Search

7Li and 27Al MAS-NMR, magnetic susceptibility, and complex impedance measurements have been performed to study the local structure and electrical resistivity in Al doped spinel LiMn2−xAlxO4 (x = 0, 0.05) exposed to a ball-milling process. The milling process decreased the effective magnetic moment of the Mn species, arising from the appearance of Mn4+, and lead to the suppression of the antiferromagnetic correlation. A hopping time and an activation energy for hopping charge carrier, estimated from electrical resistivity, relatively became larger above milling time of 2.5 h. 7Li and 27Al MAS-NMR spectra were dependent on milling time, and changes in the spectrum intensities were related to the distribution of Al/Li site occupation. Consequently, we concluded that structural disorder caused by the moderate milling process stimulated a migration of Al3+ ions from the 8a site to the 16d one and the increase of Li+ ions at the 8a site on the diffusion pathway. Such a mutual site migration between the 8a and 16d site for Li+ and Al3+ ions would be favorable to Li+ ion diffusion in the milled samples.

We performed linear and non-linear resistivity measurements for fine YBa2Cu3O7−δ ceramics under magnetic field. Successive superconducting transition was observed in the linear resistivity and the lower temperature part of the resistivity curve became convex upward at high fields. A non-linear resistivity peak was observed for this lower temperature region and its offset temperature Toffset and the onset temperature Tonest coincided with the temperature of zero resistivity and inflexion point in linear resistivity, respectively. At lower field, Tonset was less sensitive to the field than Toffset, which implies the existence of the peculiar inter-grain ordering phase at finite fields., 2019 International Conference on Strongly Correlated Electron Systems (SCES2019), September 23-28, 2019, Okayama Convention Center in Okayama, Japan

The ceramic YBa2Cu4O8 superconductor composed of submicron grains is considered a random Josephson-coupled network containing the so-called π junctions and shows successive phase transitions. With decreasing temperature, first the intragrain superconductive transition occurs inside each grain at T c1 and then the chiral-glass transition occurs among the grains at T c2 (< T c1). The third transition at T c3 (< T c2) is the intergrain superconducting transition. We measured the nonlinear susceptibility and resistivity of the ceramic YBa2Cu4O8 superconductor to determine the field dependences of the transition temperatures T c2 and T c3. The phase diagram of the intergrain ordering is discussed in light of the result predicted by Kawamura., 29th International Symposium on Superconductivity (ISS2016), 13–15 December 2016, Tokyo, Japan

We report the results of Sb nuclear quadrupole resonance (NQR) measurements on the filled skutterudite CeFe4Sb12 synthesized under high pressure (HP) and compare them with those for the sample synthesized under ambient pressure (AP) to understand the influence of the Ce-site filling fraction. The NQR spectra for the HP sample are much sharper than those for the AP sample, suggesting that the Ce filling fraction is higher. Also, the nuclear spin-lattice relaxation rate 1/T 1 follows an exponential decrease 1/T 1 ∝ exp(−Δ/k B T), with the gap being Δ/k B = 270 K at temperatures above 100 K, which is larger than that for the AP sample. This result suggests that the c-f hybridization is enhanced by increasing the Ce-site filling fraction.

The magnetic memory phenomena in a ceramic YBCO superconductor are studied by zero-field-cooled and thermoremanent magnetization after specific cooling protocols. The ceramic YBCO is considered as random Josephson-coupled networks of 0 and π junctions and shows successive phase transitions. The magnetic glass behavior, similar to those of spin-glasses, is observed below the lower transition temperature. The memory effect occurs strongly in an intermediate stop temperature and suppressed with an increased probing field. The results can be understood by the screening effect of the ceramic superconductor with a finite self- inductance.

Intergrain ordering phenomena of frustrated ceramic YBa 2 Cu 4 O 8 (Y124) and dual phase mixed ceramic containing CuO are studied by both magnetic and resistivity observations. Magnetically estimated intergrain ordering temperature T c2 and zero-resistivity attainment temperature T z are both decreased with CuO content. However, it is also revealed that mutual location of T c2 and T z is changed. For the mixed ceramic with torn intergrain network structure, T z stays above T c2 , indicating local dissolution of frustration by cluster structure. On the other side, remaining resistivity tails blow T c2 , indicating intrinsic fluctuation effect on the resistivity caused by frustration in the ordered state. It is known then that, superconductive ceramic samples which is free from structural phase inhomogeneity is essential in order to clarify the frustration effect.

We report the results of 31P-nuclear magnetic resonance (NMR) measurements on the layered cobalt oxyphosphide Sr2Sc(Co1−xFex)PO3 in order to investigate the magnetic properties at low temperatures from a microscopic view point. The 31P-Knight shifts measured at the resonance peak maximum of Sr2Sc(Co1−xFex)PO3 have positive values and are T-independent in an entire temperature range, and the absolute value decreases with increasing Fe content. Also, the nuclear spin-lattice relaxation rate 1/T1 is almost proportional to the temperature at low temperatures. The magnitude of 1/T1T decreases with increasing the Fe content, which suggests the decrease of the density of states around the Fermi level.

Superconductivity caused by oxygen reduction treatment for sintered Pr2Ba4Cu7O15–δ (Pr247) synthesized using citrate pyrolysis precursor method is examined experimentally. The sintered sample tends to be heterogeneous structure including Pr124 phase and stacking fault structure rich in CuO single chains. In this work, such a heterogeneous sample and nearly pure phase one were prepared, and the electric conductive behaviors were examined comparatively for various reduction grade sample series. In enough reduced stages, superconductivity with onset temperature 25–28 K was observed for both heterogeneous and purer samples. At transition process by the reduction, superconductivity of the heterogeneous samples can arise from lower oxygen-reduction% than purer Pr247 material. The results may possibly suggest superconductivity by CuO double chains besides Pr247 crystal in our heterogeneous ceramic system.

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 .

We report the results of 121,123Sb-nuclear quadrupole resonance (NQR) measurements on CeFe 4 Sb 12 in order to investigate the magnetic properties at low temperatures from a microscopic viewpoint. Above 50 K, the nuclear spin-lattice relaxation time T 1 shows an activated temperature dependence 1 / T 1 ∝ exp ( - Δ / k B T ) with an energy gap Δ / k B = 190 K . On the other hand, below 30 K, 1 / T 1 is proportional to the temperature. This behavior is explained by a pseudogap model, which is suggested to be induced by the effect of the hybridization between Ce 4 f and conduction electrons.

Sintered material of Pr2Ba4Cu7O15−δ (Pr247) was synthesized at ambient pressure condition by using citrate pyrolysis precursor method, and the superconductivity caused by oxygen reduction treatment has been confirmingly observed and examined. By resistivity and magnetization experiments, it has been found that the critical temperature Tc of the reduced samples could reach about 10 K higher than previously reported data for pure Pr247. Besides, from TEM observations, the present material was found to be heterogeneous system containing Pr247, Pr124 and novel stacking structure phases rich in CuO single chains. These results have suggested that oxygen–reduction at such region rich in the CuO single chains may affect the superconductivity of the adjacent crystal region containing CuO double chains, and might enhance the superconductive critical temperature.

Practical synthetic conditions of PrBa 2 Cu 4 O 8 (Pr124) and Pr 2 Ba 4 Cu 7 O 15− δ (Pr247) were clarified for our citrate pyrolysis precursor method. The decided optimum calcination temperatures were 875–880 °C for Pr124 and 885 ± 0.5 °C for Pr247. Calcination time at least ∼80 h is necessary for both the syntheses of Pr124 and Pr247, and further duration is effective in avoiding impureness by PrBaO 3 for Pr124 synthesis. Especially for the case of Pr247, the allowed temperature condition is very tight considering inevitable temperature graduation in an ordinary electric furnace. So further experimental devising for temperature homogeneity must be necessary to obtain single phase Pr247. Basically, however, it was confirmed that Pr124 and Pr247 can form under ambient pressure condition with our synthetic method consistently.

Ceramic high-T c superconductors may be viewed as a weakly coupled random Josephson network containing π-junctions, and a chiral-glass transition is predicted to be occurred. Grain-size and pressure dependences of the chiral-glass transition have been investigated on YBa 2 Cu 3 O 7-y ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at T c1 and furthermore among the grains at T c2 (< T c1 ). The increase in T c2 with increase of synthesis temperature is considered as the change of strength of inter-grain Josephson coupling accompanying the growth of grain size. The T c2 also increases with increase of pressure, however, the sharpness of the transition at T c2 does not change. In spite of difference in T c2 's, the increasing rates of t c2 by pressure are nearly the same.

We investigated the effects of oxidation and reduction on the successive superconducting transition in a ceramic material composed of YBa2Cu4O8 (Y124) submicron grains by linear and non-linear resistivity measurements. This material shows two-step superconducting transition and the lower one is identified as an inter-grain ordering. A sintering process fuses and joins the adjacent grains, so the lower transition was suppressed with increasing sintered time. Although Y124 is a stoichiometric ceramic, a reduction treatment suppressed the temperature of zero resistivity and broadened the non-linear resistivity peak. This suggests an oxygen vacancy was induced on the grain surface.

Under zero magnetic field, a quadrupolar order parameter at q_Q=(1/2,1/2,1/2) in a typical antiferro-quadrupole (AFQ) ordering compound CeB6 has been observed for the first time by means of a resonant X-ray scattering (RXS) te\ chnique. The RXS is observed at the 2p->5d dipole transition energy of the Ce L3-edge. Using this RXS technique to observe the pure order parameter of the AFQ state, the magnetic phase diagram of Phase II is first determined., 7 pages, 4 figures

A superconductive sign found in electric resistivity behavior of mixed ceramic Pr124/Pr123 is searched and discussed, to advance preceding discussion about the superconductive region induced by intergrain charge transfer effect in ceramic Pr247 containing the related impurity phases. Observed reduction effects for largely impure ceramic of Pr247 with Pr124 and Pr123 and for simply dual-phase sample of Pr124/Pr123 inform us that the oxygen-reduction brings not only superconductive electron doping but also normal resistivity enhancement. Milder heat treatment condition is expected to assist possible attainment of zero-resistivity for Pr124/Pr123.

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.

We report the results of the 31 P NMR measurements on the Os-containing filled skutterudite phosphides R Os 4 P 12 ( R = La and Pr) in order to investigate the magnetic properties at low temperatures from microscopic viewpoints. In LaOs 4 P 12 , the nuclear spin–lattice relaxation rate 1/ T 1 is proportional to the temperature in a wide temperature range. On the other hand, in PrOs 4 P 12 at low temperatures below 15 K, 1/ T 1 obeys the relation (1/ T 1 ) obs = A ×exp (-Δ CEF / k B T ) + B ×(1/ T 1 ) La , where Δ CEF (=44 K) is the crystal electric field (CEF) energy splitting and (1/ T 1 ) La corresponds to the relaxation rate for LaOs 4 P 12 . From the contribution due to quasiparticles, we estimate [(1/ T 1 T ) Pr /(1/ T 1 T ) La ] 1/2 ∼1.2. It means that the effective density of states (DOS) for PrOs 4 P 12 is ∼1.2 times compared with that for LaOs 4 P 12 , because 1/ T 1 T is proportional to the square of the DOS.

We have succeeded in observing the de Haas–van Alphen effect in LaOs 4 P 12 which is a superconductor with the transition temperature T C =2.0 K and a reference compound for PrOs 4 P 12 . We have found the multiply-connected and almost spherical Fermi surfaces whose cyclotron effective masses are ranging 1.1–4.7 m 0 . The Fermi surface topology is close to that of PrOs 4 P 12 , indicating the localized nature of 4 f -electrons in PrOs 4 P 12 . From the comparison with LaOs 4 P 12 , the large mass enhancement in PrOs 4 P 12 has been clarified.

The intergrain ordering in a superconductive ceramic of YBa 2 Cu 4 O 8 at zero external magnetic field is investigated by AC resistivity measurements. The third harmonic in-phase voltage component V ' 3ω to a weak AC current shows a distinct negative peak across the intergrain transition temperature T C2 , but not across the intragrain one T C1 ( T C2 < T C1 ). A divergence of the nonlinear resistivity ρ 2 is concluded at T C2 in the limit of vanishing AC current, while the linear resistivity ρ 0 remains finite even at and below T C2 . Such characteristic critical behaviors of the intergrain ordering of the present superconductive ceramic strongly suggest a chiral glass ordering recently predicted to occur in high- T C superconductive ceramics.

Superconducting characteristics of borocarbide intermetallic compounds have been investigated by the measurement of nuclear-spin lattice relaxation rate 1/T1 of 13C on YNi2B2C. The present NMR study has revealed that borocarbide compounds are characterized as a gap function with point nodes described in terms of s+g-wave superconductor.

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.

We have succeeded in growing single crystals of CeFe 4 Sb 12 by a Sb-self-flux method and measured the electrical resistivity and magnetization. The residual resistivity is about two times smaller than that of polycrystalline ones, suggesting that the sample quality is highly improved. The magnetization measurements at low temperature revealed that the sample contains almost no magnetic impurities which are always observed in polycrystalline samples. The weak temperature dependence and a broad maximum around 115 K in magnetic susceptibility, which is a typical feature of valence fluctuation compounds, have been observed.

The effect of pressure on two-dimensional Heisenberg antiferromagnet Cu(HCOO)2·4H2O has been investigated through X-ray diffraction, susceptibility and magnetization curve measurements. The pressurization induces the paramagnetic behavior above 1.4 kbar. The paramagnetic component increases continuously with increasing pressure, and the antiferromagnetic ordering and field-induced phase transition vanish above 2.8 kbar. The X-ray diffraction pattern at 6.6 kbar is not that of Cu(HCOO)2·4H2O but Cu(HCOO)2·2H2O. Therefore, the pressurization induces the crystal structural change from Cu(HCOO)2·4H2O to Cu(HCOO)2·2H2O with dehydration.

We report the results of 31P NMR measurements on YFe4P12, a new member of the family of the filled skutterudite compounds, which displays superconductivity below T c = 7 K . In the normal state, the nuclear spin-lattice relaxation time T 1 obeys the Korringa relation T 1 T = constant below 40 K, but it changes its behavior at higher temperatures. The Korringa constant ( T 1 T ) - 1 for YFe4P12 is smaller than that for LaFe4P12, suggesting that the density of states at Fermi energy N ( E F ) for YFe4P12 is lower than the case of LaFe4P12. In the superconducting state, T 1 - 1 ( T ) shows a small coherence peak just below T c , followed by an exponential decrease below 4 K revealing on energy gap 2 Δ ( 0 ) / k B T c = 3.3 . Our results indicate that YFe4P12 is a conventional type s-wave superconductor.

Cu(1) NQR spectra of the CuO chain site in Y1 _ xPrxBa2Cu3O7 (x = 0.1 and 0.2), and the 63Cu(1) NQR spin-lattice relaxation rates 1/T1 in Y0.9Pr0.1Ba2Cu3O7 have been measured. The half widths are about three times wider than those of the pure YBa2Cu3O7 . The peak frequencies are lowered by the Pr doping. This seems to indicate a decrease of on-site holes at the Cu(1) site. The relaxation rate 1/T1 is found to obey the same temperature dependence, 1/T1 = aT + bT3 as reported for pure YBa2Cu3O7.

The Pr content x dependence of the superconducting transition temperature Tc is examined for various oxygen concentrations by quenching the samples of the Pr-substituted Ln1−xPrxBa2Cu3O7−y, where Ln is Y, Nd and Gd. In the case of Ln = Y, the initial rate of decrease of Tc by Pr substitution (−dTcdx) decreases with decreasing oxygen concentration for 6.6 < 7 − y < 7.0, then becomes constant for 7 − y < 6.6. In contrast, the effect of Tc-suppression by Pr in the case of Ln = Nd does not depend upon the oxygen concentration, and is much stronger than that in the case of Ln = Y. In the case of Ln = Gd, intermediate behavior between those of Y and Nd is observed, corresponding to the ionic size of Ln.

The characteristic of intergrain phase ordering transition in a superconductive ceramic of YBa 2 Cu 4 O 8 is examined at zero external DC field by AC magnetic measurements. From the nonlinear response at the weak AC field limit, a negative divergence of nonlinear susceptibility χ 2 is concluded at the intergrain transition temperature T c2 but no remarkable anomaly is found at the intragrain one T c1 . This critical phenomenon may suggest the onset of chiral glass ordering at T c2 and necessarily a frustration in the intergrain Josephson type coupling network.

The superconductive ordering of a YBa 2 Cu 4 O 8 ceramic with a single phase and of homogeneous sub-micron size grains, is studied by systematic DC magnetic measurements under weak magnetic fields. Successive phase transitions of a hierarchical nature are identified to occur at two temperatures T C1 (≃ 80 K) and T C2 (≃ 37 K). The transition at T C1 is that from the normal into the intra-grain superconductive state. The transition at T C2 is that into the inter-grain superconductive state and results in an infinite superconductive network over the whole sample. Taking a possible Josephson junction type coupling into account, the second transition at T C2 is speculated to be a long range ordering among the superconductive grains and similar to that for a three-dimensional XY ferromagnet.

We have carried out the 11 B NMR experiments on single crystalline sample of CeB 2 C 2 in order to elucidate the magnetic properties. In the paramagnetic state, the NMR spectrum consists of a sharp peak with satellite lines. The Knight shift, K , decreases upon cooling, which is opposed to the magnetic susceptibility, χ. From the K –χ plot, the hyperfine field at the B sites is estimated to be -1.0 kOe/µ B , which is suggested to be dominated by the atomic dipolar field. On the other hand, in the AFM phase, the spectrum was broadened and splitted into many resonance peaks by the internal magnetic field at B sites. Also, the temperature dependence of the nuclear spin–lattice relaxation rate, 1/ T 1 , shows a divergent behavior around T N , originating from the critical magnetic fluctuation towards a magnetic phase transition.

Under zero magnetic field, antiferro-quadrupole ordering in CeB 6 has been observed for the first time by means of resonant x-ray scattering (RXS) technique. The RXS is observed at the 2 p →5 d dipole transition energy of the Ce L 3 -edge. A possibility of determination of the various type of the induced quadrupole ordered state at (1/2,1/2,1/2) is also discussed on the basis of the model calculation of azimuthal anlge dependence of the RXS intensity.

The effect of Ag addition on the crystal growth has been investigated in Bi cuprate superconductors. Bi(Pb)-Sr-Ca-Cu-Ag-O superconducting thin films were prepared by the RF sputtering method. the XRD pattern shows that these films are amorphous before annealing. Dispersed isolated atoms of Ag grow up to a pure metallic sphere of Ag during annealing. Neither silver alloy nor silver compound is formed. This was confirmed by EPMA and XRD. The lattice parameter c of the matrix does not change with increasing Ag content

We have investigated the Fermi-surface (FS) properties in the Pr-based filled skutterudite ${\text{PrOs}}_{4}{\text{P}}_{12}$ and its reference compound ${\text{LaOs}}_{4}{\text{P}}_{12}$ by means of de Haas--van Alphen experiments and the band-structure calculations. The topology of FS in ${\text{PrOs}}_{4}{\text{P}}_{12}$ is close to that in the reference compound ${\text{LaOs}}_{4}{\text{P}}_{12}$, indicating a localized nature of $4f$ electrons in ${\text{PrOs}}_{4}{\text{P}}_{12}$. Whereas the localized nature of $4f$ electrons, we have confirmed a highly enhanced cyclotron effective mass of up to $18{m}_{0}$ in ${\text{PrOs}}_{4}{\text{P}}_{12}$, which is enhanced about 3.8 times compared to that in ${\text{LaOs}}_{4}{\text{P}}_{12}$. No nesting property with $q=(1,0,0)$ in the FS of ${\text{PrOs}}_{4}{\text{P}}_{12}$ has been confirmed in contrast to good nesting properties in ${\text{PrFe}}_{4}{\text{P}}_{12}$ and ${\text{PrRu}}_{4}{\text{P}}_{12}$ which exhibit unusual ordered states at low temperatures. A role of both the $4f$ electron's contributions and the FS nesting property for unusual phase transitions are discussed.

The intensity ratio of the X-ray characteristic lines L α to K β ,L α /K β decreases exponentially with increasing Cu valence.In ordre to obtain the correct results,we need to make a correction owing to the effects of atomic number,absorption and fluorescence(ZAF correction).The valences of Cu in high-T c oxide superconductors Ba 2 NdCu 3 O 7-δ and Bi 2 Sr 2 CaCu 2 O 8+δ are estimated

Pr content ( x ) and oxygen concentration (7 − y ) dependences of T c have been investigated in the R 1− x Pr x Ba 2 Cu 3 O 7− y system (R = rare earth elements). The NQR spectra associated with the chain sites, Cu(1), have also been measured. It has been revealed that the hole transfer from the chain O sites to plane O sites plays an important role to determine T c . The relation between the decrease of T c and the hole numbers is discussed.

The NQR spectra and the nuclear spin-lattice relaxation rate 1/T1 associated with the Cu(2) sites in Y1−xPrxBa2Cu3O7 with x = 0.1 and 0.2 have been investigated. The spectrum of Y0.8Pr0.2Ba2Cu3O7 at 4.2 K is found to be broadened in the high-frequency side, and the half-width becomes two times of that for pure YBa2Cu3O7. It is also noted that several distinct lines contribute to the spectrum with increasing Pr concentration. These spectral features are quite different from the case of oxygen-depleted YBa2Cu3O7−δ, in spite of the similar behavior in the decrease of Tc. The temperature dependence of 1/T1T for x=0.1 shows a maximum around 106 K. This temperature is much higher than Tc = 86 K, indicating that the Pr doping induces the pseudo-spin-gap behavior. From the Curie-Weiss fit of 1/T1T(T), it is concluded that the Pr doping is considered to enhance the 2D AF spin fluctuations from the decrease of the Weiss temperature.

We report the results of 31P-nuclear magnetic resonance (NMR) measurements on heavy fermion compound EuNi2P2 in order to investigate the magnetic properties at low temperatures from a microscopic view point. The Knight shift has a negative value in an entire temperature range, and the absolute value increases with decreasing temperature but exhibits a broad maximum around 40 K, which is similar to the behavior of the magnetic susceptibility. Also, the nuclear spin-lattice relaxation rate 1/T1 is almost constant at high temperatures above 200 K, which is reminiscent of the relaxation mechanism dominated by the interaction of the 31P nucleus with fluctuating Eu-4f moments. Below 200 K, 1/T1 gradually decreases on cooling due to the change of the valence in the Eu ion. At low temperatures, 1/T1 does not obey the Korringa relation, in contrast to typical heavy fermion compounds. The nuclear spin-spin relaxation rate 1/T2 shows the similar behavior as 1/T1 at high temperatures. But, below 50 K, 1/T2 increases upon cooling due to the development of the magnetic excitation.

We report the results of 121,123Sb-nuclear quadrupole resonance (NQR) measurements on the fully filled skutterudite YbFe4Sb12 synthesized under high pressure, and compare them with those for an Yb-deficient sample synthesized under ambient pressure in order to elucidate the intrinsic behavior and the effects of the Yb-site filling fraction on magnetic properties. The NQR spectra for the fully filled sample remain narrow down to 1.5 K, indicating no sign of a phase transition, in contrast to the results of the Yb-deficient sample. The nuclear spin–lattice relaxation rate divided by temperature, 1/T1T, is composed of two terms as 1/T1T = (1/T1T)χ + (1/T1T)PG. The first term (1/T1T)χ is scaled to the magnetic susceptibility, which is similar to the behavior of nearly ferromagnetic metals. The pseudogap behavior, related to the second term (1/T1T)PG, is consistent with the optical conductivity measurements, and is a characteristic of the filled skutterudites with a divalent filler ion.

We report on the results of the 195Pt NMR measurements for M Pt 4 Ge 12 ( M = Sr,Ba ) to clarify the microscopic electronic states at low temperatures. In SrPt 4 Ge 12 and BaPt 4 Ge 12 , the isotropic component of the Knight shift, that is, the contribution from the Pt 5 s states is smaller than that in LaPt 4 Ge 12 . In the superconducting state, 1 / T 1 shows a small coherence peak just below T C , followed by an exponential decrease at low temperatures revealing on energy gap Δ / k B T C = 1.6 , which suggests a conventional BCS superconductor.

Ceramic YBa 2 Cu 3 O 7− δ composed of sub-micron size grains has shown successive phase transitions under zero field. The first transition occurs inside each grain at T c1 and the second transition occurs among the grains at T c2 ( T c1 ). We measured the current–voltage characteristics and estimated the linear and nonlinear resistivity. The linear resistivity remains finite below T c2 . The nonlinear resistivity has a broad peak above T c2 and diminishes continuously toward T c2 with decreasing temperature. These results are in good agreement with the theoretical analysis and suggest that chiral-glass ordering occurs at T c2 .

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 negative divergence of nonlinear susceptibility is observed at T c2 and the divergence of nonlinear resistivity is also observed near (just above) T c2 . We have investigated pressure dependence of the successive phase transition in YBa 2 Cu 3 O 7-δ ceramics both under external uniform pressure for the sintered sample and in the pelleterizing process between calcination and sintering. With increasing pressure in the pelleterizing process, the T c2 increases and both the absolute values of linear resistivity and the divergence of nonlinear resistivity (just above T c2 ) decrease. The nonlin...

We have carried out the 31 P NMR measurements on Ce-based filled skutterudites CeFe 4 P 12 and CeRu 4 P 12 , which display semiconductive behaviors due to the strong hybridization between Ce 4f and conduction electrons. At higher temperatures, the nuclear spin-lattice relaxation rate 1 / T 1 obeys an activation behavior 1 / T 1 ∝ exp [ - E g / k B T ] with an energy gap of E g / k B = 1300 and 1000 K for CeFe 4 P 12 and CeRu 4 P 12 , respectively. These are in good agreement with the values measured by various measurements.

Ceramic high- T C superconductors may be viewed as a weakly coupled random Josephson network containing the so-called π-junctions, and a chiral-glass transition is predicted to be occurred in such d-wave ceramics. In the present work, pressure dependence of the chiral-glass transition have been investigated on YBa 2 Cu 3 O 7− y and YBa 2 Cu 4 O 8 ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at T C1 and furthermore among the grains at T C2 ( T C1 ). The T C2 increases with increase of pressure, while the sharpness of the transition at T C2 does not change. In spite of differences in not only T C2 ’s but also T C1 ’s, the increasing rates of T C2 by pressure are nearly the same.