Heavy fermion behaviors in the Pr-based filled skutterudites

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Abstract

In filled skutterudites, which have attracted much attention as candidate materials for thermoelectric applications, rare Pr-based heavy-fermion (HF) behaviors have been revealed recently. We review recent studies focusing on PrFe4P12 and PrOs4Sb12; the former shows HF behaviors in high-fields where a quadruple ordering is suppressed, and the latter shows the first known Pr-based HF superconductivity as well as an anomalous field-induced quadruple ordering. We suggest that quadrupole degrees of freedom on the Pr-ions commonly play key roles in these exotic states.

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 TA=6.5  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 (χ) shows a sharp drop below TA. 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 ΔC/T0.5 J/K2 mol at the superconducting transition temperature Tc=1.85  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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