Keitaro Kuwahara

The Unconventional Superconductivity of Skutterudite PrOs4Sb12: Time-Reversal Symmetry Breaking and Adjacent Field-Induced Quadrupole Ordering

This review presents a summary and evaluation of the experimental properties of unconventional superconductivity in PrOs 4 Sb 12 . After a brief introduction of filled skutterudites, we argue that the normal-state properties of PrOs 4 Sb 12 are quite different from ordinary heavy-fermion superconductors, in the sense that the 4 f -electrons seem to be well localized even at low temperatures where the superconductivity appears. This is reflected in the existence of a field-induced antiferro-quadrupole ordering of Pr 4 f -electrons. It is shown that the 4 f collective excitations can be described as quadrupolar excitons . Their strong temperature dependence in close correlation with the superconductivity suggests a conjecture that the superconducting mechanism may involve quadrupolar-exciton-mediated pairing of some kind. Although the structure of the superconducting order parameter is far from conclusive, outstanding experimental observations reported to date reveal the uniqueness of the unconventional sup...

Nonmagnetic Ground State in Fully Filled PrxFe4Sb12 (x=1.0) Synthesized under High Pressure

We have succeeded in synthesizing the filled skutterudite Pr x Fe 4 Sb 12 with the Pr-site filling fraction x close to 1 under high pressure. The residual resistivity of ∼24 µΩ·cm is several times smaller than that of samples with x ∼0.8 synthesized under ambient pressure by independent research groups, indicating that highly improved sample quality can be achieved by reducing the Pr-site vacancy. From the transport, magnetic, and thermal measurements of the present sample with x ∼1, we have found a singlet ground state of the crystalline electric field (CEF) for 4 f -electrons and no phase transition down to 0.15 K, in contrast to the reported magnetic ordering near 5 K in the samples with x ∼0.8. The effect of the Pr-site vacancy on the CEF level scheme and the 3 d -electron density of states near the Fermi level is discussed as a possible origin of such a drastic change in the magnetic ground state.

Itinerant-Electron Weak Ferromagnetism in La-Based Filled Skutterudite LaFe4As12

High-quality samples of LaFe 4 As 12 with a residual resistivity ratio of 64 have been successfully synthesized under high pressure ∼4 GPa at 1100 K. The magnetization, specific heat, electrical resistivity, and Hall coefficient of the samples were measured. All the investigated properties exhibit evident anomalies at 5.2 K, thereby indicating a ferromagnetic transition. The large Rhodes–Wohlfarth ratio (≡ the ratio of the effective moment deduced from the Curie constant to the saturation moment per magnetic ion) of 32 suggests that LaFe 4 As 12 is the first La-based filled skutterudite that exhibits itinerant-electron weak ferromagnetism, which is realized by the high density of states at the Fermi level D ( E F ). The ferromagnetism is unexpected within the simple Stoner-type argument, because D ( E F ) lies between those of an ordinary Pauli paramagnet LaFe 4 P 12 and an enhanced Pauli paramagnet LaFe 4 Sb 12 in the LaFe 4 X 12 (X = P, As, Sb) series. We discuss this unexpected ferromagnetic transition...

Crystal Field Effect on Superconducting Transition in PrxOs4Sb12

We have succeeded in growing Pr x Os 4 Sb 12 single crystals under high pressure (∼4 GPa) for the first time, where the Pr site filling fraction is supposed to be enhanced. From the systematic measurement of the transport, magnetic, and thermal properties on the grown single crystals, we found a marked decrease in the onset superconducting transition temperature T c and a considerable increase in the 4 f -electron crystal field (CF) energy splitting between the ground state and the first excited state (Δ CF ), compared with those reported on the ambient-pressure grown single crystals. Furthermore, the mass enhancement is reduced in the high-pressure grown samples. The correlation between Δ CF and both T c and mass enhancement indicates the crucial role of conduction electron scattering accompanied by the CF excitation in the superconductivity of this compound.

Structural Transformation on the Pressure-Induced Metal–Insulator Transition in PrFe4P12

We report the first X-ray diffraction experiments of PrFe 4 P 12 under high pressure. We discovered a symmetry lowering from cubic to orthorhombic or lower at the metal–insulator transition temperature, accompanied by a jump in the lattice constant, which indicates a first-order phase transition. The superlattice reflections at q =(1,0,0) observed in the low-pressure nonmagnetic ordered phase are found to disappear in the insulating phase. The temperature dependence of the lattice constant and the absence of the superlattice reflections are well explained by taking into account an antiferromagnetic ordering.

Pressure-Induced Antiferromagnetic Order in Filled Skutterudite PrFe4P12 Studied by Single-Crystal High-Pressure Neutron Diffraction

The order parameter in the pressure-induced insulating phase of the filled skutterudite compound PrFe 4 P 12 has been investigated using a high-pressure single-crystal neutron diffraction technique. Clear evidence of the antiferromagnetic order with the propagation vector q = (1, 0, 0) was observed in the insulating phase above 2.7 GPa. The q = (1, 0, 0) structure is identical with that in the nonmagnetic ordered phase below 2.5 GPa and with the nesting property of the Fermi surface. The observed magnetic moment of the Pr ion is 2 µ B and almost pressure-independent up to at least 4.2 GPa. The 2 µ B magnetic moment originates from the dipole in the low-lying quasi-quartet crystal field state. The pressure-induced antiferromagnetic structure at 3.2 GPa is suppressed by applying a magnetic field of about 1.5 T, which is interpreted as a flop of magnetic moments into the alignment of the ferromagnetic state. This brings about a steep decrease in the electrical resistivity due to the disappearance of the anti...

Field-induced spin-density wave beyond hidden order in URu2Si2

URu2Si2 is one of the most enigmatic strongly correlated electron systems and offers a fertile testing ground for new concepts in condensed matter science. In spite of >30 years of intense research, no consensus on the order parameter of its low-temperature hidden-order phase exists. A strong magnetic field transforms the hidden order into magnetically ordered phases, whose order parameter has also been defying experimental observation. Here, thanks to neutron diffraction under pulsed magnetic fields up to 40u2009T, we identify the field-induced phases of URu2Si2 as a spin-density-wave state. The transition to the spin-density wave represents a unique touchstone for understanding the hidden-order phase. An intimate relationship between this magnetic structure, the magnetic fluctuations and the Fermi surface is emphasized, calling for dedicated band-structure calculations.

Magnetic Excitations in Heavy Electron State of Filled Skutterudite PrxLa1-xFe4P12 (x = 1.00 and 0.85)

PrFe 4 P 12 exhibits clear heavy electron behaviors as well as the phase transition to an antiferro-type ordering of 4 f electron state. In this paper, we report on inelastic neutron scattering results for powdered samples of Pr x La 1- x Fe 4 P 12 ( x = 1.00 and 0.85). A clear quasielastic magnetic response is observed, and no distinct signal is seen for crystal-field (CF) excitations in the heavy electron state. A model with strong hybridization between conduction electrons and Pr 3+ 4 f 2 electrons [J. Otsuki et al. : J. Phys. Soc. Jpn. 74 (2005) 200 and 2082] is consistent with the observed magnetic response. This result shows that the heavy electron behavior of Pr x La 1- x Fe 4 P 12 ( x = 1.00 and 0.85) originates from the Kondo effect associated with low-lying singlet–triplet CF states.

Renormalized Motion of Dysprosium Atoms Filling Boron Cages of DyB6

The lattice dynamics of DyB 6 are investigated by an inelastic X-ray scattering technique. The measured phonon modes are dominated by the motion of Dy ions in oversized boron cages. These soften remarkably as temperature is decreased, with the energy of the longitudinal phonon for the propagation vector q 1 =(1/2,0,0) decreasing by 29% from 300 K down to the phase transition temperature of 31 K. The dispersion relation curves show kinklike anomalies at q k =(0.38,0.38,0). The observed phenomena are very similar to those previously observed in GdB 6 [Iwasa et al. : Phys. Rev. B 84 (2011) 214308]. We note analogous phonon anomalies in the heavier rare-earth hexaboride systems, owing to the interplay between the motion of rare-earth atoms and the electronic state, particularly characterized by the Friedel oscillation.

Strong Effect of Yb Filling Fraction on the Magnetic Ground Sate of the Filled Skutterudite YbFe4Sb12

YbFe 4 Sb 12 single crystals with the Yb-site filling fraction close to 100% have been successfully synthesized by the Sb self-flux method under a pressure of 4 GPa. The grown single crystals do not exhibit ferromagnetic ordering down to the lowest temperature investigated, in contrast with the ferromagnetic transition reported for Yb-deficient single crystals grown at ambient pressure. The electronic specific heat coefficient of the present crystals is evidently smaller than those reported for Yb-deficient samples, indicating that the Fermi energy is located at the high-energy side of a peak in the electronic density of states dominated by the contribution of Fe-3d electrons. Therefore, the appearance of the ferromagnetic state in the Yb-deficient samples can be naively understood within the simple Stoner-type argument. Marked enhancement of the positive magnetoresistance was found in the temperature region where the spin-fluctuation scattering of conduction electrons dominates.