Crystal Field Potential ofPrOs4Sb12: Consequences for Superconductivity

Abstract

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...

Resistivity and Magnetoresistance of La-doped CeOs4Sb12and PrOs4Sb12

“…However, this field variation of ρ is not consistent with the now-established CEF configuration of Pr, worked out by elastic 12) and inelastic 13) neutronscattering and high-field specific heat 14) data. According to these data, the ground state is a nonmagnetic Γ 1 singlet, separated by about 8 K from the first excited Γ triplet (in T h symmetry).…”

de Haas–van Alphen Study of the Pr-based Filled Skutterudite PrOs4P12

“…In fact, the observed crystalline electric field (CEF) excitation spectra of the inelastic neutron scattering experiments in PrT 4 P 12 (T =Fe, or Ru) are very broad above the transition temperatures in contrast to the sharp ones in PrT 4 Sb 12 (T =Fe, Ru, or Os), [23][24][25][26][27] which can be ascribed to the strong c − f hybridization. Therefore, the strong c−f hybridization can be naturally expected for an origin of the large mass enhancement in PrOs 4 P 12 .…”

Pr-magnetism in the quasi-skutterudite compound PrFe2Al8

“…The 2J + 1 = 9 fold multiplet of Pr 3+ splits so that a crystal field singlet is always present for all type of crystallographic symmetry [21]. Pr-based intermetallics often display singlet ground states however, also have been found to lead to exotic behaviour such as the observation of heavy fermion superconductivity [22,23]. Not only that, in certain Pr based systems like PrAu 2 Si 2 -related compounds, frustration effects on the magnetic ground state were argued to be introduced by crystal field effects [24].…”