Search

We report on the first observation of the de Haas–van Alphen (dHvA) effect in CrAs under hydrostatic pressures of up to 2.2 GPa, exceeding the critical pressure Pc ∼ 0.7 GPa, where the antiferromag...

To investigate the electronic state of CeRu2Al10, which exhibits an unusual antiferromagnetic order below TN = 27 K, we measured the electrical resistivity ρ, the transverse magnetoresistance (TMR)...

We have investigated the Fermi surface properties in the Pr-based heavy-fermion superconductor PrTi2Al20 and its reference compound LaTi2Al20 by means of the de Haas–van Alphen effect experiments and the band-structure calculation. The topology of Fermi surface in PrTi2Al20 is close to that of the reference compound LaTi2Al20, indicating a localized nature of 4f-electrons. Whereas the localized nature of 4f-electrons, we have confirmed a highly enhanced cyclotron effective mass up to 12 m0 (m0 is a rest mass of electron), which is enhanced about 5 times compared to that in LaTi2Al20.

We performed 75As nuclear quadrupole resonance (NQR) measurements on single-crystalline Ru1−xRhxAs to study the relationship between the superconductivity observed in a limited Rh-substitution regi...

We have studied the multi-band electronic structure of ferromagnetic CeRuPO (TC = 15 K) by means of angle-resolved photoemission spectroscopy (ARPES). The ARPES results show that three hole bands exist around the zone center and two of them cross the Fermi level (EF). Around the zone corner, two electron bands are observed and cross EF. These hole and electron bands, which can be assigned to the Ru 4d bands, are basically consistent with the band-structure calculation including their orbital characters. However, one of the electron bands with Ru 4d 3z2 − r2 character is strongly renormalized indicating correlation effect due to hybridization with the Ce 4f orbitals. The Ru 4d 3z2 − r2 band changes across TC suggesting that the out-of-plane 3z2 − r2 orbital channel plays essential roles in the ferromagnetism.

Magnetization, specific heat, and electrical resistivity measurements have been performed on polycrystalline samples of a new cubic compound, Ce6Pd13Zn4. This compound exhibits metallic behavior and is classified as a Kondo-lattice system. The trivalent Ce ions are responsible for the antiferromagnetic transition at TN = 3.3 K and the phase transition at \(T'_{\text{N}} = 1.3\) K with the formation of superzone gaps. The increase in magnetic susceptibilities below TN and the considerably large value of the specific heat divided by temperature (1.25 J·Ce-mol−1·K−2) imply the existence of non-ordered Ce magnetic moments due to the geometrical frustration on the octahedral Ce sublattice.

We report on the systematic evolution of normal-state properties and superconducting characteristics in filled-skutterudite compounds (Pr 1- x La x )Os 4 Sb 12 via Sb-NQR experiments. In the superconducting state for Pr-rich compounds, 1/ T 1 P r exponentially decreases down to T =0.7 K with no coherence peak below T c as well as in PrOs 4 Sb 12 . A remarkable finding is that the residual density of states (RDOS) at the Fermi level below T c is induced by La substitution for Pr. However, the RDOS does not increase and T c does not decrease with increasing La content up to x =0.2. These results are proposed to be understood in terms of a multiband-superconductivity model that assumes a full gap for part of the FS and an anisotropic gap for a small FS. The novel strong-coupling superconductivity in PrOs 4 Sb 12 is inferred to be mediated by the local interaction between 4 f 2 -derived crystal-electric-field states with the electric quadrupole degree of freedom and conduction electrons. This coupling causes ...

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.

Filled skutterudites show various interesting physical properties, such as unconventional superconductivity, anomalous magnetism, metal–insulator transition, heavy fermion behavior and so on. We have investigated the multipole ordering state of SmRu 4 P 12 by ultrasonic measurements under extreme conditions of low temperature, high pressure and high magnetic field by pulsed method, so far. Recently, the angle dependence of the elastic constant has been measured. The elastic constant shows interesting and peculiar angle dependence below the metal–insulator transition temperature in the magnetic field. It will enable us to reveal characteristic multipole ordered state in SmRu 4 P 12 . In this paper, we will report recent status of the multipole ordered state of SmRu 4 P 12 , and discuss the recent results of our elastic constant measurements and the relation to the octupole ordering scenario proposed for this system.

We review inelastic neutron scattering experiments performed on single crystal of the heavy fermion superconductor PrOs 4 Sb 12 . The low lying excitation spectrum consist of a singlet–triplet crystal field transition modulated by quadrupolar intersite interactions. The evolution of this so-called quadrupolar exciton on crossing the superconducting transition was studied both as a function of temperature and magnetic field. Our measurements point out the coupling between the quadrupolar fluctuations and the superconductivity.

Last five years we have mainly investigated phonons in the following cage compounds by Raman scattering: RT 4 X 12 , A 8 Ga 16 Ge 30 , La 3 Pd 20 Ge 6 , RB 6 , KOs 2 O 6 . From this systematic investigation we have clarified the following dynamical properties of the guest ion trapped in the large cages. The vibration of the guest ion is highly anharmonic due to weak atomic interaction between the guest ion and cage atom. This anharmonicity universally decreases the vibration energy of the guest ion with decreasing temperature. The existence of carriers decreases the interaction between the guest atoms and changes the dispersion curve of the guest ion mode to flat, which is the experimental evidence of the weak interaction between the guest atoms. For thermoelectricity, it is found in A 8 Ga 16 Ge 30 that the off-center rattling decreases lattice thermal conductivity.

We report measurements of the dc magnetization of a series of Pr 1- x La x Fe 4 P 12 single crystals (0 ≤ x ≤0.15). The nonmagnetic order (A phase) evolves to a short-range one at a very small La concentration of about 3%, above which ferromagnetic ordering occurs. Both the ferromagnetic transition temperature and spontaneous magnetization increase abruptly above x > 0.03, although the Weiss temperature decreases substantially with increasing x . These results suggest a competition between the A phase and ferromagnetic state, and the role of Fermi surface nesting on the A phase transition is emphasized. The effect of La impurities on the high-field ordered state (B phase) in the [111] direction is also presented.

We have succeeded in observing the de Haas–van Alphen effect in PrOs 4 P 12 which shows no anomaly down to 50 mK in contrast to PrFe 4 P 12 and PrRu 4 P 12 . The Fermi surface (FS) topology is reasonably explained by the energy band structure calculation based on a full potential linearized augmented-plane-wave method within the local-density-approximation. Whereas the localized nature of 4 f -electrons, we have confirmed a highly enhanced cyclotron effective mass up to 18 m 0 in PrOs 4 P 12 , which is enhanced 3.8 times compared to that of LaOs 4 P 12 . No nesting property with q =(1,0,0) in the FS of PrOs 4 P 12 has been confirmed, which suggests both the 4 f -electron's contribution and the FS nesting property play the role for the phase transitions in PrFe 4 P 12 and PrRu 4 P 12 .

Field-orientation dependent specific heat C ( H ,θ) and the magnetization M ( H ,θ) have been measured on the Pr-based filled skutterudite compounds PrOs 4 Sb 12 and PrFe 4 P 12 . In PrOs 4 Sb 12 , fourfold angular oscillations of C ( H ,θ) have been observed in the normal state above the upper critical field H c2 as well as in the vortex state. Field variation of the normal-state oscillation in C ( H ,θ) strongly suggests that antiferroquadrupolar fluctuations exist in the wide region above H c2 . The oscillation exhibits a sign reversal below H c2 , whose origin is attributed to the nodal structure in which the superconducting gap takes minima (point nodes) along [100] directions. In PrFe 4 P 12 , an exotic ordered phase (A-phase) below T A ( H =0)=6.5 K has been examined. Both M ( H ,θ) within the A-phase and T A ( H ,θ) show a smooth angular variation with a cubic anisotropy for H rotated in the (001) and (1 1 0) planes. The results strongly suggest that cubic symmetry is preserved in the A-phase; the...

We report resistivity, nuclear quadrupole resonance (NQR), and magnetization measurements under high pressures in a novel heavy fermion compounds SmOs 4 Sb 12 . The resistivity and nuclear spin–lattice relaxation rate 1/ T 1 show the coherent behavior below a characteristic temperature T * = 20-25 K at ambient pressure. Nuclear spin–spin relaxation rate 1/ T 2 was found to show a peak of electrical origin around T * . Some charge fluctuation is considered to play an important role in the Kondo effect of SmOs 4 Sb 12 . On the other hand, the magnetization at low temperature does not increase under pressures, although the transition temperature increases. The NQR signals are strongly suppressed around the transition temperature and vanishes at low temperatures. These suggest that the ordered state in SmOs 4 Sb 12 is not simple ferromagnetism.

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.

We have performed ultrasonic experiments on clathrate compounds ROs 4 Sb 12 and R 3 Pd 20 Ge 6 to explore rattling and tunneling of a guest rattler's motion. We have found frequency dependence of the elastic constants and ultrasonic attenuations, which is called as ultrasonic dispersion, due to thermally activated rattling. At low temperatures, Curie-type elastic softening obeying 1/ T with antiferro-type intersite interaction, which corresponds to quantum tunneling, appears and continues even to the lowest 20 mK in LaOs 4 Sb 12 and La 3 Pd 20 Ge 6 . The understanding of the local charge fluctuation, rattling and tunneling, being coupled to conduction electrons in metals is a basic issue in the aspect of electron–phonon interaction in strongly correlated phenomena such as heavy fermion state, superconductivity and multi-channel Kondo screening effect.

We report nuclear quadrupole resonance (NQR) measurement at ambient pressure and resistivity measurement under high pressure in the new heavy fermion (HF) compound SmOs 4 Sb 12 . The NQR spectrum and the nuclear spin–lattice relaxation rate (1/ T 1 ) clearly demonstrate the bulk nature of ferromagnetic ordering below T C ∼2 K. The temperature dependence of 1/ T 1 exhibits the characteristic behaviors of a Kondo lattice. The application of pressure decreases Kondo temperature and stabilizes the magnetic ordering, which is the same tendency as that found in some Yb-based compounds. The obtained phase diagram indicates SmOs 4 Sb 12 to be in the vicinity of its ferromagnetic critical point.

We have studied the origin of the successive phase transitions in filled skutterudite SmRu 4 P 12 based on the results of elastic constant measurements. The temperature dependence of the elastic constants shows a sharp drop at the metal–insulator (M–I) transition of T MI = 16.5 K in a zero field. The successive transition at T N = 14 K is indistinct in a low field. The anomaly at T MI disappears with increasing field. A clear softening toward T N appears in the elastic constants in a high magnetic field. This result suggests a breakdown of the time reversal symmetry in the temperature range between T MI and T N . Such a breakdown is considered to be due to the coupling between the dipole moment and the octupole moment, which is allowed only in T h symmetry.

We have carried out nuclear quadrupole resonance (NQR) measurements in the filled skutterudite compounds Pr(Os 1- x Ru x ) 4 Sb 12 ( x =0.1, 0.2), in order to gain insights into the symmetry of the superconducting gap function. Upon replacing Os with Ru, the spin–lattice relaxation rate 1/ T 1 becomes proportional to temperature ( T ) at low T far below T c , and the magnitude of (1/ T 1 T ) low- T increases with increasing Ru content. These results indicate that a finite density of states is induced at the Fermi level by the impurity, and thus suggest that there exist nodes in the gap function of PrOs 4 Sb 12 .

Electronic state evolution in the metal-non-metal transition of PrRu4P12 has been studied by X-ray and polarized neutron diffraction experiments. It has been revealed that, in the low-temperature non-metallic phase, two inequivalent crystal-field (CF) schemes of Pr3+ 4f^2 electrons with Gamma_1 and Gamma_4^(2) ground states are located at Pr1 and Pr2 sites forming the bcc unit cell surrounded by the smaller and larger cubic Ru-ion sublattices, respectively. This modulated electronic state can be explained by the p-f hybridization mechanism taking two intermediate states of 4f^1 and 4f^3. The p-f hybridization effect plays an important role for the electronic energy gain in the metal-non-metal transition originated from the Fermi surface nesting., 5 pages, 5 figures. Accepted by J. Phys. Soc. Jpn

Bulk-sensitive photoemission spectroscopy using hard x-ray up to h ν=5450 eV has been performed for heavy-fermion Pr compounds. A significant spectral intensity of the |3 d 9 4 f 3 > final state was observed in Pr 3 d core-level spectra of PrFe 4 P 12 at both h ν=2450 and 5450 eV. The spectra obtained by the configuration-interaction cluster model calculation reproduce the experimental spectra for well-hybridized PrFe 4 P 12 and localized Pr metal. Results of the calculations suggest that the Kondo behavior observed in PrFe 4 P 12 arise from the mixing of the |4 f 2 > and |4 f 3 > configurations due to the c – f hybridization. Whereas a contribution possibly from a disordered surface to the spectrum can be negligible for the fractured PrFe 4 P 12 , they cannot be disregarded for the scraped PrSn 3 and Pr metal even in the high-energy x-ray region.

High-field magnetization measurements have been performed on the filled skutterudite compounds PrFe 4 P 12 and PrOs 4 Sb 12 with the cubic crystal structure. Magnetization curves along the main symmetry axes of PrFe 4 P 12 show clear anisotropy at high fields. On the basis of this result, we discuss the possible crystalline electric field (CEF) scheme in PrFe 4 P 12 which reproduce the anisotropic feature at high magnetic fields. On the other hand, the magnetization of PrOs 4 Sb 12 for H ∥ at 0.1 K shows weak anomalies at 4.5 and 15 T, which correspond to the phase boundaries of the field-induced antiferroquadrupolar ordering phase. The observed magnetization curve is well explained by the CEF scheme determined by the previous neutron scattering experiments.

Specific heat and transport measurements have revealed an unconventional heavy-fermion (HF) state in SmOs4Sb12 single crystals. The electronic specific-heat coefficient (gamma=0.82 J/K^2mol) and the coefficient (A) of the quadratic temperature dependence of electrical resistivity are largely enhanced, although the ratio A/gamma^2 is reduced from the Kadowaki-Woods ratio of HF materials. Both gamma and A do not show any significant decrease in applied field in contrast with Ce-based HF compounds, suggesting an unconventional origin of the heavy quasiparticles. A weak ferromagnetic ordering sets in below about 3 K, probably originating in the itinerant quasiparticles., Comment: 5 pages, 8 figures. To appear in J. Phys. Soc. Jpn., vol.74 (2005)

We have made ultrasonic measurements on a filled skutterudite PrOs 4 Sb 12 showing a heavy fermion superconductivity at T C =1.85 K. The softening of the elastic constant with decreasing temperatur...

Novel Kondo-like behaviors realized in the Pr-based filled skutterudites are reviewed based on the electronic transport properties, specific heat and de Haas–van Alphen (dHvA) experiments, along wi...

Dc magnetization and specific heat measurements on the filled skutterudite PrFe 4 P 12 reveal the presence of a new ordered state (B phase) in high magnetic fields applied along the [111] direction. The present data suggest that the B phase exists in a limited angular range around the [111] direction. A possible origin of the B phase is discussed within the framework of the Γ 1 –Γ 4 model.

Magnetic excitations in a single crystal of heavy-fermion superconductor PrOs 4 Sb 12 have been studied by inelastic neutron scattering (INS) at low temperatures under high magnetic fields. The crystal field (CF) excitations at 0.7 meV observed in a previous INS study of a polycrystal sample were confirmed in this study on a single crystal. The observed excitations in the zero magnetic field soften around a wave vector (1,0,0), which is the modulation vector of the field-induced antiferro-quadrupolar ordering, suggesting the presence of quadrupolar instability. The observed field dependence of excitations is better reproduced in a singlet ground-state model rather than in a non-Kramers doublet ground-state model. In a high resolution experiment, furthermore, the change of the excitation spectra at (1,0,0) was observed on passing through the superconducting transition temperature, indicating that the excitation and the superconductivity are closely related in PrOs 4 Sb 12 .

We have studied the optical conductivity of the filled skutterudite CeOs 4 Sb 12 . The optical conductivity spectra show a strong temperature dependence and reveal an energy gap of ∼30 meV below 60...

The element specific magnetic susceptibilities of some rare-earth compounds are estimated by measuring magnetic circular dichroism at rare-earth M 4,5 absorption edges. The temperature dependences ...

We investigated the magnetic phase diagram of the first Pr-based heavy fermion superconductor PrOs 4 Sb 12 by means of high-resolution dc magnetization measurements in low temperatures down to 0.06 K. The temperature dependence of the magnetization M ( T ) at 0.1 kOe exhibits two distinct anomalies at T c1 =1.83 K and T c2 =1.65 K, in agreement with the specific heat measurements at zero field. Increasing magnetic field H , both T c1 ( H ) and T c2 ( H ) move toward lower temperatures without showing a tendency of intersecting to each other. Above 10 kOe, the transition at T c2 ( H ) appears to merge into a line of the peak effect which is observed near the upper critical field H c2 in the isothermal M ( H ) curves, suggesting a common origin for these two phenomena. The presence of the field-induced ordered phase (called phase A here) is confirmed for three principal directions above 40 kOe, with the anisotropic A-phase transition temperature T A : T A [100] > T A [111] > T A [110] . The present results ...

Neutron diffraction experiments on the heavy-fermion superconductor PrOs 4 Sb 12 revealed that a small antiferromagnetic moment, parallel to [010] ( y -direction), is induced in the field-induced phase ( H ∥ z ). The analysis, based on the Γ 1 singlet ground state crystal field model, shows that antiferro-order of O y z -type quadrupole moments of Pr ions is formed in the field-induced phase. This strongly suggests that the crystal field ground-state of Pr ions in PrOs 4 Sb 12 is the Γ 1 singlet and the quadrupolar ordering is induced due to level crossing with an excited state under a magnetic field.

We have measured the Pr 4 d –4 f high-resolution resonant photoemission and the Pr 3 d core-level photoemission spectra of PrFe 4 P 12 and related compounds PrSn 3 , PrAl 2 and Pr metal. The Pr 4 f...

Core excited magnetic circular dichroism (MCD) signals have been measured on PrFe 4 P 12 and CePd 3 under the states without magnetic order. The inverse values of the MCD magnitudes have been plott...

We have grown a single crystal with the residual resistivity ratio of 81, and studied its electronic properties of the cubic antiferromagnetic compound UGa 3 . From the transverse magnetoresistance...

We have investigated the de Haas-van Alphen (dHvA) effect and the transverse magnetoresistance in filled skutterudite LaFe 4 P 12 and NdFe 4 P 12 . The Fermi surface of LaFe 4 P 12 consists of a nearly spherical hole sheet and a multiply connected one. These features are explained reasonably well by a full potential linearized augmented-plane-wave (FLAPW) band structure calculation. The Fermi surface of NdFe 4 P 12 is similar to those in LaFe 4 P 12 except the exchange splitting in the dHvA frequencies. The cyclotron effective masses are enhanced roughly twice compared to those of LaFe 4 P 12 due to the electron-magnon interaction.

We have studied the magnetic and Fermi surface properties in a ferromagnet UGa 2 . The electrical resistivity, elastic constant, thermal expansion coefficient, thermoelectric power, magnetic susceptibility and Hall coefficient indicate highly anisotropic properties, reflecting the hexagonal structure. The 5 f 2 - and 5 f 3 -CEF schemes, based on the localized 5 f -electron nature, are discussed to understand the magnetic susceptibility and magnetization data. We have also done the magnetoresistance and de Haas-van Alphen (dHvA) experiments to investigate the Fermi surface properties. Both data are very complicated, implying that the Fermi surface is a multiply-connected one. They are explained neither by the localized 5 f 2 - and 5 f 3 -core models nor by the spin-polarized 5 f -itinerant model. The cyclotron mass is, however, rather light, ranging from 0.17 to 6.2 m 0 , which is consistent with the electronic specific heat coefficient of 11 mJ/K 2 ·mol.

We have investigated high field magnetization on single-crystalline CeFe 2 Ge 2 which is classified as a heavy fermion with a non-magnetic ground state. A clear metamagnetic anomaly, which resembles those reported in CeRu 2 Si 2 , was observed around 300 kOe for the field along the c -axis. The T 3/2 dependence of the electrical resistivity in a wide temperature range from around 15 K down to 2 K, followed by T 2 dependence at lower temperatures, is apparently similar to that in CeNi 2 Ge 2 which is close to a quantum critical point.

We have measured the electrical resistivity, thermoelectric power, magnetic susceptibility, magnetoresistance and specific heat of a Pauli paramagnetic compound UB 2 . The magnetoresistance shows a...

Sb nuclear quadrupole resonance (NQR) experiment has been performed on filled skutterudite compounds CeOs 4 Sb 12 and CeRu 4 Sb 12 in order to investigate an electronic state from a microscopic point of view. Five NQR transitions originate from 121 Sb and 123 Sb were observed and the nuclear quadrupole frequency and asymmetry parameter were estimated. 1/ T 1 T 's shows exponential-like decrease with decreasing temperature above 100 K, indicating a presence of c – f hybridization gap for both compounds. 1/ T 1 T 's start to increase and show characteristic temperature dependence 1/ T 1 T ∝ T - n ( n =1/2 and 1) for CeOs 4 Sb 12 and CeRu 4 Sb 12 below ∼30 K, indicating a development of the spin fluctuation. The non-Fermi-liquid behavior in CeRu 4 Sb 12 is suggested to be caused by a magnetic instability.

We report the results of the 31 P NMR measurements on filled skutterudite phosphides with heavy lanthanide GdFe 4 P 12 and TbFe 4 P 12 with ferromagnetic transitions at T C = 23 K and 10 K, in order to investigate the magnetic properties from microscopic viewpoints. The Knight shift components have the Curie–Weiss temperature dependencies at high temperatures, similar to those of the magnetic susceptibilities. Also, the nuclear spin–lattice relaxation rate 1/ T 1 is almost constant at high temperatures due to the interaction of the 31 P nucleus with fluctuating local moments at lanthanide sites, but decreases rapidly at low temperatures below T C , indicating the suppression of the spin fluctuations by magnetic ordering.

In the s -wave superconductor LaRu 4 P 12 ( T c =7.2 K), a clear Redfield pattern was observed below T c , which arises from a triangular lattice. We observed distinct frequency dependence of the spin–lattice relaxation rate 1/ T 1 in the vortex state in contrast to the normal state. In addition, from the T -dependence of 1/ T 1 T measured at different frequencies, 1/ T 1 T at the vortex core is considered to be greater than that in the normal state, indicating that the LDOS in the vortex core region is higher than that in the normal state. This enhancement of the LDOS in the core region can be understood in terms of the bound states inside a vortex core.

We report experimental results of La-NMR in the filled skutterudite La T 4 P 12 and La T 4 Sb 12 ( T = Fe, Ru, Os). 1/ T 2 in La T 4 P 12 shows monotonic increase with temperature, which can be modeled qualitatively in terms of the nuclear dipolar interaction and the contribution from electron spins. In contrast, a distinct peak of 1/ T 2 was observed in La T 4 Sb 12 although no peak was observed in La T 4 P 12 . Considering these observations, we consider the possible origin of the different 1/ T 2 behavior in these compounds.

Elastic property of the filled skutterudite compound PrFe 4 P 12 was investigated under hydrostatic pressures by means of ultrasonic measurements for the first time. PrFe 4 P 12 undergoes a mysterious phase transition at 6.5 K, which origin is still an important issue in controversial. A clear elastic increase below 6.5 K is significantly suppressed, while the transition temperature is hardly changed under applied hydrostatic pressure up to 1.2 GPa. The present data indicate that a coupling constant between the order parameter and elastic strain becomes rapidly small with increasing hydrostatic pressure. We argue the possible primary order parameter active below 6.5 K, holding the expectation to be with total symmetry Γ 1 .

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.

Optical conductivity [σ(ω)] spectra have been measured for eight Ce-filled skutterudite compounds Ce T 4 X 12 ( T = Fe, Ru, Os; X = P, As, Sb) to systematically study their microscopic electronic structures around Fermi level. The σ(ω) spectra show large differences among X = P, As and Sb, while they show smaller differences among T = Fe, Ru and Os for a given X element. This indicates that the choice of X has larger effects on the electronic structures of Ce T 4 X 12 than that of T . The spectral shape and temperature dependence of σ(ω) suggest that the electronic structures of Ce T 4 P 12 are similar to that of a band semiconductor and those of Ce T 4 Sb 12 to that of a heavy fermion metal or a Kondo semiconductor. In the case of Ce T 4 As 12 , the σ(ω) spectra suggest intermediate characteristics between Ce T 4 P 12 and Ce T 4 Sb 12 , namely they are similar to those shown by a semiconductor with a very narrow energy gap.

Double superconducting transition in PrOs 4 Sb 12 has been studied carefully by performing the local magnetization measurements using micro-Hall probe and the magneto-optical imaging. Local magnetization curves for each element of the Hall probe reveal that there exists a significant difference in the transition temperature at each point; two phases which give different T c 's are stabilized in the crystal. Magneto-optical imaging at T < T c2 and T c2 < T < T c1 also reveals a non-uniform superconducting nature. The characteristics of superconducting double transition in PrOs 4 Sb 12 are discussed based on the local field inductions at multi-points.