Iwasa Kazuaki

Novel Features Realized in the Filled Skutterudite Structure

Full varieties of strongly correlated electron phenomena have been found in the filled skutterudite systems, by changing the constituent elements. The unique crystal structure leads to novel features even in the systems containing rare earth elements with plural number of 4 f -electrons. According to the intensive competitions and cooperation among experimental and theoretical research works, remarkable progress has been made in sorting out several basic origins that bring about the variety of this system, such as the strong c – f hybridization, the small CEF level splitting, the orbital degree of freedom (multipoles), and the positional degree of freedom within a pnictogen cage. However, there still remain many attractive questions to be answered.

Well-Defined Crystal Field Splitting Schemes and Non-Kramers Doublet Ground States of f Electrons in PrT2Zn20 (T = Ir, Rh, and Ru)

Inelastic neutron scattering studies of Pr\(T_{2}\)Zn20 (\(T\) = Ir, Rh, and Ru) were carried out in order to reveal \(f\) electron states indispensable for the low-temperature properties of Pr\(T_{2}\)Zn20 associated with multipole degrees of freedom. Sharp excitation peaks, which are located within excitation energies of less than 10 meV, are attributed to crystal field splitting level schemes of \(4f^{2}\) in Pr3+ ions. Data of the Ir-based compound is explained by a non-Kramers doublet ground state in the local symmetry \(T_{d}\) for this system. Results of the Rh-based compound are explained by the scheme for the Pr-site local cubic symmetry \(T\) in the low-temperature structural phase, which is lower than \(T_{d}\). In the Ru-based compound, the doublet is split as a result of the loss of local cubic symmetry through structural transformation. These experimental results confirm the non-Kramers doublet ground state that causes the quadrupole ordering or orbital Kondo effect predicted for this series...

Magnetic Correlation in the Ordered Phase of CeOs4Sb12

We performed neutron diffraction and magnetization measurements to reveal the nature of phase transition of a Kondo semiconductor CeOs 4 Sb 12 . Weak antiferromagnetic (AFM) reflections characterized by a wave vector q = (1 0 0) were observed, and the ordered moment value is around 0.07µ B /Ce at zero magnetic field. The AFM structure is rapidly suppressed by magnetic field of 1 T, in contrast to the transition temperature suggested to be enhanced by magnetic field. Only ferromagnetic correlation is observed above 1 T. Such drastic variation of the magnetic correlation is closely connected into the transport properties; the AFM ordered phase below 1 T and the induced ferromagnetic state above 1 T correspond to the semiconducting and metallic states, respectively.

Newly Proposed Inelastic Neutron Spectrometer POLANO

In Japan Proton Accelerator Research Complex (J-PARC), the construction of direct geometry chopper spectrometer dedicated for inelastic scattering with polarization analysis is newly planed. The concepts of the instrument are compact design, higher neutron flux, higher energy polarization up to 150–200 meV, and medium resolution in order to achieve a new paradigm in the momentum-energy space beyond the reactor based spectrometers for dynamical studies in materials.

Field Induced Magnetic Moment Distribution in the Ordered Phase of PrFe4P12

Neutron diffraction experiments have been performed to investigate the electronic phase transitions in the Pr-based filled skutterudite PrFe 4 P 12 . This compound undergoes a transition from a characteristic heavy-electron state to a nonmagnetic phase below 6.5 K. Neutron diffraction experiments reveal an antiferromagnetic alignment of the magnetic-field induced dipole moments. The directions of the antiferromagnetic components are considered to be always parallel to the magnetic field applied along either [0 0 1], [1 1 0] or [1 1 1]. This implies that the symmetry of the ordered state can be categorized as Γ 1 . This phenomenon is consistent with the scenario of a scalar-type order parameter for the Pr 4 f electron state.

Magnetic Phase Diagrams of YVO3 and TbVO3 under High Pressure

Magnetic states in R VO 3 ( R = Y, Tb) were investigated using low-temperature and high-pressure neutron-diffraction techniques, and the pressure–temperature magnetic phase diagrams were clarified up to 6.2 GPa. We elucidated that, on application of pressure, the magnetic ground state changes from C -type spin ordering ( C -SO) to G -type spin ordering ( G -SO); this corresponds to the orbital ground-state switching from G -type orbital ordering ( G -OO) to C -type orbital ordering ( C -OO). It is also interesting that the G -SO transition occurs simultaneously with the C -OO transition, but not with the G -OO one. When the transition temperature of C -OO exceeds that of G -OO under high pressure, the G -OO phase vanishes completely, and a simultaneous spin and orbital order–disorder phase transition occurs. Such a transition is the first case in perovskite-type transition-metal oxides.

Magnetic Field and Temperature Dependence of X-ray Superlattice Intensity in Multipole Ordered Phase of Pr(Ru1-xRhx)4P12 (x = 0, 0.03)

Superlattice reflections in the multipole ordered phase of PrRu 4 P 12 and Pr(Ru 0.97 Rh 0.03 ) 4 P 12 has been measured using a synchrotron x-ray scattering technique. The Thomson scattering intensity of (111) at 1.7 K does not show significant variation with magnetic field up to 7 T in contrast to the reported variation of electrical resistivity. This means that the lattice distortion in the ordered phase is robust and the staggered ordering remains after split of triplet ground state at half of Pr sites. The nonmetallic staggered ordering is not destroyed completely by applying magnetic field. Energy scan at (111) reflection of Pr(Ru 0.97 Rh 0.03 ) 4 P 12 show temperature dependence around resonant energy of Pr L 3 edge. This may allow us to observe anisotropy of electronic ordering of Pr sites and gives us a hint for direct observation of higher rank multipole order in PrRu 4 P 12 .

Magnetic Properties of Electron-Doped LaCoO3

We studied electron-doped \(\text{LaCo}_{1 - y}\text{Te}_{y}^{6 + }\text{O}_{3}\) by magnetization measurements and neutron scattering. The effective Bohr magneton, estimated by Curie–Weiss fitting around room temperature, is independent of y. This suggests that magnetic Co3+(HS), not nonmagnetic Co3+(LS), is mainly replaced by doped magnetic Co2+(HS). At the lowest temperatures, a Brillouin-function-like saturating behavior persists in the magnetization curves even in the high-y samples, and neither a clear magnetic reflection nor magnetic dispersion is observed by neutron scattering. These findings indicate that the magnetic correlation is very weak, in contrast to the well-known hole-doped LaCoO3 accompanied by a drastic transition to a ferromagnetic metal. However, we also found that the low-y samples exhibit nonnegligible enhancement of the saturated magnetization by ∼2μB per a doped electron. All these characteristics are discussed in the light of the activation and inactivation of a spin-state bloc...

Softening of Phonon by Filled Rare-Earth Ion Motion Common to Skutterudite with Sb Cages

We investigate phonon modes of the rare-earth filled skutterudite compounds CeRu 4 Sb 12 , CeOs 4 Sb 12 , PrRu 4 Sb 12 , and PrOs 4 Sb 12 by using an inelastic neutron scattering technique. These materials exhibit similar low-lying motions of filled ions inside the Sb cages that softens with decreasing temperature. Potentials at the rare-earth sites are considerably anharmonic. Despite that this behavior appears in all the four materials, magnitude of the softening of PrOs 4 Sb 12 is largest among them. This fact implies that difference of carrier state gives rise to renormalization effect on the rare-earth-ion vibrations.