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We have investigated the Hall effect of the multilayers prepared on substrates with submicron V-grooves; for both the current-at-an-angle-to-plane (CAP) geometry and the current-perpendicular-to-plane (CPP)-like geometry. No difference was found in the Hall resistivity, when only the current direction was changed with respect to the V-grooves on a single sample. By a comparison of the results on an ordinary multilayer made on a flat substrate, we found an enhanced extraordinary Hall resistivity of the CAP-type multilayers, which supports the theoretical prediction reported previously.

We report $^{123}$Sb nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs$_4$Sb$_{12}$ under high pressure. The temperature dependence of NQR frequency and the spin-lattice relaxation rate $1/T_1$ indicate that the crystal-electric-field splitting $\Delta_{\rm CEF}$ between the ground state $\Gamma_1$ singlet and the first excited state $\Gamma_4^{(2)}$ triplet decreases with increasing pressure. Ac-susceptibility measurements indicate that the superconducting transition temperature ($T_{\rm c}$) also decreases with increasing pressure. However, above $P$ $\sim$ 2 GPa, both $\Delta_{\rm CEF}$ and $T_{\rm c}$ do not depend on external pressure up to $P$ = 3.82 GPa. These pressure dependences of $\Delta_{\rm CEF}$ and $T_{\rm c}$ suggest an intimate relationship between quadrupole excitations associated with the $\Gamma_4^{(2)}$ level and unconventional superconductivity in PrOs$_4$Sb$_{12}$. In the superconducting state, 1/$T_1$ below $T_{\rm c}$ = 1.55 and 1.57 K at $P$ = 1.91 and 2.63 GPa shows a power-law temperature variations and are proportional to $T^5$ at temperatures considerably below $T_{\rm c}$. These data can be well fitted by the gap model $\Delta(\theta) = \Delta_0\sin\theta$ with $\Delta_0$ = 3.08 $k_{\rm B}T_{\rm c}$ and 3.04 $k_{\rm B}T_{\rm c}$ for $P$ = 1.91 and 2.63 GPa, respectively. The results indicate there exists point nodes in the gap function.

Through the characterization of many samples of the filled skutterudite compound PrOs4Sb12, we found that even though the double superconducting transition in the specific heat (Tc1 ~1.89 K and Tc2 ~ 1.72 K) appears in samples of good quality, there are various pieces of evidence that cast doubt on its intrinsic nature. First, three samples exhibit a single sharp transition of ~15 mK width at Tc~1.7 K. The quality of the samples exhibiting a single transition is clearly better; the width of the transition is smallest and the normalized specific heat jump ((C-Cnormal)/Cnormal)Tc is larger than the sum of the two specific heat jumps when a double transition exists. Second, the ratio of the two specific heat jumps Delta C(Tc1)/Delta C(Tc2) exhibits a wide variation with samples not only among different batches but also even within a batch. Finally, this ratio was strongly reduced by polishing a sample down to ~100 µm, which points to bulk inhomogeneities as an origin of the spurious transition. Our x-ray diffraction analysis points to Pr-vacancy problem in the samples, but it does not show that they are the origin of the double transition. We provide the superconducting phase diagram under magnetic field of a sample exhibiting a single transition and fit the curve Hc2(T) with a two-band model taking into account the appropriate values for the gap as deduced from thermal conductivity measurements.

We have investigated the exotic metal-insulator (M-I) transition in $\mathrm{Pr}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}$ by using hard-x-ray and soft-x-ray photoemission spectroscopies. The significantly large hybridization strength between conduction and $\mathrm{Pr}\phantom{\rule{0.2em}{0ex}}4f$ electrons $(c\text{\ensuremath{-}}f)$ is found not to change much between the metal and insulating phases. These results suggest that the M-I transition does not noticeably affect the $c\text{\ensuremath{-}}f$ hybridization nature and $\mathrm{P}\phantom{\rule{0.2em}{0ex}}3p--\mathrm{Pr}\phantom{\rule{0.2em}{0ex}}4f$ mixed states are highly stable.

Semiconducting skutterudite $\mathrm{Ce}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ is investigated by synchrotron x-ray photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}3d$ core-level PES and $3d\text{\ensuremath{-}}4f$ XAS, in combination with single-impurity Anderson model (SIAM) calculations, confirm features due to ${f}^{0}$, ${f}^{1}$, and ${f}^{2}$ configurations. The $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}3d\text{\ensuremath{-}}4f$ resonant-PES spectra provide the $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}4f$ density of states (DOS), which indicates the absence of a Kondo resonance at the Fermi level, but can still be explained by SIAM with a small gap in non-$f$ DOS. While $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}4f$ partial DOS from band structure calculations is consistent with the main $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}4f$ DOS, the importance of SIAM for core-level PES and XAS spectra quantifies the mixed valence for semiconducting $\mathrm{Ce}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$, derived from strong hybridization between non-$f$ conduction and $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}4f$ DOS.

We present measurements of the local tunneling density of states in the low temperature ordered state of $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$. The temperature dependencies of the Fermi level density of states and of the integrated density of states at low bias voltages show anomalies at $T\ensuremath{\simeq}6.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the onset of multipolar ordering as detected by specific heat and other macroscopic measurements. In the ordered phase, we find a local density of states with a V-shape form, indicating a partial gap opening over the Fermi surface. The size of the gap according to the tunneling spectra is about $2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$.

We report experimental results of nuclear magnetic resonance (NMR) at the La site and nuclear quadrupole resonance (NQR) at the Sb site in the filled skutterudite $\mathrm{La}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$. We found that the nuclear spin-lattice relaxation rate divided by temperature $1∕{T}_{1}T$ at the La site exhibits a different temperature dependence from that at the Sb site. Although $1∕{T}_{1}T$ at the Sb site is explained by the Korringa mechanism, $1∕{T}_{1}T$ at the La site exhibits a broad maximum around $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, showing the presence of an additional contribution at the La site. The additional low-lying excitations observed at the La site can be understood with the relaxation from anharmonic phonons due to the rattling motion of the La atoms.

We have investigated the anisotropy of the magnetoresistance in the Pr-based HF compound PrFe4P12. The large anisotropy of effective mass and its strong field dependence have been confirmed by resistivity measurements. Particularly for H||[111], where the effective mass is most strongly enhanced, the non-Fermi liquid behavior has been observed. Also, we have found the angular dependence of the magnetoresistance sharply enhanced at H||[111], which is evidently correlated with both the non-Fermi liquid behavior and the high-field ordered state (B-phase)., Comment: 3 pages, 3 figures. J. Phys. Soc. Jpn. Vol.77, No.8, in press

S. Imada,1 H. Higashimichi,1 A. Yamasaki,2 M. Yano,1 T. Muro,3 A. Sekiyama,1 S. Suga,1 H. Sugawara,4 D. Kikuchi,5 and H. Sato5 1Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan 2Department of Physics, Faculty of Science and Engineering, Konan University, Kobe 658-8501, Japan 3Spring-8/JASRI, Sayo 579-5198, Japan 4Faculty of the Integrated Arts and Sciences, The University of Tokushima, Minami-jyousanjima-machi 1-1, Tokushima 770-8502, Japan 5Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Received 1 June 2007; revised manuscript received 3 September 2007; published 24 October 2007

We report {121, 123}Sb nuclear quadrupole resonance measurements under pressure in a novel heavy fermion (HF) system SmOs4Sb12. The nuclear spin-spin relaxation rate 1/T{2} exhibits a distinct peak near the coherent temperature of the Kondo effect. The isotope effect of 121Sb and 123Sb indicates that the peak in 1/T{2} is electrical in origin. The connection between the peak in 1/T{2} and the development of coherency of the Kondo effect is robust even under pressure. It is conjectured that charge fluctuation plays an important role in forming the HF state in SmOs4Sb12.

Zero- and longitudinal-field muon spin relaxation experiments have been carried out in the alloy series $\mathrm{Pr}{({\mathrm{Os}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x})}_{4}{\mathrm{Sb}}_{12}$ and ${\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{La}}_{y}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ to elucidate the anomalous dynamic muon spin relaxation observed in these materials. The damping rate $\ensuremath{\Lambda}$ associated with this relaxation varies with temperature, applied magnetic field, and dopant concentrations $x$ and $y$ in a manner consistent with the ``hyperfine enhancement'' of $^{141}\mathrm{Pr}$ nuclear spins first discussed by Bleaney [Physica (Utrecht) 69, 317 (1973)]. This mechanism arises from Van Vleck-like admixture of magnetic ${\mathrm{Pr}}^{3+}$ crystalline-electric-field-split excited states into the nonmagnetic singlet ground state by the nuclear hyperfine coupling, thereby increasing the strengths of spin-spin interactions between $^{141}\mathrm{Pr}$ and muon spins and within the $^{141}\mathrm{Pr}$ spin system. We find qualitative agreement with this scenario and conclude that electronic spin fluctuations are not directly involved in the dynamic muon spin relaxation.

Temperature, pressure, and magnetic field dependence of the Raman scattering spectra of Pr-based filled skutterudites PrFe4P12 and PrOs4Sb12 have been measured. In the insulator phase of PrFe4P12 under a pressure, an additional peak is observed. On the other hand, in the A-phase below T=6.5 K at ambient pressure, no additional peak is observed. By means of a factor group analysis, we have evaluated the crystallographic symmetry of the insulator and A-phases as Immm and R3, respectively. In the spectra of PrOs4Sb12, the crystal field excitation and Einstein oscillations of Pr ion are observed in addition to the first order Raman active phonons. The Einstein oscillation is observed in the second-order Raman scattering spectrum owing to a strong anharmonicity of Pr vibration with the large displacement.

Sm-based heavy-fermion compound ${\mathrm{SmOs}}_{4}{\mathrm{Sb}}_{12}$ has been investigated by soft x-ray ($h\ensuremath{\nu}=1070--1600\text{ }\text{ }\mathrm{eV}$) and hard x-ray (HX; $h\ensuremath{\nu}=7932\text{ }\text{ }\mathrm{eV}$) spectroscopy. The HX photoemission spectroscopy clearly demonstrates that the strongly mixed-valence state and the heavy-fermion state coexist in the bulk. It is found that the Sm valence decreases below 100 K, indicating that the Kondo coherence develops with approaching the proposed Kondo temperature. Our theoretical analyses suggest that the origin of the coexistence in ${\mathrm{SmOs}}_{4}{\mathrm{Sb}}_{12}$ is the coincidence of two conditions, namely, (i) the energy difference between Sm divalent and trivalent states is very small and (ii) the hybridization between Sm $4f$ and conduction electrons is weak.

We have investigated the elastic properties of the Ce-based filled skutterudite antimonides CeRu$_{4}$Sb$_{12}$ and CeOs$_{4}$Sb$_{12}$ by means of ultrasonic measurements. CeRu$_{4}$Sb$_{12}$ shows a slight increase around 130 K in the temperature dependence of the elastic constants $C$$_{11}$, ($C$$_{11}$-$C$$_{12}$)/2 and $C$$_{44}$. No apparent softening toward low temperature due to a quadrupolar response of the 4$f$-electronic ground state of the Ce ion was observed at low temperatures. In contrast CeOs$_{4}$Sb$_{12}$ shows a pronounced elastic softening toward low temperature in the longitudinal $C$$_{11}$ as a function of temperature ($T$) below about 15 K, while a slight elastic softening was observed in the transverse $C$$_{44}$ below about 1.5 K. Furthermore, CeOs$_{4}$Sb$_{12}$ shows a steep decrease around a phase transition temperature of 0.9 K in both $C$$_{11}$ and$ C$$_{44}$. The elastic softening observed in $C$$_{11}$ below about 15 K cannot be explained reasonably only by the crystalline electric field effect. It is most likely to be responsible for the coupling between the elastic strain and the quasiparticle band with a small energy gap in the vicinity of Fermi level. The elastic properties and the 4$f$ ground state of Ce ions in CeRu$_{4}$Sb$_{12}$ and CeOs$_{4}$Sb$_{12}$ are discussed from the viewpoint of the crystalline electric field effect and the band structure in the vicinity of Fermi level.

We report a muon spin rotation study in single-crystalline samples of the heavy fermion superconductor $\mathrm{Pr}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$. The muon Knight shift is independent of temperature passing through ${T}_{c}$ down to $20\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ at 3 and $17\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, indicating that the local spin susceptibility does not decrease in the superconducting state. This result is evidence that spin-triplet superconductivity is realized in $\mathrm{Pr}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$.

We report the results of high-resolution dilatometry and dc magnetization measurements of a small single crystal of CeOs4Sb12, which exhibits an anomalous ordered state. Both linear thermal expansion coefficient and magnetization results reveal the presence of a new phase (phase A) transition at TA = 0.87 K, slightly below the antiferromagnetic (AFM) transition temperature TN = 1.05 K. In addition, the magnetostriction and magnetization results provide unambiguous thermodynamic evidence of the presence of a field-induced phase transition to a high-field ordered phase (phase B), as previously suggested by neutron scattering experiments. The resulting magnetic field–temperature (H–T) phase diagram of CeOs4Sb12 consists of three ordered phases: A, B, and AFM phases. From a thermodynamic point of view, it is likely that the AFM and high-field ordered states coexist within the A phase, and that the B phase is of nonmagnetic origin.

We report the detailed analysis of the 101Ru nuclear quadrupolar resonance spectrum in the ferromagnetically ordered state of EuRu4Sb121 and propose that Eu 4f moments align in the [111] direction. The localized character of Eu 4f electrons is suggested from the temperature dependence of the nuclear spin–lattice relaxation rate.

Magnetization, specific heat, and electrical resistivity measurements have been performed for polycrystalline samples of hexagonal CeRh6Ge4 and CeRh6Si4. The Ce ions in CeRh6Ge4 are trivalent above 100 K, while those in CeRh6Si4 display an intermediate-valence state below room temperature. We discovered that CeRh6Ge4 is a new ferromagnetic (FM) compound with the transition temperature TC = 2.5 K. The −lnT dependence of the magnetic part of the electrical resistivity, the small magnetic entropy released at TC (0.19Rln2), and the relatively large electronic specific-heat coefficient (0.25 J·mol−1·K−2) imply that CeRh6Ge4 is a Kondo-lattice compound. We argue that CeRh6Ge4 is a rare Ce-based FM compound with a proximity to a quantum critical point.

We have studied the de Haas-van Alphen (dHvA) effect under pressure for some typical compounds of strongly correlated Ce compounds CeSb, CeB6, CeRu2Si2, CeRu2Ge2 and CeCo2. The pressure dependence of the dHvA frequencies is much larger than that expected from compressibility or that of normal metals. The pressure dependence of the effective masses have been measured in CeSb and CeB6, and it is difficult to explain that in terms of the increase of mixing with pressure.

Raman scattering spectra of filled skutterudite RT 4 X 12 (R=La, Ce, Pr, Nd, Sm and Y, T=Fe, Ru and Os, and X=Sb and P) and unfilled skutterudite CoP 3 have been measured. All first-order Raman active phonons, which are due to the vibration of pnictogens, are observed. In addition, the crystal field excitations in PrRu 4 P 12 spectra and the second-order phonons including rare-earth vibrations in SmRu 4 P 12 and ROs 4 Sb 12 are also observed. The second-order phonons can be observed for the sample with the larger cage space. The peak intensity of the second-order phonons decreases with decreasing temperature and vanishes at low temperature. Such a temperature dependence suggests that the second-order phonons are thermally excited and due to independent vibrations of the rare-earth ions, that is rattler motion.

The antiferroquadrupolar ordering of ${\mathrm{PrFe}}_{4}{\mathrm{P}}_{12}$ is explored by probing thermal and thermoelectric transport. The lattice thermal conductivity drastically increases with the ordering, as a consequence of a large drop in carrier concentration and a strong electron-phonon coupling. The low level of carrier density in the ordered state is confirmed by the anomalously large values of the Seebeck and Nernst coefficients. The results are reminiscent of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ and suggest that both belong to the same class of partial metal-insulator transitions. The magnitude of the Nernst coefficient, larger than in any other metal, indicates a new route for Ettingshausen cooling at Kelvin temperatures.

Dynamical conductivity spectra s(w) have been measured for a diverse range of heavy-fermion (HF) Ce and Yb compounds. A characteristic excitation peak has been observed in the mid-infrared region of s(w) for all the compounds, and has been analyzed in terms of a simple model based on conduction (c)-f electron hybridized band. A universal scaling is found between the observed peak energies and the estimated c-f hybridization strengths of these HF compounds. This scaling demonstrates that the model of c-f hybridized band can generally and quantitatively describe the charge excitation spectra of a wide range of HF compounds., 5 pages, 1 table, 3 figures, to appear in J. Phys. Soc. Jpn. 76 (2007)

Raman scattering spectra have been measured for the unfilled skutterudite CoP3 and for the filled skutterudites RT4X12 (R= La, Ce, Pr and Nd, T=Fe and Os, and X=Sb and P). For the P‐skutterudite, the energies of two Ag phonons and one Eg phonon decrease with rare‐earth filling, while, for the Sb‐skutterudite, the energies of the observed phonons do not change significantly. In order to clarify this filling effect microscopically, a normal mode analysis has been performed using a GF matrix method. The following differences due to the filling effect have been found for the P‐ and Sb‐skutterudites: the interaction between pnictogens on the cage becomes weaker for the P‐skutterudite, while it does not change significantly for the Sb case.

We report on Sb nuclear-quadrupole-resonance (NQR) study in filled-skutterudite compounds (Pr_1-xLa_x)Os_4Sb_12. The Sb-NQR spectra have split into two sets, arising from different Sb_12 cages containing either Pr or La, which enables us to measure two kinds of nuclear spin-lattice relaxation time T_1^Pr and T_1^La. In the normal state, the temperature (T) dependence of 1/T^Pr_1T showed almost the same behavior as that for PrOs_4Sb_12 regardless of. In contrast, 1/T^La_1T markedly decreases with increasing La concentration. In the superconducting state for x=0.05 and 0.2, 1/T_1^Pr exponentially decreases down to T=0.7 K with no coherence peak below T_c as well as in PrOs_4Sb_12. A remarkable finding is that the residual density of states at the Fermi level below T_c is induced by La substitution for Pr. 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 the presence of point nodes for a small 4f^2-derived FS inherent in PrOs_4Sb_12. For x=0.8 and 1,1/T^La_1 exhibits a coherence peak and the nodeless energy gap characteristic for weak-coupling s-wave superconductors. With increasing Pr content, T_c and the energy gap increases. The novel strong-coupling superconductivity in PrOs_4Sb_12 is inferred to be mediated by the local interaction between 4f^2-derived crystal-electric-field states with the electric quadrupole degree of freedom and conduction electrons. This coupling causes a mass enhancement of quasi-particles for a part of FS and induces a small FS, which is responsible for point nodes in the superconducting gap function. Note that the small FS does not play any primary role for the strong-coupling superconductivity in PrOs_4Sb_12., Comment: 9 pages, 11 figure. to be published in J. Phys. Soc. Jpn

We have performed ultrasonic measurement on the filled skutterudite compound SmRu4P12 at low temperatures. We observed a clear softening below around 2 K in the elastic constant (C11‐C12)/2 even below the metal‐insulator transition temperature of 16.5 K. It strongly suggests that this softening is not originated from the ground state property of Sm ion formed by the crystalline electric field effect. We discuss the obtained elastic softening based on a deformation potential model.

We performed local magnetization measurements using micro‐Hall probe in filled skutterudite PrOs4Sb12, which shows a superconducting state with broken time‐reversal symmetry. It turns out that there appears a remarkable dip in the local magnetization hysteresis near zero field, which is characteristic of chiral superconductors, while temperature dependence of local magnetization shows no distinctive anomaly related to the spontaneous field. Possible situations of the spontaneous field is discussed from the results of local magnetization.

We have performed 59 Co NQR study in superconducting CeCo 2 with T c ∼ 1.4 K in order to investigate the superconducting property. Nuclear spin-lattice relaxation rate of Co, 1/ T 1 was measured down to 150 mK. 1/ T 1 is proportional to temperature ( T ) in the normal state, and decreases exponentially in the superconducting state with the Hebel-Slichter peak just below T c . T -dependence of 1/ T 1 in the superconducting state is a typical feature of s-wave superconductivity with 2 Δ / k B T c ∼ 2.26, which is remarkably smaller than 3.5 in the BCS superconductor.

We have measured the temperature dependence of the specific heat ( C ) on a NdGa 2 single crystal down to 100 mK in the magnetic field B along [120] axis up to 8 T to investigate the low-energy excitation in the incommensurate antiferromagnetic (AF) phase. Below about 0.5 K, C / T shows a largely enhanced value of 150 mJ/K 2 mol in B = 0 T. In the field-induced ferromagnetic (F) phase above 2 T, the value of C / T is suppressed to 10 mJ/K 2 mol. In correlation, large negative magnetoresistance ( Δϱ / ϱ 0 ∼ −40%) is observed. Fluctuating Nd magnetic moments around the nodes in the incommensurate AF phase would be a possible origin for the observed large C / T .

We have measured the temperature dependence of electrical resistivity, Hall coefficient and thermoelectric power using a high-quality single crystal CeCo2. All transport properties exhibit clear differences compared to that of YCo2, indicating itinerant 4f character in Ce. In superconductive mixed state, we have measured the field dependence of the flux flow resistivity at constant temperature, and have found an anomalous pinning structure related to the peak effect reported for the AC-susceptibility measurements.

The thermal conductivity of the heavy fermion superconductor PrOs4Sb12 was measured down to Tc/40 throughout the vortex state. At lowest temperatures and for magnetic fields H ~ 0.07Hc2, already 40% of the normal state thermal conductivity is restored. This behaviour (similar to that observed in MgB2) is a clear signature of multiband superconductivity in this compound.

We report inelastic neutron scattering experiments performed to investigate the low energy magnetic excitations on single crystals of the heavy-fermion superconductor PrOs(4)Sb(12). The observed excitation clearly softens at a wave vector Q=(1,0,0), which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and its intensity at Q=(1,0,0) is smaller than that around the zone center. This result directly evidences that this excitonic behavior is derived mainly from nonmagnetic quadrupolar interactions. Furthermore, the narrowing of the linewidths of the excitations below the superconducting transition temperature indicates the close connection between the superconductivity and the excitons.

We report on a novel phase transition at $T$ = 0.9 K in the Ce-based filled-skutterudite compound CeOs$_{4}$Sb$_{12}$ via measurements of the nuclear-spin lattice relaxation rate $1/T_{1}$ and nuclear quadrupole resonance (NQR) spectrum of Sb nuclei. The temperature ($T$) dependence of $1/T_1$ behaves as if approaching closely an antiferromagnetic (AFM) quantum critical point (QCP), following the relation $1/T_{1}\propto T/(T-T_{\rm N})^{1/2}$ with $T_{\rm N} = 0.06$ K in the range of $T=1.3-25$ K. The onset of either the spin-density-wave (SDW) or charge-density-wave (CDW) order at $T_{\rm 0} = 0.9$ K, that is, of the first order, is evidenced by a broadening of the NQR spectrum and a marked reduction in $1/T_1$ just below $T_{\rm 0}$. The $f$-electron-derived correlated band realized in CeOs$_{4}$Sb$_{12}$ is demonstrated to give rise to the novel phase transition on the verge of AFM QCP., 4pages, 5figures

Magnetic excitations of the filled skutterudite $\mathrm{Pr}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}$ exhibiting a metal-insulator (M-I) transition at ${T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}=63\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ were studied by inelastic neutron scattering experiment. The spectra at temperatures much lower than ${T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}$ are described as well-defined crystal-field excitations. With approaching ${T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}$, the excitation peaks broaden and shift considerably together with the temperature variation of the carrier number and the atomic displacement in the transition. The $4f$ electron states evolve from the well-localized state in the insulator phase to the strongly hybridized itinerant state by $p\text{\ensuremath{-}}f$ mixing near ${T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}$. The hybridization is responsible for the M-I transition of $\mathrm{Pr}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}$.

We have investigated the electrical resistivity of the heavy fermion superconductor PrOs$_{4}$Sb$_{12}$ in the mixed state. We found unusual double minima in the flux-flow resistivity as a function of magnetic field below the upper critical field for the first time, indicating double peaks in the pinning force density ($F_{\rm P}$). Estimated $F_{\rm P}$ at the peak exhibits apparent dependence on applied field direction; composed of two-fold and four-fold symmetries mimicking the reported angular dependence of thermal conductivity ($\kappa$). The result is discussed in correlation with the double step superconducting (SC) transition in the specific heat and the multiple SC-phases inferred from the angular dependence of $\kappa$., Comment: 5 pages, 7 figures, to appear in J. Phys. Soc. Jpn. Vol. 74, No. 6 or 7

We report the de Haas--van Alphen (dHvA) effect and magnetoresistance in the filled-skutterudite superconductor ${\mathrm{LaRu}}_{4}{\mathrm{P}}_{12}$, which is a reference material of ${\mathrm{PrRu}}_{4}{\mathrm{P}}_{12}$ that exhibits a metal-insulator (M-I) transition at ${T}_{\mathrm{MI}}\ensuremath{\simeq}60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The observed dHvA branches for the main Fermi surface (FS) are well explained by the band-structure calculation, using the full potential linearized augmented-plane-wave method with the local-density approximation, suggesting a nesting instability with $\mathbf{q}=(1,0,0)$ in the main multiply connected FS as expected also in ${\mathrm{PrRu}}_{4}{\mathrm{P}}_{12}$. Observed cyclotron effective masses of $(2.6\ensuremath{-}11.8){m}_{0}$, which are roughly twice the calculated masses, indicate the large mass enhancement even in the La-skutterudites. Comparing the FS between ${\mathrm{LaRu}}_{4}{\mathrm{P}}_{12}$ and ${\mathrm{PrRu}}_{4}{\mathrm{P}}_{12}$, an essential role of $c\text{\ensuremath{-}}f$ hybridization cooperating with the FS nesting in driving the M-I transition in ${\mathrm{PrRu}}_{4}{\mathrm{P}}_{12}$ is clarified.

We have measured both magnetoresistance and Hall effect in $\mathrm{Ce}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ to clarify the large resistivity state ascribed to the Kondo insulating one and the origin of the phase transition near $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reported in the specific-heat measurement. We found unusual temperature $(T)$ dependence both in the electrical resistivity $\ensuremath{\rho}\ensuremath{\sim}{T}^{\ensuremath{-}1∕2}$ and the Hall coefficient ${R}_{H}\ensuremath{\sim}{T}^{\ensuremath{-}1}$ over the wide temperature range of about two orders of magnitude below $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which can be explained as a combined effect of the temperature dependences of carrier density and carrier scattering by spin fluctuation. An anomaly related with the phase transition has been clearly observed in the transport properties, from which the $H\text{\ensuremath{-}}T$ phase diagram is determined up to $14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Taking into account the small entropy change, the phase transition is most probably the spin density wave one. Both the electrical resistivity and Hall resistivity at $0.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is largely suppressed about an order of magnitude by magnetic fields above $\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, suggesting a drastic change of electronic structure and a suppression of spin fluctuations under magnetic fields.

We have studied the electrical resistivity of the filled skutterudite compound $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ under high pressure. The antiferroquadrupolar ordering temperature ${T}_{Q}$ decreases monotonically with increasing pressure up to $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Above $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, we have found that a metal-insulator $(M\text{\ensuremath{-}}I)$ transition appears. The insulating state is easily suppressed by a magnetic field. The observed Kondo effect and a field-induced heavy-fermion state at high pressure suggest that the quadrupolar interactions survive in the insulating region. The quadrupolar interactions might play an essential role in the $M\text{\ensuremath{-}}I$ transition. $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ is an unusual compound in that it shows a pressure-induced transition from a metal to an insulator.

$^{31}\mathrm{P}$-NMR studies have been carried out to investigate magnetic properties in the Pr-based filled skutterudite compound $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$. This compound shows an unusual phase transition at ${T}_{A}\ensuremath{\sim}6.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is now regarded as antiferroquadrupole (AFQ) ordering from neutron-diffraction experiments. Splitting of the P-NMR spectrum due to the appearance of two P sites with different hyperfine fields was observed below ${T}_{A}$. From the field dependence, the splitting seems to disappear in zero magnetic field, indicating that the different hyperfine fields are not due to magnetic order, but to the appearance of two inequivalent P sites below ${T}_{A}$. This is ascribed to the distortion of the P cage surrounding a Pr ion below ${T}_{A}$, which is associated with the Pr orbital ordering. The nuclear spin-lattice relaxation rate $(1∕{T}_{1})$ shows the typical behavior of Kondo systems, where onset of local-moment screening due to the coupling between conduction and localized $\mathrm{Pr}\text{\ensuremath{-}}4f$ electrons is observed below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Far above a critical field ${H}_{A}\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, the Korringa behavior ${T}_{1}T=\mathrm{const}$ is observed below $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $100\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. The Korringa value below $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is one order of magnitude larger than that in $\mathrm{La}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$, which has no $4f$ electrons. Our NMR experiment shows that the heavy-fermion state is realized far above ${H}_{A}$ in $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$. The magnetic fluctuations in $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ are discussed from a comparison with typical heavy-fermion compounds. In magnetic fields below ${H}_{A}$, $1∕{T}_{1}$ shows a sharp decrease below ${T}_{A}$, however, in smaller magnetic fields less than $10\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, $1∕{T}_{1}$ stays constant far below ${T}_{A}$ with a relatively large value. The temperature and field dependence of $1∕{T}_{1}$ reveals the presence of low-energy spin fluctuations in the low-temperature and low-field region. These unusual magnetic fluctuations are considered to originate from the magnetic dynamics of Pr-nuclear spins since the fluctuating magnetic field $\ensuremath{\sim}1.4\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ and frequency $\ensuremath{\sim}3.5\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ are so small. The nuclear ordering temperature is estimated to be $\ensuremath{\sim}0.08\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ using the nuclear exchange fluctuations derived from the observed $1∕{T}_{1}$ of P. We show that $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ is a quite unique compound in which nuclear magnetism can be detected by P NMR thanks to the nonmagnetic ground state of the $\mathrm{Pr}\text{\ensuremath{-}}4f$ moments.

We show that in recently discovered heavy fermion superconductors, quasi two-dimensional CeCoIn5 and skutterudite PrOs4Sb12, multiple superconducting phases with different symmetries manifest themselves below T-c. The ultrasound velocity measurements revealed that in CeCoIn5 the Fulde-Ferrel-Larkin-Ovchinikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices, appears at low temperature and high field. The angle resolved magnetothermal transport measurements revealed that in PrOs4Sb12 a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state. These results open up a new realm for the study of the superconductivity with multiple phases

Bulk electronic structures of Pr-based filled skutterudites and their reference materials have been investigated by bulk-sensitive photoemission spectroscopy using soft and hard X-rays. For Pr 3 d – 4 f resonance, a strong intensity in the vicinity of Fermi level ( E F ) is observed only for PrFe 4 P 12 . Furthermore, PrFe 4 P 12 has the significant spectral weight of the | 3 d 9 4 f 3 〉 final state in Pr 3 d core-level spectrum. We interpret that the observed prominent intensity just below E F arises from a Kondo resonance due to the strong hybridization between the Pr 4 f and conduction states.

We have measured the electrical resistivity, thermoelectric power, Hall coefficient, and magnetoresistance (MR) on single crystals of PrOs$_{4}$Sb$_{12}$, LaOs$_{4}$Sb$_{12}$ and NdOs$_{4}$Sb$_{12}$. All the transport properties in PrOs$_{4}$Sb$_{12}$ are similar to those in LaOs$_{4}$Sb$_{12}$ and NdOs$_{4}$Sb$_{12}$ at high temperatures, indicating the localized character of 4$f$-electrons. The transverse MR both in LaOs$_{4}$Sb$_{12}$ and PrOs$_{4}$Sb$_{12}$ tends to saturate for wide field directions, indicating these compounds to be uncompensated metals with no open orbit. We have determined the phase diagram of the field induced ordered phase by the MR measurement for all the principle field directions, which indicates an unambiguous evidence for the $\Gamma_{\rm 1}$ singlet crystalline electric field ground state., Comment: 7 pages, 10 figures, to appear in Physical Review B

The measurements of magnetization and ultrasonic velocity have been performed on single-crystalline CeRu 2 to clarify the anomalous superconducting mixed state on the basis of the FFLO state. Our experimental results suggest that a novel pinning mechanism works in the FFLO state.

A high-field magnetization study of single crystal NdCu2 has been carried out along the a-, b- and c-axes up to 50 T below 6 K. A metamagnetic transition is found along all the axes, and the magnetic phase diagrams are obtained. The magnetization along the a-axis reaches 3.1μB/Nd at 51 T, exhibiting de Haas—van Alphen oscillation with a frequency of 1.55 × 106 and 1.5 × 106 Oe.

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of Pr(Os$_{1-x}$Ru$_{x}$)$_{4}$Sb$_{12}$ down to 0.1 K. Both $\lambda$ and superfluid density $\rho_{s}$ exhibit an exponential behavior for the $x$$\geq$0.4 samples, going from weak ($x$=0.4,0.6), to moderate, coupling ($x$=0.8). For the $x$$\leq$0.2 samples, both $\lambda$ and $\rho_{s}$ vary as $T^{2}$ at low temperatures, but $\rho_{s}$ is s-wave-like at intermediate to high temperatures. Our data are consistent with a three-phase scenario, where a fully-gapped phase at $T_{c1}$ undergoes two transitions: first to an unconventional phase at $T_{c2}$$\lesssim$$T_{c1}$, then to a nodal low-$T$ phase at $T_{c3}$$, Comment: Changed title, enlarged numbering in figures 5 pages, 4 figures, 1 table

We report that the flux-line lattice in the cubic superconductor Pr(Os4Sb12 is strongly distorted from an ideal hexagonal lattice at very low temperatures in a small applied field. We attribute this to the presence of gap nodes in the superconducting state on at least some Fermi-surface sheets.

Pr $4f$ electronic states in Pr-based filled skutterudites $\text{Pr}{T}_{4}{X}_{12}$ ($T=\text{Fe}$ and Ru; $X=\mathrm{P}$ and Sb) have been studied by high-resolution bulk-sensitive Pr $3d\ensuremath{\rightarrow}4f$ resonance photoemission. A very strong spectral intensity is observed just below the Fermi level in the heavy-fermion system ${\text{PrFe}}_{4}{\mathrm{P}}_{12}$. The increase of its intensity at lower temperatures is observed. We speculate that this is the Kondo resonance of Pr, the origin of which is attributed to the strong hybridization between the Pr $4f$ and the conduction electrons.