Heavy fermion behaviors in the Pr-based filled skutterudites
Introduction
The filled skutterudites, named after a mine town Skutterud in Norway, have attracted much attention as candidate materials for thermoelectric applications [1], [2], [3]. The family of compounds has the general formula RT4X12, where R is the rare-earth or uranium ions; T is Fe, Ru, or Os; and X is P, As, or Sb. A remarkable feature in the crystal structure shown in Fig. 1 is that the R ions are located at the center of the X12 icosahedron cages. This allows local vibrations of the R ions called “rattling” [4], which is considered to reduce the thermal conductivity, improving the efficiency for thermoelectric materials.
The heavy-fermion (HF) behaviors based on 4f-electrons have been observed mainly in Ce- or Yb-based compounds. In contrast, Pr3+ ions had been considered to be quite stable in intermetallic compounds since in many cases the magnetic properties are well understood in terms of the well localized 4f2 configuration. Therefore, it is surprising that HF behaviors have been revealed recently in several Pr-based filled skutterudites, as summarized in Table 1. In this paper, we discuss present understanding on the Pr-based exotic electronic states focusing on PrFe4P12and PrOs4Sb12.
Section snippets
PrFe4P12: quadrupole ordering and HF behaviors
In PrFe4P12, an ordered state sets in below K [10]. Large specific heat anomaly evidences the phase transition originates in 4f electrons. At the early stage, it was interpreted as an antiferromagnetic (AFM) ordering of the Pr magnetic moments [10] since the magnetic susceptibility () shows a sharp drop below . However, recent studies revealed that the ordered state is not of magnetic origin; most probably it is an ordered phase of antiferro-electric-quadrupole (AFQ) moments, as
PrOs4Sb12: HFSC and field-induced quadrupole ordering
Among the Pr-based filled skutterudites listed in Table 1, PrOs4Sb12has the largest lattice constant, suggesting the weakest hybridization effect. Nevertheless, it has been found that this compound is a HF superconductor [37]. Large specific heat jump J/K2 mol at the superconducting transition temperature K [37] and the enhanced cyclotron-effective masses [38] reflect the existence of heavy quasiparticles, which form Cooper pairs. The magnetic field versus temperature phase
Concluding remarks
It has been found that quadrupole ordering appears commonly in both of the typical Pr-based HF compounds PrFe4P12 and PrOs4Sb12. This fact suggests the scenario that quadrupole degrees of freedom on the Pr-ions play a key role for the HF state formation in this class of materials. In Ce- and Yb-based compounds, it is well understood that HF states often provides quite large thermoelectric power S[59]. We have found that Sis also largely enhanced in the HF state of PrFe4P12, as mentioned above.
Note added in proof
Recently, in PrFe4P12, the existence of a new ordered phase has been found for H//[111] above 7.5 T. Refer to T. Tayama, J. Custers, H. Sato, T. Sakakibara, H. Sugawara, H. Sato, J. Phys. Soc. Jpn. 73 (2004) 3258.
Acknowledgements
We thank our collaborators listed in the references below. This work was supported by a Grant-in-Aid for Scientific Research Priority Area “Skutterudite” (No. 15072206) from MEXT of Japan.
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