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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.

PrOs 4 Sb 12 is the first Pr-based heavy fermion superconductor exhibiting intriguing behavior both in the superconducting and normal states. To clarify the origin of the behavior, intensive studies have been made by variety of experimental techniques. As a result of intensive studies, the normal state properties have been mostly clarified, however, there still remain important questions on the superconducting state of this material. We discuss the present status of the understanding of superconducting state in relation with the normal state properties of this material.

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.

Local magnetization measurements were performed by micro Hall-probe technique in the superconducting state of PrOs 4 Sb 12 , in which the time-reversal symmetry is broken. We have observed positive, i.e. ferromagnetic like, local magnetic induction just below the superconducting transition temperature, which changes into a usual Meissner signal as temperature goes down. The signal is dependent on the measuring points of the sample surface, and may be interpreted as a sign of the dilute magnetic moment arising from spontaneous current in the superconducting state. The characteristics of the superconductivity in PrOs 4 Sb 12 including the presence of possible spontaneous magnetization is discussed based on the local magnetization measurements.

CeRh3B2 is a ferromagnet with exceptionally high Curie temperature ( T C = 115 K) among the Ce compounds with no transition metal magnetic element. Magnetic circular dichroism of soft X-ray photoabsorption (XMCD) has revealed that the orbital angular momentum of Ce 4f electron is quenched to some extent. In addition, preferential occupation of the Ce 4f orbital pointing to the c-axis has been confirmed by the soft X-ray photoabsorption measurements with different geometries. On the other hand, resonant photoemission around the Ce 3d → 4f photoabsorption edge has been adopted in order to disentangle the Ce 4f and Rh 4d components in the valence band. The Rh 4d spectrum is qualitatively consistent with the band structure calculation. Although the Ce 4f spectrum has a peak near the Fermi level, the spectral shape is qualitatively different from that of the typical Kondo materials and is rather similar to that of itinerant Ce 4f systems.

In the filled skutterudite compounds (RT 4 X 12 : R rare earth, T=Fe, Ru, Os, and X=pnictogen-ions), various attractive features have been found by replacing the constituent elements, e.g., metal–insulator transition in PrRu 4 P 12 , competing heavy fermion and multipolar ordered states in PrFe 4 P 12 , heavy fermion superconductivity in PrOs 4 Sb 12 , etc. Most of these features are realized probably by the unique crystal structure; R is embedded in the cage made of 12X and eight T ions, so that the strong c–f hybridization becomes a common feature irrespective of the constituent elements. The strong c–f hybridization in skutterudites enables the strongly correlated electron behaviors even in the systems containing rare-earth elements with more than one 4f-electron. The effect of X-replacement appears most apparently as a change in c–f hybridization, as is naturally inferred from the large difference in lattice constant. In addition to the c–f hybridization effect, it was recognized that f-electron's multipolar degrees of freedom and relatively small crystalline electric field splitting are important factors to understand the aforementioned novel phenomena.

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 performed local magnetization measurements in the superconducting state of PrOs 4 Sb 12 using micro-Hall probe. For a magnetic field applied along one of the principal axes, a remarkable dip structure in the magnetization hysteresis loop was observed close to zero local magnetic induction. The anomaly is most pronounced when the magnetic field is applied only in a certain axis of the crystal, and is not observed along other two axes. We believe that the anomaly close to zero induction is universal to the spin-triplet superconductors with broken time-reversal symmetry.

The resonant inverse photoemission study of Pr metal, PrAl2 and PrFe4P12 was performed in the Pr 4d–4f excitation region. The Pr 4f spectra strongly depend on the excitation energy due to the resonant effect and have two broad peaks originated from f 3 final states. The intensity of the shoulder in the vicinity of the Fermi energy for PrFe4P12, which mainly contains f 2 final states on the analogy of the Ce 4f spectra, is much stronger than that for Pr metal and PrAl2 due to the strong hybridization between Pr 4f electrons and valence-band electrons in PrFe4P12. This result is consistent with the conclusion derived from the photoemission spectroscopy.

We have measured the temperature dependence of magnetic circular dichroism (MCD) of 4d–4f resonant X-ray emission for Gd and DyCo 2 . A parameter to correct the self-absorption effect for the MCD spectra is introduced to estimate the 4f magnetization. However, the discrepancy between the magnetizations estimated from the MCD measurement and the SQUID measurement still remains, especially for DyCo 2 at the low temperature.

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.

We report the experimental results on resistivity, Hall coefficient and thermoelectric power in REFe4P12 single crystals (RE: La, Ce, Pr and Nd), with a special emphasis on the magnetic field effect on PrFe4P12 that is one of the most attractive skutterudites. In all the transport properties of PrFe4P12, clear anomalies, anisotropic with respect to the magnetic field direction, have been observed associated with the metamagnetic transition. Unusual features found in the magneto-transport properties point to the unique ground state of this compound.

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 magnetic characteristics of monatomic scale epitaxial multilayers Fe x /Au 2− x utilizing the electrical and Hall resistivity measurements. The results indicate the existence of two distinct conduction electron scattering centers, one with ferromagnetic perpendicular anisotropy and the other with paramagnetic (or superparamagnetic) character. For x =0.5, the anomalous Hall contribution associated with the former is found to show a temperature-driven sign change at low temperatures. Possible two-dimensional Fe cluster formation is discussed in relation with the two scattering centers.

We have performed $^{31}\mathrm{P}$-NMR measurements on single-crystalline CeRuPO under pressure in order to understand the variation in magnetic character against pressure. The nuclear magnetic resonance (NMR) spectra for $\text{H}\ensuremath{\perp}c$ and $\text{H}\ensuremath{\parallel}c$ at 2.15 GPa split below the ordered temperature, which is microscopic evidence of the change in the magnetic ground state from the ferromagnetic state at ambient pressure to the antiferromagnetic (AFM) state under pressure. The analysis of NMR spectra suggests that the magnetic structure in the AFM state is the stripe-type AFM state with the AFM moment ${m}_{\mathrm{AFM}}\ensuremath{\perp}c$ axis, and it changes with a magnetic field perpendicular to the $c\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{axis}$. In addition, the dimensionality of magnetic correlations in the spin and $k$ space is estimated. We reveal that three-dimensional magnetic correlations in CeRuPO are robust against pressure, which is quite different from the suppression of the magnetic correlations along the $c\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{axis}$ by Fe substitution in $\text{Ce}({\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\text{PO}$.

This work was supported by Grants-in-Aid for Scientific Research (No. 21204456, No. 21102515, and No. 2430087) from JSPS and MEXT, and by a Basic Science Research Grant from the Sumitomo Foundation.

We have investigated the magnetic properties of the Heusler compound YbPd2Sn, which exhibits coexistence of superconductivity (Tc=2.3 K) and antiferromagnetism (TN=0.22 K). In the temperature dependence of the specific heat in applied magnetic fields, we found an anomalous broadening of the Schottky peak caused by the Zeeman splitting of the Γ7 crystalline–electric-field (CEF) ground state of the Yb3+ ions. This fact indicates the existence of antiferromagnetic (AF) short-range correlations, which are hardly destroyed by magnetic field. An anomalous temperature dependence of the upper critical field Bc2 with a maximum around 0.8 K can be explained by taking into account the pair breaking effect of Cooper pairs caused by the Yb-moment fluctuation through an exchange interaction. We propose that a weak magnetic coupling between the cubic Pd sublattice responsible for superconductivity and the NaCl-type Yb–Sn sublattice responsible for the AF magnetic ordering play an essential role for the coexistence.

We report the results of 69Ga- and 71Ga-NMR measurements on NdGa2 at temperatures between 0.1 and and in applied magnetic fields between zero and 74 kOe. NdGa2 orders antiferromagnetically below and undergoes several metamagnetic transitions in external magnetic fields. In zero applied magnetic field and below the temperature dependence of the spin-lattice relaxation rate T1 -1 ( T ) shows a large linear-in-T term, about two orders of magnitude higher than for the reference compound LaGa2. This strong enhancement confirms the presence of low-energy excitations in the antiferromagnetic phase of NdGa2 as was previously indicated by specific heat data. Above , T1 -1 ( T ) is dominated by an exponential term, which we associate with excitations between the lowest energy levels of the f-electron system. The separation of these energy levels is determined by exchange, crystal-field and Zeeman interactions.

The bias and temperature dependences of tunneling magnetoresistance (TMR) woe investigated for Fe/Al2O3/CoFe with a relatively small tunneling impedance (resistance times area: R × A∼1.8 × 10-9Ωm2). Despite a rather thinner and lower tunneling barrier, the change in conductance under a magnetic field obeys an a-bT3/2 dependence, suggesting the dominance of the contribution of the decrease in effective spin polarization with temperature. In addition to a monotonic decrease in the magnitude of the TMR with increasing bias voltage, there is a slight change in the field dependence in low fields.

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.

We have carried out soft and hard X-ray spectroscopy of Sm-based filled skutterudites SmFe 4 P 12 and SmOs 4 Sb 12 . Sm 3d–4f resonant photoemission and Sm 3d–4f X-ray absorption spectra suggest that SmOs 4 Sb 12 has much more strongly mixed Sm valence state than SmFe 4 P 12 in the bulk. On the other hand, the Sm trivalent state is dominant on the surface of SmOs 4 Sb 12 .

Recent studies of the Nernst effect in a number of heavy-fermion systems have led to a previously unsuspected result. In some circumstances, the Nernst signal of quasi-particles becomes very large and can easily overwhelm the well-known Nernst effect produced by the movement of the superconducting vortices under the influence of a thermal gradient. In particular, the Nernst coefficient attains an exceptionally large magnitude in the ordered states of URu 2 Si 2 and PrFe 4 P 12 . In all these cases, the order of magnitude of the Nernst signal appears compatible with the Boltzmann picture which links the Nernst coefficient to the energy-dependence of the Hall angle.

We present the relevant elastic constants for a single crystal of the La-substituted Pr 1 - x La x Os 4 Sb 12 system for x = 0.4 by means of ultrasonic measurements. A characteristic softening toward low temperature was observed below around 30 K in the temperature dependence of the elastic constants. In addition, a clear increase was observed around 40 K. These results can be reasonably explained by the 4f electric state of the Pr ion formed by the crystalline electric field effect combined with a rattling motion of an off-center Pr ion in a cage of Sb-icosahedron.

We have carried out electrical resistivity measurements on PrFe 4 P 12 under high pressure using some kinds of pressure-transmitting mediums in order to investigate the effect of uniaxial stress on a pressure-induced metal–insulator (M–I) transition. In the measurement under uniaxial stress, the M–I transition appears at a lower pressure than in the case of quasi-hydrostatic pressure, indicating that the uniaxial stress is favorable to the insulating phase. On the other hand, the M–I transition appears suddenly with finite M–I transition temperature between 2.55 and 2.66 GPa in the measurement under hydrostatic pressure. This fact suggests that the transition from the antiferroquadrupolar metallic phase to the insulating phase is of first order.

We present dc magnetization data for the heavy fermion superconductor PrOs 4 Sb 12 under pressures up to 1.45 GPa . With increasing pressure, the superconducting region becomes smaller, while the antiferroquadrupolar region moves significantly to lower fields. Our results strongly suggest that the crystal field energy splitting of a Γ 1 singlet ground state and a Γ 4 ( 2 ) triplet first excited state decreases with an increase in pressure.

We have performed measurements of the angle-resolved specific heat C ( H , ϕ ) on PrOs 4 Sb 12 and the well-known s-wave superconductor CeRu 2 in order to gain information about their respective superconducting (SC) gap structure. PrOs 4 Sb 12 exhibits pronounced minima (maxima) in C ( H , ϕ ) along the [1 0 0] ([1 1 1]) directions. CeRu 2 shows minima (maxima) along the [1 1 0] ([1 0 0]) directions. The field dependence of the oscillation amplitude of C ( ϕ ) , | A | , shows similar behavior for both suggesting finite minima in the SC gap, where C ( ϕ ) is smallest. On the other hand, in comparison the temperature dependence of | A | shows aberrant behavior.

The ceramic YBa 2 Cu 3 O 7– δ samples composed of sub-micron size grains are considered as random Josephson-coupled networks of 0 and π junctions and show successive phase transition. With decreasing temperature, a transition occurs at first inside each grain at T c1 (just the same with the bulk superconducting transition temperature T c ) and successively another transition occurs among the grains at T c2 ( T c1 ). The various ceramic samples were prepared with not only changing the calcination and the sintering temperature but also changing the pressure in pelleterizing process between calcination and sintering, using sub-micron size powder prepared by the coprecipitation method. With increasing sintering temperature (equal to calcination temperature), T c2 increases and both the absolute values of linear resistivity and the divergence of nonlinear resistivity (just above T c2 ) decrease. With increasing the density of sample by increasing the pressure in the pelleterizing process, the T c2 and the absolute value of conductivity increase monotonously. These effects may be considered as the changes in Josephson-couple strength between superconducting grains in the ceramic specimens.

Sb 121 , 123 nuclear spin-lattice relaxation rate ( 1 / T 1 ) 121 , 123 has been measured for the filled skutterudite superconductor PrOs 4 Sb 12 with T c = 1.85 K in order to probe the multipole fluctuations. Within the conventional analyses for NMR relaxation process, we found that 1 / T 1 for H ∥ [ 0 0 1 ] is suppressed by magnetic fields. However, the ratio of ( 1 / T 1 ) 121 to ( 1 / T 1 ) 123 is close to 3.4, implying that the magnetic relaxation process is dominant under the magnetic field. These results cannot be explained by a simple magnetic relaxation mechanism due to the spin–dipolar relaxation process.

We have succeeded in growing single crystals of the filled skutterudite compound SmRu 4 P 12 . To clarify the magnetic anisotropy, we have measured the specific heat C and the magnetization M as functions of temperature T , magnetic field H , and magnetic fields directions. In the H dependence of M at T = 15.5 K , between the two characteristic temperatures T MI = 17.5 and T * = 14 K , we have observed clear magnetic anisotropy under high magnetic fields. The anisotropy decreases with decreasing H , and becomes within the experimental uncertainty below 1 T. T MI determined by the specific heat under magnetic fields is basically field independent up to 8 T, while T * exhibits evident anisotropy. These results could be a clue to understand the ordered state in this material.

We have performed resonant X-ray scattering study on PrRu 4 P 12 at the Pr L 2 and L 3 edges. Below the metal–insulator transition temperature ( T MI ≃ 62 K ) , resonant signal appears in the superlattice reflections in addition to the lattice distortion, which demonstrates that different electronic states of the Pr atoms are alternatingly ordered in the BCC unit cell. Polarization analysis of the scattered X-ray shows that the intensity changing the polarization from π to σ is absent or very weak. This means that the difference of the anomalous scattering factor between two Pr atoms in the BCC unit cell, which reflects the character of the intermediate 5d states of the resonant process, is almost isotropic.

We report on superconducting (SC) properties in a series of filled-skutterudite compounds (Pr 1− x La x )Os 4 Sb 12 through the Sb nuclear-quadrupole-resonance (NQR). In the SC state, the nuclear spin-lattice relaxation rate 1/ T 1 Pr at Pr-cage decreases exponentially with no coherence peak below T C , consistent with the results for the pure PrOs 4 Sb 12 . In the Pr-rich compounds of x = 0.05 and 0.2, the residual density of states (RDOS) at the Fermi level are induced below T C due to the La substitution. It is concluded that the RDOS is not due to the impurity effect that used to be observed in unconventional superconductors with line-node gap. Rather, a part of the Fermi surface that contributes to 5.5% of the total is suggested to become gapless for x = 0.05 and 0.2, yielding the RDOS. For the La-rich compounds of x = 0.4 , 0.8 and 1, as the Pr-substitution for La increases, T C increases and a size of energy gap increases. The Pr-substitution for La makes the pairing interaction for forming the Cooper pairs strong and causes an anisotropy in its energy-gap structure.

We report results of La-NMR and P-NMR/Sb-NQR measurements on the La-based filled skutterudite LaT 4 X 12 ( T = Fe , Ru , Os ; X = P , Sb ) . The temperature dependence of 1 / T 1 at the La site is different from that at the X site in LaOs 4 Sb 12 , LaFe 4 Sb 12 , and LaOs 4 P 12 , although no site-dependence in 1 / T 1 is observed in LaRu 4 Sb 12 , LaRu 4 P 12 , and LaFe 4 P 12 . We consider that rattling phonon contributes to additional 1 / T 1 at the La site, resulting in that a temperature dependence of 1 / T 1 at the La site differs from that at the X site.

121 Sb nuclear magnetic resonance (NMR) of filled skutterudite compound CeOs 4 Sb 12 has been carried out to investigate a spin fluctuation below T ∼ 25 K . In spite of a powdered sample, many sharp peaks, similar to a data for single crystal, were observed because of an orientation of the sample from the anisotropy of the magnetization. A numerical calculation well reproduces resonance fields for Sb(1) sites with H ∥ V zz . The nuclear spin-lattice relaxation rate divided by temperature 1 / T 1 T shows continuous decrease with increasing magnetic field, indicating a suppression of the spin fluctuation by the field.

We have performed AC specific heat measurements on the filled skutterudite PrOs 4 Sb 12 at low temperature and under high pressure. The double superconducting transition was followed up to 4.3 GPa in a diamond-anvil cell with argon as a pressure-transmitting medium. At low pressure, the slope ∂ T c 1 / ∂ P is at least 20% smaller than ∂ T c 2 / ∂ P and both of them are consistent with previous thermal expansion measurements by Oeschler et al. However, above 1 GPa the behavior of the two transitions is similar, with T c 1 - T c 2 stabilizing around 200 mK. So the superconducting phase diagram under pressure (as well as under magnetic field) does not imply an intrinsic double superconducting transition. We discuss the origin of the double transition in the light of the pressure dependence of the T c 's and propose an explanation for the disparity between the slopes of T c at low pressure.

We measured the high-field magnetization M for skutterudite compounds of a paramagnet PrFe 4 Sb 12 and a ferromagnet SmOs 4 Sb 12 to investigate the characteristic magnetic properties. In PrFe 4 Sb 12 , we observed a peak structure in the d M / d H curve, which might be a level crossing effect between the singlet ground state and the excited state with a splitting energy of about 20 K in the crystalline electric field (CEF) scheme. In SmOs 4 Sb 12 , the anisotropy of magnetization for H ∥ [ 1 0 0 ] and [1 1 0] was not observed, which is inconsistent with the quartet ground state in the CEF scheme.