Proc. Int. Workshop NASCES11 J. Phys. Soc. Jpn. 80 (2011) SB019 c 2011 The Physical Society of Japan SB019-1 New Aspects of Electronic Ordered States due to f 2 Configuration of Pr-Based Systems Kazuaki Iwasa ∗ , Kotaro Saito, and Lijie Hao 1 , Department of Physics, Tohoku University, Sendai 980-8578, Japan. 1 China Institute of Atomic Energy, Beijing 102413, P. R. China. (Received April 30, 2011) Strong hybridization between conduction electrons and f 2 electrons in a trivalent Pr ion is a recent attractive sub- ject. For example, the Pr-filled skutterudite compounds have been extensively studied. Pr(Ru 1−x Rh x ) 4 P 12 exhibits the reentrant-like behavior of the metal-nonmetal transition, and PrFe 4 P 12 the heavy-fermion state followed by the non- metallic ordering. In this article, we review briefly these phenomena characterized by the staggered structures of the totally symmetric order parameters of f 2 state. We also demonstrate the higher-order magnetic scattering of polarized neutron to investigate quantum state levels in the f electron systems, which is expected to be carried out at the pulsed neutron facility J-PARC MLF. KEYWORDS: f electron systems, electronic hybridization, neutron scattering 1. Introduction Interplay between localized f and conduction electrons is essential for various physical properties in the rare-earth or actinide compounds. Heavy electrons due to a singlet ground state arising from screening of local magnetic moments is a typical phenomenon, which has often been found in the Ce-based 4 f 1 systems. 1) It is evidenced by the quasielas- tic or damped magnetic spectra in neutron scattering. 2) Hy- bridization due to the f 2 configuration has also been an attrac- tive subject. A non-Kramers doublet crystal-field (CF) split- ting state in a cubic lattice carries no magnetic dipoles but quadrupoles. Screening of such quadrupole by carriers, called as the two-channel or the orbital Kondo effect, was studied for several U- and Pr-based materials. 3–5) The f 2 configuration provides also a singlet state without any degeneracy even un- der cubic symmetry. Recently, the unconventional electronic states involving the f 2 singlet state are found in some of the Pr 3+ -based skutterudite compounds. PrRu 4 P 12 undergoes a metal-nonmetal transition at 63 K without any anomaly of the magnetic susceptibility. 6) The non-metallic state is characterized by the staggered arrange- ment of inequivalent CF splitting schemes of Pr 3+ 4 f 2 elec- trons accompanying the crystal-structure superlattice. 7, 8) The Fermi surface of the conduction band dominated by the p state originating from the phosphorus molecular orbitals has the nesting instability. The non-metallic phase is suggested to be the charge density wave associated with p- f hybridization and the antiferro-type ordering of non-magnetic multipoles of Pr f 2 state. 9, 10) Because the ordered multipole is a totally sym- metric one preserving the cubic point group at the Pr sites, the evolution of the order parameter is seen as the drastic temper- ature variation of the inequivalent CF splitting schemes. It is noticeable that the Rh-doped systems show the reentrant-like behaviors as suppression of the electrical resistivity 11) and the structural superlattice below 15 K. 12) The inelastic neutron scattering study on the Rh-doped system is shown later. PrFe 4 P 12 has also attracted much attention, because of the non-magnetic ordering of Pr f 2 electrons at 6.5 K from the ∗ E-mail address: iwasa@m.tohoku.ac.jp heavy electron phase. 13, 14) Only the quasielastic magnetic ex- citation was observed in the heavy electron phase. 15) In the low-temperature ordered phase, it varies into the pronounced excitation peaks, 16) and the staggered arrangement of mag- netic moments are induced at the Pr sites. 17) These properties are also discussed in terms of the hybridization effect and the totally symmetric order parameter, as reviewed in the follow- ing section. In this paper, we show the recent data of magnetic excita- tions of Pr(Ru 1−x Rh x ) 4 P 12 across the reentrant-like anomaly. The spectrum in the low-temperature metallic phase is not identical with those in the disordered and the non-metallic phases, indicating a sequence of three different phases. We also simulate the higher-order magnetic neutron scattering for a CF level scheme, which allows to detect additional excita- tions beyond the conventional dipole approximation. 2. Experimental Polycrystalline samples of Pr(Ru 1−x Rh x ) 4 P 12 were synthe- sized by the Sn-flux method described in the literatures (9.4 and 5.4 g of x = 0 and 0.05, respectively). 18, 19) The concen- tration x for the Rh substitution means the atomic ratio in the starting compositions prior to the synthesis. The crystal structures were confirmed as the filled-skutterudite one by a powder x-ray diffraction technique. Inelastic neutron scatter- ing experiments were carried out at the thermal neutron triple- axis spectrometer TOPAN installed at the beam hole 6G of the research reactor JRR-3, Japan Atomic Energy Agency (Tokai, Japan). Spectra were obtained by varying the incident neutron energy with the scattered neutron fixed to 13.5 meV. Both en- ergies were selected by the pyrolytic graphite crystals. Higher order contamination was eliminated by a pyrolytic graphite filter installed after the sample. A set of horizontal collima- tors were open-60’-60’-open. Sample temperatures were con- trolled between 3.6 and 70 K by the helium closed-cycle re- frigerators. 3. Experimental Results Figure 1 depicts the inelastic spectra of PrRu 4 P 12 . Clear temperature dependence of the spectrum was confirmed to J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by 34.228.24.229 on 06/16/20