrease with decreasing temperature and locks in again at ␦ = 1 3 over a narrow temperature range ͑6.2Ͻ T Ͻ 6.5 K͒. The system then undergoes a strongly first-order transition to the ferromagnetic ground state ͑␦ =0͒ at T c = 6.2 K. The ferromagnetism partially relieves the cross-tie site frustration, with ordered moments on the spine-site and cross-tie sites of 2.73 B and 1.54 B , respectively. The spin direction for all spins is along the a axis ͑Ising-like behavior͒. A dielectric anomaly is observed around the ferromagnetic transition temperature of 6.2 K, demonstrating that there is significant spin-charge coupling present in CVO. A theory based on group theory analysis and a minimal Ising model with competing exchange interactions can explain the basic features of the magnetic ordering.