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We report NMR spectrum analysis for CrAs, which was recently reported to be superconducting under pressure. The NMR spectrum obtained by the powdered single crystals shows a typical powder pattern reproduced by the electric field gradient (EFG) parameters and isotropic Knight shift, indicating anisotropy of Knight shift is not remarkable in CrAs. For the oriented sample, the spectrum can be understood by considering that the crystals are aligned for H ∥ b . The temperature dependence of Knight shift was successfully obtained from NMR spectrum with large nuclear quadrupole interaction.

We report on detailed longitudinal field (LF) muon spin relaxation studies in SmRu 4 P 12 in order to characterize magnetic fluctuations in a magnetically ordered state below T MI = 16.5 K . The magnitude and fluctuation rate of the fluctuating field are derived as functions of temperature from LF dependence of the longitudinal relaxation rate. Possible low-energy excitations of magnetic multipoles are discussed.

Wide varieties of strongly correlated electron phenomena are performed on the stage of a “filled skutterudite structure”. Especially when one of the players contains a plural number of 4f electrons, the orbital degrees of freedom play a major role as a new type of nonmagnetic and/or weak-magnetic phenomena. Several examples found in Pr- and Sm-based filled skutterudites are introduced in relation to muon spin relaxation (μSR) experiments.

We have investigated an off-center local oscillator of a guest atom in cage of clathrate compounds R3Pd20Ge6 ( R = La , Ce, Pr, Nd) and a filled skutterudite PrOs4Sb12 using ultrasonic measurements. The observation of ultrasonic dispersions in transverse C 44 mode of R3Pd20Ge6 and in transverse ( C 11 - C 12 ) / 2 mode of PrOs4Sb12 revealed the rattling of the off-center oscillator characterized by a relaxation time with Arrhenius-type temperature dependence. A low-temperature softening of C 44 in La3Pd20Ge6 below 3 K down to 20 mK and a softening of ( C 11 - C 12 ) / 2 in PrOs4Sb12 above the heavy Fermion superconducting transition indicate the charge fluctuation due to the degenerate off-center oscillators.

A general survey of the variety of phenomena exhibited by the filled skutterudite compounds is presented from the view points of electrical properties and electronic structure, putting emphasis on the Pr- and Sm-based ones. Until now, Pr-based ones, such as heavy Fermion superconductivity in PrOs4Sb12, metal-insulator transition in PrRu4P12 and field-induced heavy Fermion state above antiferro-quadrupolar ordered state in PrFe4P12, have been most intensively investigated. Quite a few attractive features have been found also in Ce- and Sm-based skutterudites; such as recently found novel heavy Fermion state robust against magnetic fields in SmOs4Sb12. We discuss the novelty and variety of such attractive features in correlation with the unique crystal structure of the filled skutterudite.

We have made ultrasonic measurements on a filled skutterudite PrOs4Sb12 showing a heavy Fermion superconductivity at T C = 1.85 K . The elastic constants ( C 11 - C 12 ) / 2 and C 44 of PrOs4Sb12 exhibit low-temperature softenings below 20 K down to T C , indicating a quadrupole fluctuation of Pr 3 + ion. A minimum of ( C 11 - C 12 ) / 2 in magnetic fields along the [1 1 0] direction around 8 T shows a level crossing of one of a excited Γ 4 ( 2 ) triplet to a ground state Γ 1 singlet. An ultrasonic dispersion in ( C 11 - C 12 ) / 2 around 30 K suggests a thermally activated off-center rattling of Pr ion in a cage of Sb-icosahedron. The observed relaxation time shows the Arrhenius-type temperature dependence τ = τ 0 exp ( E / k B T ) with an attempt time τ 0 = 8.8 × 10 - 11 s and an activation energy E = 168 K . The charge fluctuation of the off-center motion and the quadrupole fluctuation of the 4f electron may participate in the formation of the heavy fermion and the superconductivity in PrOs4Sb12.

Phase diagrams and the ground-state magnetic properties of PrOs 4 Sb 12 , PrRu 4 P 12 and PrFe 4 P 12 are studied by means of low-temperature magnetization and specific heat measurements. Field-induced ordering observed in PrOs 4 Sb 12 for all directions above 4.5 T is well explained by a level crossing in the Γ 1 – Γ 4 ( T h ) pseudo-quartet states with inclusion of an antiferroquadrupolar (AFQ) interaction. In PrRu 4 P 12 , a Curie-like increase of the susceptibility observed down to below 300 mK indicates that a half of Pr 3 + ions have the magnetic Γ 4 ( T h ) triplet ground state, in accordance with a dramatic change in the crystal field splitting below the metal–insulator transition at 63 K recently observed by an inelastic neutron scattering experiment. In PrFe 4 P 12 , a new ordered phase is observed for H ( ∥ [ 1 1 1 ] ) > 7.5 T above the AFQ phase below 0.6 K, which is probably responsible for unusual heavy electron behavior observed in this field direction.

The elastic properties of the filled skutterudite compound Pr x La 1-x Fe 4 P 12 for x = 0.95 and 0.85 have been investigated in magnetic fields by means of ultrasonic measurements. The elastic constant C 11 shows a softening towards low temperatures for both x = 0.95 and 0.85. The observed elastic softening in C 11 for x = 0.85 is gradually suppressed by increasing the applied field. The results for x = 0.85 is well reproduced qualitatively in terms of the quadrupolar susceptibility based on the crystalline electric field level scheme with Γ 3 non-Kramers doublet. However, C 11 for x = 0.95 shows unexpected behavior in applied field. We discuss the obtained field dependence of elastic constants of Pr x La 1-x Fe 4 P 12 for x = 0.95 and 0.85 from the viewpoints of the low-lying state of 4f-levels in Pr ions.

The magnetic susceptibility, χ, of CeCo2 compound has been measured under pressure up to 2 kbar at T=78 and 300 K , and a pronounced pressure effect, d ln χ/ d P≃−5 Mbar −1 , is revealed. In order to analyze the experimental magnetovolume effect, d ln χ/ d ln V , the volume-dependent electronic structure of CeCo2 has been calculated ab initio by employing a relativistic full-potential LMTO method. The calculations have brought out strong hybridization of the itinerant 4f (Ce) and 3d (Co) states, which leads to a filling of the peculiar bonding band. It is demonstrated that the magnetic properties of CeCo2 are consistent with itinerant behavior of the 4f electrons of cerium. The large volume derivative of the effective electron–electron interaction parameter, d ln I/ d ln V≃−1.7 , implies a predominant role of the short-range correlations, in accordance with the Hubbard model.

We have investigated the thermoelectric power ( S ), Hall coefficient ( R H ), and magnetoresistance (MR) of Pr x Fe 4 Sb 12 ( x ∼ 1 ) synthesized under high pressure. The dependence of S and R H on the Pr-site filling fraction x become apparent at low temperatures. In MR, we confirmed that the electrical resistivity is enhanced by the 4f-electron crystalline electric field effect below ∼ 10 K and suppressed by the 3d-electron spin fluctuation effect above ∼ 10 K . The change of magnetic state in Pr x Fe 4 Sb 12 with x , realized as a combined effect of 4f-electrons and the high electronic density of state at Fermi level, is well reflected in the transport properties.

We have succeeded in observing the de-Haas van Alphen effect in the Pr-based filled skutterudite compound PrOs4P12. We have found the multiply-connected and almost spherical Fermi surfaces, whose cyclotron effective masses are ranging 1.2 – 18 m 0 .

We have synthesized the filled skutterudite Sm x Fe 4 Sb 12 under high pressure to improve the Sm-site filling fraction x. From the magnetic susceptibility, electrical resistivity and specific heat measurements, we confirmed a ferromagnetic transition at T C ∼ 43 K . We found no drastic change in the magnetic properties caused by the reduced number of the Sm-site vacancy, in contrast with the significant change of the magnetic ground state in Pr x Fe 4 Sb 12 ; from ferromagnetic to non-magnetic one. Such a different response of the magnetic properties to the change in rare earth site vacancy is discussed.

We report that the nuclear spin–spin relaxation rate 1 / T 2 shows characteristic peaks in the skutterudite compounds of ROs 4 Sb 12 ( R = La , Pr, Nd, and Sm) and PrRu 4 Sb 12 . The broadening of Sb-NQR spectrum accompanied with the peak in 1 / T 2 suggests that some charge fluctuation exists in these systems and slows down around the peak in 1 / T 2 . The peaks in 1 / T 2 appear around 120 K in all the systems we measured, while the other peaks around 20 K are observed in ROs 4 Sb 12 ( R = Pr , Nd, and Sm). The 12 Sb sites are not equivalent at low temperatures in these compounds.

We report nuclear quadrupole resonance (NQR) studies on Ru-sites of filled skutterudite compounds R Ru 4 P 12 ( R = La , Nd , Sm ) in zero magnetic field. For NdRu 4 P 12 , the Ru 101 -NQR signal disappears below T C = 1.6 K , which is indicative of the transformation into the ferromagnetically ordered ground state. For SmRu 4 P 12 , a single Ru 101 -NQR line splits into two lines below the metal-insulator transition temperature T MI = 16.5 K with the intensity ratio of 3 : 1 . This result gives evidence that there are two inequivalent Ru-sites with different electric field gradient. The intensity ratio leads to a conclusion that the symmetry is lowered below T MI from a cubic Im 3 ¯ space group to a trigonal R 3 ¯ one.

In filled skutterudite compounds and clathrate compounds, guest atoms, which are accommodated in polyhedral cages, exhibit an anharmonic oscillation so-called the rattling motion. The rattling motion is suggested by inelastic neutron scattering experiments in filled skutterudite compound LaFe 4 Sb 12 . To investigate an influence of rattling motion of La atoms to the lattice system, we have measured temperature dependence of elastic modulus C 44 on a LaFe 4 Sb 12 single-crystalline sample in the T range between 4.2 and 150K for ultrasonic frequencies from 30 to 220 MHz. We found ultrasonic frequency dependence in C 44 , suggesting the rattling motion of La atoms between 30 and 80 K. We obtained a relaxation time 3.1 x 10 -11 s and an excitation energy ∼300K of the rattling motion.

We have studied the effect of pressure in the filled skutterudite PrRu4P12, which shows a metal–insulator (MI) transition at T MI = 63 K , via simultaneous measurements of electrical resistivity ( ρ ) and AC-calorimetry ( C AC ). Schottky-like anomalies in C AC disappear under pressure, suggesting a change of the ground state. The resistivity below T MI is strongly suppressed with increasing pressure, in contrast to the weak pressure dependence of T MI . Above 10 GPa, ρ ( T ) shows metallic behavior with small anomalies at T MI . We discuss the likely change of ground state in PrRu4P12 with pressure from triplet to singlet.

We have carried out nuclear quadrupolar resonance (NQR) measurements on SmRu 4 P 12 and YbAl 3 C 3 , both of which have attracted much interest in terms of multipolar ordering. Although no structural anomalies have been detected by diffraction experiments so far, the NQR resonance frequencies ν Q 's of both compounds show marked anomalies just below each phase transition temperature, which probably probes the order parameters. The results provide important information to shed light on the ordered structures in the low temperature phases of these compounds.

We performed muon Knight shift measurements on randomly aligned single crystalline samples of SmRu 4 P 12 in a magnetic field of 6 T. A marked increase in the muon spin relaxation rate was observed below a metal-insulator transition temperature, interpreted as an occurrence of finite local fields due to a dipole and/or octupole ordering. An expected magnetic anomaly at around 14 K in high fields was still obscure in a field of 6 T.

We have performed inelastic neutron scattering experiment on a powder sample of the ferromagnetic filled skutterudite compound NdOs 4 Sb 12 in order to clarify the crystal-field (CF) states of 4f electrons in this system. At lowest temperature 15 K, two inelastic peaks have been observed at energy transfers 23.0 and 30.2 meV. The intensity of these two peaks decreases with increasing temperature, as expected for excitations from the CF ground state. This observation of CF excitations evidences the localized character of 4f electrons in NdOs 4 Sb 12 . From this result and the bulk magnetic susceptibility of NdOs 4 Sb 12 , possible CF level schemes are discussed.

By nuclear specific heat measurements in the anomalous ordered phase of SmRu 4 P 12 , the size of the ordered Sm dipole moment m Sm has been determined accurately. The strongly reduced value of m Sm agrees perfectly with a mean-field model calculation, providing evidence that the dipole moments appear as a secondary order parameter in an octupole ordering. In the quite rare examples of octupole ordering in NpO 2 and Ce 1 - x La x B 6 , no dipole moments are induced. In this sense, the ordered state in SmRu 4 P 12 provides a new example of octupole ordering with such unprecedented character.

Optical conductivity spectra [ σ ( ω ) ] have been measured for many heavy-fermion (HF) Ce and Yb compounds. A characteristic mid-infrared peak has been observed in σ ( ω ) of these compounds, and has been analyzed in terms of the conduction (c)-f electron hybridized bands. A universal scaling is found in terms of the observed peak energies for these HF compounds. This optical scaling is discussed in comparison with other universal relations found for HF compounds such as the Kadowaki–Woods relation.

Low-energy magnetic excitations in single crystals of the heavy-fermion superconductor PrOs4Sb12 have been studied by inelastic neutron scattering. The clear softening of excitations at a wave vector Q = ( 1 , 0 , 0 ) , which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and the intensity analyses of excitations directly evidence that the nonmagnetic quadrupolar fluctuations are dominant in this system. Furthermore, the narrowing of the linewidths of the excitations in the superconducting phase indicates the close connection between the superconductivity and the excitations. The preliminary data under magnetic fields are also presented.

Inelastic neutron scattering and X-ray diffraction experiments were carried out to study the dynamics of the crystal lattice of PrOs 4 Sb 12 , in which the rattling motion of Pr ions has been inferred. A transverse acoustic phonon propagating along the cubic [0 1 1] axis exhibits large softening by about 30% from 300 to 10 K. Temperature dependence of the phonon intensity is quite close to that based on the Debye–Waller factor for the Pr ions. This fact indicates that the mode with the characteristic softening is dominated by the Pr-ion vibration.

Raman scattering spectra of the single-crystalline PrRu 4 P 12 have been measured in the temperature region between 1.5 and 300 K. Below T MI , new peaks appear at phonon region and these peaks are assigned as the Raman-active phonons with Pm 3 ¯ symmetry. While, the broad spectra in the lower-energy region than 200 cm - 1 become sharp below T MI and the detailed polarization measurements conclude that these are the crystal field excitations (CF) due to two-different Pr sites. Furthermore, temperature dependence of the CF energy levels for Pr2 has been clarified.

The filled skutterudite PrFe4P12 undergoes an antiferro-quadrupolar (AFQ) ordering below T Q = 6.5 K . In a recent resistivity measurement under high pressures, it was reported that T Q is suppressed with applied pressure and an insulating phase appears above 2.4 GPa. To obtain microscopic information about the AFQ phase and the insulating state under pressure, we performed X-ray diffraction experiments. We have succeeded in observing superlattice reflections at the wave vector q = ( 1 , 0 , 0 ) with the intensity being ∼ 10 - 4 of the fundamental Bragg's ones. At 1.4 GPa, T Q is suppressed to 5.5 K, while it is found out that the calculated Fe-ion displacement is almost the same as that at ambient pressure. In the insulating state, on the other hand no superlattice reflections at q = ( 1 , 0 , 0 ) are observed within the experimental error.

Ultrasonic attenuation measurements have been firstly performed for a large single crystal of PrOs 4 Sb 12 with the dimensions of 5.97 × 0.6 × 0.6 mm 3 . Remarkable frequency dependence around 20–40 K has been observed in the elastic constant and attenuation coefficient of the longitudinal C 11 mode associated with E g symmetry strain in part, which results from a thermally activated off-center rattling with E g symmetry of a Pr ion inside a Sb icosahedron cage. Parameters of a characteristic time τ 0 = 3.1 × 10 - 11 s and an activation energy E = 225 K were obtained. This E g rattling involving a local charge fluctuation inside a Sb cage periodically arranged may couple to the conduction electrons. As a result, the electron–phonon coupling would lead to heavy fermion and its superconductivity in PrOs 4 Sb 12 .

We have performed ultrasonic measurements on the filled skutterudite compound PrRu 4 P 12 at temperatures down to 0.5 K. A characteristic softening towards low temperature was observed in elastic constants C 11 , ( C 11 - C 12 ) / 2 and C 44 . A further decrease of the softening seems to be expected in the further low temperature in all the elastic constants. We will show the elastic behavior at the low temperatures, and discuss them based on the 4f ground state of Pr ion split by the crystalline electric field effect.

The superconducting (SC) gap of the Pr-based skutterudite PrOs 4 Sb 12 has been investigated by means of field-angle-dependent specific heat C ( H , ϕ ) experiments. At fixed temperatures, H was varied and rotated in the basal planes. A 4-fold oscillation of C ( H , ϕ ) is observed in the entire H–T SC phase diagram. The minima are located along the [ 1 0 0 ] directions suggesting the excistence of gap nodes or minima along these directions. The oscillation amplitude | A | becomes maximum at H / H c 2 ≈ 0.3 . Below, | A | → 0 . The temperature variation of | A | follows a quadratic dependence.

We have determined the lower and upper critical fields, H c1 and H c2 by measuring the DC magnetization, the AC susceptibility and the resistivity in fields for a superconductor U 6 Fe. The coherence length and the penetration depth are estimated as 50 and 2060 A, respectively.

We have measured the electrical resistivity, Hall effect and transverse even voltage in the flux-flow state of CeRu2, CeCo2 and Yb3Rh4Sn13. In all the transport properties, we found a clear anomaly related with the peak effect, which was argued in relation to the inhomogenous superconducting phase proposed by Fulde-Ferrell and Larkin-Ovchinnikov. The pinning force and the transverse even voltage depend largely on sample quality. A deviation from the ordinary BCS prediction, suggesting an anisotropy in the superconducting gap, has been found in the specific head of CeCo2/

We have measured temperature dependence of magnetic susceptibility, resistivity, Hall coefficient, and lattice constant of Ce 3 Ir 4 Sn 13 in order to clarify the origin of anomalies at 0.6 and 2 K in specific heat. The susceptibility measurement indicates that the anomaly at 0.6 K is due to an antiferromagnetic ordering. The Hall coefficient, resistivity, and lattice constant exhibit a sharp change at 2 K, indicating that the transition at 2 K involves a change of the band structure accompanied by a lattice expansion below the transition temperature.

We have performed ultrasonic measurement on heavy-fermion superconductor PrOs 4 Sb 12 . A characteristic elastic softening towards the low temperature was observed in the temperature dependence of elastic constants C 11 , ( C 11 - C 12 ) / 2 and C 44 . It is likely to that a coupling between elastic strain and the conduction electrons plays a crucial role in heavy-fermion states at low temperatures, leading to a resultant elastic anomaly. In addition to the crystalline electric field effect on Pr ions, we discuss the origin of the softening toward the low temperature obtained in these elastic constants based on the deformation potential coupling involving narrow bands with high density of states.

We present a precise ( H – T ) phase diagram of the double superconducting transition in the specific heat of the heavy fermion superconductor PrOs 4 Sb 12 , down to 350 mK, on a sample which exhibits two sharp distinct anomalies at T c 1 = 1.89 K and T c 2 = 1.72 K . Comparison with an existing phase diagram based on transport measurements will be reviewed. The intrinsic or extrinsic nature of this double transition is discussed in light of some characterisations of the superconducting transition of different samples and with respect to the historical case of UPt 3 . Finally, we give a detailed analysis of H c 2 which shows the importance of the multiband character of the electronic structure of PrOs 4 Sb 12 and gives strong support to an even parity order parameter.

We have measured 31 P NMR in the filled skutterudite PrFe 4 P 12 . In the antiferro-quadrupole ordered phase, field-induced, site-dependent line splitting was observed. We propose that the order parameter is of Γ 23 type and both of the quadrupoles O 2 0 and O 2 2 have finite magnitude. In the high-field phase, the nuclear spin-lattice relaxation rate 1 / T 1 exhibits remarkable anisotropy. For the field along 〈 1 1 1 〉 , we observed non-Fermi-liquid behavior characterized by the enhanced, almost temperature-independent 1 / T 1 .

We have studied the electrical resistivity in a skutterudite compound PrFe 4 P 12 under high pressure. An antiferro-quadrupolar ordering temperature T Q decreases monotonously with increasing pressure up to 2.4 GPa. Above 2.4 GPa, the metal–insulator (M–I) transition appears. By applying magnetic field, this M–I transition is suppressed easily and heavy-fermion state is realized. The observed Kondo effect and the field-induced heavy-fermion state at high pressure suggest that the quadrupolar interactions of the f electrons of Pr 3 + survive even in the insulator phase.

Raman-active phonons are completely assigned for RFe4P12 and ROs4Sb12 ( R = La , Ce, Pr and Nd) at 300 K. Normal mode analysis concludes that the force constant between rare-earth and pnictogen at icosahedron is strong compared with other rare-earth compounds. This is the experimental evidence to support the strong hybridization effect in the filled skutterudite.

We have measured the de Haas–van Alphen (dHvA) effect in PrFe 4 P 12 under high pressures up to 3.0 GPa, and found that both the dHvA frequency and cyclotron mass of ψ -branch considerably decrease with increasing pressure.

We have studied the zero-field muon spin relaxation (ZF- μ SR) on a filled-skutterudite superconductor LaOs 4 Sb 12 ( T c = 0.74 K ) to compare with the time-reversal-symmetry (TRS)-broken superconductor PrOs 4 Sb 12 . The relaxation rate is independent of temperature down to 20 mK, suggesting that the absence of TRS breaking in LaOs 4 Sb 12 . This finding provides convincing evidence for the Pr 4 f -electrons playing an essential role for the realization of the TRS broken superconductivity in PrOs 4 Sb 12 .

Ultrasonic measurement was performed on Kondo semiconductor CeOs4Sb12 to investigate the elastic property. A distinct elastic softening toward low temperatures was observed in C 11 , whereas no softening was observed in C 44 . The obtained results were not explained reasonably based on the crystalline electric field effect, indicating that the softening is ascribed to another origin. Since CeOs4Sb12 is likely to form the narrow quasiparticle band, the coupling between elastic strain and the band may play a crucial role. We discuss the origin of the distinct softening in C 11 from the view of this coupling.

Raman scattering spectra of the single-crystalline PrRu 4 P 12 have been measured in the temperature region between 5 and 293 K. At 293 K, eight sharp peaks have been observed. These peaks are completely assigned as the Raman-active phonons with the Im 3 ¯ symmetry. Below T MI ( = 60 K ) , new additional peaks appear in the phonon frequency region. Some peaks can be assigned to CEF excitations by an energy comparison. The observed phonon number suggests that the possible symmetry is Pm 3 ¯ at 5 K. Furthermore, a quantitative change of the spectra has been observed at ∼ 35 K.

Muon spin relaxation (μSR) experiments have been carried out down to 20 mK in the filled skutterudite PrRu4P12 that exhibits a metal–insulator transition around T MI = 60 K . No precession in the μSR data at zero external magnetic field (ZF) suggests no magnetic order down to 20 mK. The ZF relaxation rate of the μ + spin polarization shows no sizable change across TMI within the experimental accuracy. However, it increases below 30 K monotonically with decreasing temperature. Indications have been found on the existence of dynamically fluctuating internal fields, presumably associated with the 4f electron dynamics.

121 Sb nuclear magnetic resonance (NMR) measurements have been carried out on a single-crystalline PrOs 4 Sb 12 for magnetic field (H) up to 15 T. A well-split 121 Sb-NMR spectrum was observed for H ∥ 〈 0 0 1 〉 . We calculated the electric field gradient (EFG) of Sb site by a FLAPW method based on LDA. We have identified the observed NMR lines corresponding to three different Sb sites with asymmetric EFG tensors.

We performed inelastic neutron scattering experiment to investigate a role of 4f electrons in the M–I transition of PrRu 4 P 12 at T M – I = 63 K . Below about 20 K, there are resolution-limited sharp excitations represented by two kinds of crystal-field schemes of Pr ions due to the low-temperature superlattice formation. With increasing temperature, the excitation peaks shift and broaden considerably. Such drastic evolution of the crystal-field excitations in PrRu 4 P 12 indicates strong hybridization between 4f and conduction electrons.

The order parameters in the antiferro-quadrupolar (AFQ) ordered phase of PrFe 4 P 12 have been investigated by polarized neutron diffraction measurement. By observing the interference effect of scattering amplitudes from the Fe-ion displacement and the antiferromagnetic component induced by the magnetic field applied along the [0,0,1] axis, we conclude that the dominant ordered quadrupole is O 2 0 in the low-temperature phase. By inelastic neutron scattering experiment for a single-crystal sample, we observed sharp magnetic excitations in the AFQ-ordered phase, whose maximum intensity appears at the Brillouin zone center and minimum at the zone boundary. This fact indicates the existence of ferromagnetic correlation between Pr ions in the AFQ-ordered phase.

We have investigated elastic properties of the filled skutterudite compounds LaRu4Sb12 and PrRu4Sb12 by means of ultrasonic measurement. No elastic softening towards low temperature in both (C11−C12)/2 and C44, has been observed in PrRu4Sb12. The obtained result suggests that Pr3+ Γ1 singlet ground state is realized in PrRu4Sb12 under the crystalline electric field (CEF). It also indicates that Γ1 singlet ground state is possible in other Pr-based filled skutterudite compounds.

We report the first successful observation of the de Haas–van Alphen effect in the filled skutterudite LaRu4P12. Observed Fermi surfaces (FSs) are close to those expected from the band calculation. The effective mass m c ∗ is roughly twice compared to the calculated band mass. Assuming the similarity of FS between LaRu4P12 and PrRu4P12, the FS nesting with q=(1,0,0) as a possible origin of the M–I transition in PrRu4P12 is discussed.

We have done de Haas-van Alphen experiments for NdIn3 in high fields up to 17 T. The drastic change of the Fermi surfaces at the metamagnetic transition is presented.

We report transport studies on high-quality CeRu 4 Sb 12 single crystals. At temperatures below ∼3 K , both the electrical resistivity and thermoelectric power exhibit apparent anomalies correlated with a non-Fermi-liquid ground state. We succeeded in observing Shubnikov–de Haas oscillations both in the magnetoresistance and Hall effect. The result along with the Hall effect suggests a small semi-metallic Fermi surface (FS) made of highly correlated electrons in contrast with the large FS for LaRu 4 Sb 12 .

Nature of mysterious phase transition of the filled-skutterudite PrFe4P12 has been studied by X-ray diffraction experiments. Superlattice reflections characterized by a wave vector q =(1, 0, 0) were observed below the transition temperature of 6.5 K at zero magnetic field. The observed pattern in the reciprocal space is explained by the formation of modulated structure with dominant Fe-ion displacements by about 0.02% of the lattice constant. This result is consistent with the theoretically predicted nesting of Fermi surface producing a gap formation in electronic structure.