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In contrast to the "Pr-issue" that neither PrBa2Cu3O7 (Pr123) nor PrBa2Cu4O8 (Pr124) shows superconductivity (SC), we have observed 100%-fraction of SC in an oxygen-reduced Pr2Ba4Cu7O(15-delta) (Pr247) which has a hybrid structure that Pr247 = Pr123 + Pr124. It is found that Cu nuclear-spin signals from the CuO2 planes observed at 300K are completely wiped out down at 2K. Instead, plane signals at 2K are observed at higher frequencies. This indicates that, despite the bulk SC, the CuO2 planes in Pr247 are found to be in an insulating and antiferromagnetically ordered state., Comment: 12 pages, 4 figures

Ytterbium silicide-based thermoelectric materials have attracted attention because they exhibit large power factors owing to large absolute values of their Seebeck coefficient as well as high electrical conductivity. Here, we demonstrate that, in the Yb(Si1− xGe x)2 system, by controlling the valence fluctuation, we combine two phenomena, namely, the Kondo effect and suppression of the spin fluctuation. This leads to a large density of states and the suppression of the spin fluctuation of the 4 f electrons, thereby resulting in a high Seebeck coefficient with extremely low resistivity. Consequently, Yb(Si0.5Ge0.5)2 exhibits a high-power factor of 8.4 mW K−2 m−1, which makes energy harvesting from a cold source a practical possibility.

We report on a pure $^{181}\mathrm{Ta}$ nuclear quadrupole resonance (NQR) measurement of ${\mathrm{PbTaSe}}_{2}$ at zero magnetic field, which has the advantage of directly probing the intrinsic superconducting phase and electronic states of the ${\mathrm{TaSe}}_{2}$ layer. We observed the $^{181}\mathrm{Ta}$ NQR spectrum of the intrinsic structure with space group $P\overline{6}m2$, which agrees well with density functional theory calculations. The nuclear spin relaxation rate $1/{T}_{1}$ shows an exponential decrease well below ${T}_{{}_{\mathrm{c}}}$, indicating that the superconducting state is fully gapped in the framework of BCS theory. The gap size obtained by $^{181}\mathrm{Ta}$ NQR was smaller than the value in previous reports, which may imply that the Fermi surfaces composed of $\mathrm{Ta}\phantom{\rule{4pt}{0ex}}5d$ orbitals, where the average pairing interactions are expected to be weaker than in the BCS model, are primarily probed. The temperature dependence of $1/{T}_{1}$ below ${T}_{{}_{\mathrm{c}}}$ can be reproduced well by the superposition of quadrupole and magnetic relaxation mechanisms, together with the distribution of the superconducting gap size inherent in multiple Fermi surfaces theoretically proposed in ${\mathrm{PbTaSe}}_{2}$.

Pb1−xTlxTe is an anomalous low carrier density superconductor for which the origin of the superconductivity is not understood. Thallium is the only dopant to cause superconductivity in PbTe, suggesting that these specific impurities have a unique effect on the electronic states. Systematic 125Te-NMR studies on single-crystallines Pb1−xTlxTe (x = 0, 0.35, 1.0 at%) reveal that the Tl dopants induce spatially inhomogeneous electronic states around the Tl dopants. In the superconducting sample at x = 1.0 at%, 125Te nuclear spin relaxation rate (1/T1T) in the vicinity of the Tl dopants is unexpectedly enhanced below $\sim $ 10 K. This coincides with the temperature below which the resistivity exhibits an upturn. By contrast, such anomalies were not detected in the non-superconducting sample at x = 0.35 at%. These observations can be consistently explained by charge Kondo effect, in which two nearly degenerate valence states of the Tl dopant form a resonating valence state upon cooling below 10 K. Based on these observations, we suggest that the coherent hopping of 6s-electron pair may provide the pairing interaction (i.e., a negative-U scenario), leading to the anomalously high Tc found for Pb1−xTlxTe.

We report NMR experiments on heavily electron-doped Fe-based superconductor in comparison with the results on the parent Fe-based compounds. The typical parent Fe-based compound LaFe(As1-x′P x′)O exhibits the re-emergent antiferromagnetic (AFM) order at x′ ~ 0.6 (AFM2) separated from the parent AFM order at x′ =0 (AFM1). Systematic 31P-NMR study on [Sr4Sc2O6]Fe2(As1−x P x )2 (SrSc42622), which has local lattice parameters of iron-pnictogen (FePn) layer similar to the series of LaFe(As1−x′/P x′)O, also revealed that the presence of AFM1 order is universal for most of parent Fe-based compounds. In contrast, the static AFM2 order was absent in this series, however, the dynamical low-energy AFM spin fluctuations are enhanced at around x ~ 0.8, indicating that the onset of the static AFM2 is quite sensitive to the local lattice parameters of FePn layer. In order to elucidate the further universality and diversity, we have carried out 77Se-NMR measurement on Li x (NH3)yFe2−δ Se2 (Tc = 44 K) in heavily electron-doped regime. Although the spin fluctuations at low energies does not significantly develops upon cooling, the moderate spin fluctuations were extracted at high temperatures from comparison of the temperature (T) dependences of Knight shift and nuclear relaxation rate (1/T 1 T). We discuss the universality and diversity of the relationship between the Tc and the characteristics of the spin fluctuations in the Fe-based compounds from a microscopic point of the NMR measurements.

We report a systematic NMR study on [Sr4Sc2O6]Fe-2(As1-xPx)(2), for which the local lattice parameters of the iron-pnictogen (FePn) layer are similar to those of the series LaFe(As1-x'Px')O, which exhibits two segregated antiferromagnetic (AFM) order phases, AFM1 at x' = 0 - 0.2 and AFM2 at x' = 0.4 - 0.7. Our results reveal that the parent AFM1 phase at x = 0 disappears at x = 0.3 - 0.4, corresponding to a pnictogen height (h(pn)) from the Fe plane of 1.3-1.32 angstrom, which is similar to that of LaFe(As1-x'Px')O and various parent Fe pnictides. By contrast, the AFM2 order reported for LaFe(As0.4P0.6)O does not appear at x similar to 0.8, although the local lattice parameters of the FePn layer and the microscopic electronic states are quite similar. Despite the absence of the static AFM2 order, reemergent dynamical AFM spin fluctuations were observed at approximately x similar to 0.8, which can be attributed to the instability of the AFM2 phase. We suggest this re-enhancement of AFM spin fluctuations plays a significant role in enhancing the T-c to 17 K for x = 0.8 - 1. Finally, we discuss the universality and diversity of the complicated magnetic ground states from a microscopic point of view, including the difference in the origins of the AFM1 and AFM2 phases, and their relations with the high superconducting transitions in Fe pnictides.

The phase diagram of LaFeAs$_{1-x}$P$_x$O system has been extensively studied through hole- and electron-doping as well as As/P-substitution. It has been revealed that there are three different superconducting phases with different Fermi surface (FS) topologies and thus with possibly different pairing glues. One of them is well understood as spin fluctuation-mediated superconductivity within a FS nesting scenario. Another one with the FSs in a bad nesting condition must be explained in a different context such as orbital or spin fluctuation in strongly correlated electronic system. In both phases, $T$-linear resistivity was commonly observed when the superconducting transition temperature $T_{\rm c}$ becomes the highest value, indicating that the strength of bosonic fluctuation determines $T_{\rm c}$. In the last superconducting phase, the nesting condition of FSs and the related bosonic fluctuation are moderate. Variety of phase diagram characterizes the multiple orbital nature of the iron-based superconductors which are just near the boundary between weak and strong correlation regimes., 18 pages, 9 figures, accepted in PRB

We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental results indicate the close relationship between the AFMSFs and the SC from the hole-doped state to heavily electron-doped state, which shed light on a unique SC phase emerged in the heavily electron-doped regime being formally equivalent to non-SC compound Ba(Fe0.5Co0.5)Fe2As2., Comment: 5 pages, 4 figures

We report $^{63}$Cu- and $^{205}$Tl-NMR studies on six-layered ($n$=6) high-$T_c$ superconducting (SC) cuprate TlBa$_2$Ca$_5$Cu$_6$O$_{14+\delta}$ (Tl1256) with $T_c\sim$100 K, which reveal that antiferromagnetic (AFM) order takes place below $T_{\rm N}\sim$170 K. In this compound, four underdoped inner CuO$_2$ planes ($n$(IP)=4) sandwiched by two outer planes (OPs) are responsible for the onset of AFM order, whereas the nearly optimally-doped OPs responsible for the onset of bulk SC. It is pointed out that an increase in the out-of-plane magnetic interaction within an intra-unit-cell causes $T_{\rm N}\sim$ 45 K for Tl1245 with $n$(IP)=3 to increase to $\sim$170 K for Tl1256 with $n$(IP)=4. It is remarkable that the marked increase in $T_{\rm N}$ and the AFM moments for the IPs does not bring about any reduction in $T_c$, since $T_c\sim 100$ K is maintained for both compounds with nearly optimally doped OP. We highlight the fact that the SC order for $n\ge5$ is mostly dominated by the long-range in-plane SC correlation even in the multilayered structure, which is insensitive to the magnitude of $T_{\rm N}$ and the AFM moments at the IPs or the AFM interaction among the IPs. These results demonstrate a novel interplay between the SC and AFM orders when the charge imbalance between the IPs and OP is significantly large., Comment: 4 pages, 4 figures, Accepted for publication in J. Phys. Soc. Jpn

We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-Tc$ superconductivity (HTSC) for a disorder-free CuO$_2$ plane based on an evaluation of local hole density ($p$) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO$_2$ plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO$_2$ plane in an underdoped regime of HTSC. The $T$=0 phase diagram of AFM constructed here is in quantitative agreement with the theories in a strong correlation regime which is unchanged even with mobile holes. This {\it Mott physics} plays a vital role for mediating the Cooper pairs to make $T_c$ of HTSC very high. By contrast, we address from extensive NMR studies on electron-doped iron-oxypnictides La1111 compounds that the increase in $T_c$ is not due to the development of AFM spin fluctuations, but because the structural parameters, such as the bond angle $\alpha$ of the FeAs$_4$ tetrahedron and the a-axis length, approach each optimum value. Based on these results, we propose that a stronger correlation in HTSC than in FeAs-based superconductors may make $T_c$ higher significantly., Comment: 5 pages, 4 figures, accepted for publication in J.Phys.Chem.Solids (2010)

We report a 63 Cu-NMR study on underdoped Hg-based five-layered ( n = 5) compounds HgBa 2 Ca 4 Cu 5 O 12+ y (Hg-1245) with T c = 82 K and 92 K, and review the previous Cu-NMR results on the various multilayered cuprates. On the basis of extensive experiments on n = 5 compounds, we have unraveled that (1) the antiferromagnetic metallic (AFMM) phase is robust up to the carrier-density N h ∼ 0.17 , (2) the uniformly mixed state of superconductivity (SC) and AFMM is realized at least in 0.14 N h ⩽ 0.17 , (3) T c has a peak close to the quantum critical point (QCP) at which the AFM order collapses, indicating the strong relationship between the high- T c SC and AFM order. In the four-layered ( n = 4 ) compounds, the QCP of AFM order was at N h ∼ 0.15 , which is lower than the case for n = 5 compounds, suggesting that the weaker AFM interlayer coupling makes the carrier-density at QCP decrease. In the light of recent progress in experiments that have unraveled intrinsic phase diagrams in the underdoped regime, we deal with the N h dependence of the AFM moment and an in-plane superexchange J in in order to gain insight into the magnetic characteristics in antiferromagnetic high- T c superconductors.

We report 63Cu-NMR/NQR on the bi-layered high- T c copper oxide Ba 2 CaCu 2 O 4 F 2 , which is expected to be a Mott insulator in an ideal case of a nominal content of fluorine, and less fluorinated Ba 2 CaCu 2 O 4 ( F 1.6 O 0.4 ) to clear the doping mechanism in apical-fluorine multilayered system Ba 2 Ca n - 1 Cu n O 2 n F 2 , where n is the number of the layers in a unit cell. In the case of n = 3 and, especially, n = 4 , the origin of carrier doping has been attributed to a self-doping mechanism based on the nominal stoichiometry; electrons are transferred between the inner CuO 2 plane (IP) and the outer one (OP), keeping Cu + 2 on average. It is revealed that Ba 2 CaCu 2 O 4 F 2 is an underdoped superconductor with hole carriers, doped by an inevitable deviation from the nominal content of apical fluorines. This result suggests that hole carriers are doped in the same manner for the case of n = 3 and 4, and that we should reconsider the self-doping mechanism as the carrier source in the apical-fluorine system.

We review our nuclear-magnetic resonance (NMR) and nuclear-quadrupole-resonance (NQR) studies in superconducting Sr2RuO4, which have been performed in order to investigate the gap structure and the pairing symmetry in the superconducting state and magnetic fluctuations in the normal state. The spin-lattice relaxation rate (1/T1) of a high-quality sample with Tc∼1.5K shows a sharp decrease without a coherence peak just below Tc, followed by a T3 behavior down to 0.15 K. This result indicates that the superconducting gap in pure Sr2RuO4 is a highly anisotropic character with a line-node gap. The Knight shift, which is related to the spin susceptibility, is unchanged in the superconducting state irrespective of the direction of the applied fields and various magnitude of the field. This result strongly suggests that the superconducting pairs are in the spin-triplet state, and the spin direction of the triplet pairs is considered to be changed by small fields of several hundred Oe.

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 ...

Zero- and longitudinal-field muon-spin rotation (ZF-μSR and LF-μSR) measurements were carried out on a multi-layered HgBa 2 Ca 4 Cu 5 O y (Hg-1245) superconductor with T c of 108 K. With decreasing temperature, the observed ZF-μSR function changed from a Gaussian-type to an exponential-type below 60 K and zero field muon precession was also observed below 45 K. These μSR signals are attributed to the development of the antiferromagnetic ordering in inner CuO 2 planes with a low carrier concentration. These results seem to indicate clear evidence that antiferromagnetism microscopically coexists with superconductivity in the Hg-1245 superconductor below 60 K.

We report systematic Cu-NMR studies on five-layered cuprates from under-doped HgBa2Ca4Cu5O12+δ (Hg-1245(UD)) to slightly overdoped Tl-1245(OVD), and compare with optimally-doped Hg-1245(OPT). In the under-doped Hg-1245(UD), antiferromagnetism (AFM) has been found to take place at TN = 290 K, exhibiting a large antiferromagnetic moment of 0.67–0.69 μB at three inner planes (IP’s). These values are comparable to that reported for non-doped cuprates, suggesting that the IP’s may be in a nearly non-doped regime. Most surprisingly, the AFM order is also detected with MAFM(OP) = 0.1 μB even at two outer planes (OP’s) that are responsible for the onset of superconductivity (SC) with Tc = 72 K. The high-Tc SC at Tc = 72 K can uniformly coexist on a microscopic level with the AFM at OP’s. This is the first microscopic evidence for the uniformly mixed phase of AFM and SC on a single CuO2 plane. Although, the AFM/SC mixed CuO2 planes are significantly separated by three non-doped AFM layers, the onset of AFM does not prevent the occurrence of SC with the high value of Tc = 72 K.

We report systematic 31P-NMR study on iron (Fe)-based superconductors SrFe2(As_{1-x}P_{x})2 (Sr122AsP), in which a superconducting (SC) transition temperature Tc at x=0.35 increases from Tc=26 K up to 33 K by annealing an as-grown mono-crystalline sample. The present NMR study has unraveled that Tc reaches a highest value of 33 K at x=0.35 around a quantum critical point at which antiferromagnetic (AFM) order disappears. When noting that the SC transition disappears at x=0.6 where the AFM spin fluctuations (SFs) are no longer present, we remark that the onset and increase of Tc are apparently associated with the emergence and enhancement of AFM-SFs, respectively. In the SC state, the residual density of state (RDOS) at the Fermi energy EF in the SC state becomes much smaller for the annealed sample than for the as-grown one, suggesting that some inhomogeneity and/or imperfection for the latter increases RDOS as expected for unconventional SC state with nodal gap. These findings in Sr122AsP are consistent with the unconventional s(+-)-wave Cooper pairing state that is mediated by AFM-SFs. We also discuss other key-ingredients besides the AFM-SFs to increase Tc further.

We present a scanning tunneling microscopy/spectroscopy (STM/STS) study of synthetic polycrystalline boron-doped diamond in the temperature range 0.5–4.3 K. At 4.3 K the sample-surface was very non-uniform and tunneling I(V) spectra were typical for p-type semiconductors. After cooling below the superconducting transition temperature, we detected and measured the superconducting gap of diamonds. At temperatures around 0.5 K the energy gap was around 0.8 and 1 mV (for two different samples).

We report the measurements of the critical current of superfluid 3He film for the thickness of 0.47 µm. By employing inter-digitated capacitors, 2.5×10−2 mm33He can be manipulated with 0.4% resolution. Critical current Jfc of superfluid 3He film were observed for the onset of dissipative flow. The magnitude of measured Jfc is considerably smaller than the theoretically predicted pair-breaking critical current. This resugt indicates that another mechanism must participate in the dissipation of superflow.

We report the first mobility measurement of ions trapped below the free surface of normal and superfluid 3He-B. A Corbino electrode arrangement is used to measure the mobility of ions. Applying an electrostatic step potential to the Corbino electrode, a charge distribution of ions becomes inhomogeneous, which induces the motion of ions. We observe the time development of the redistribution of ions by employing a capacitive coupling method. The mobility of ions is deduced from the transient response of ions, using a distributed-constant equivalent circuit analysis. The measured mobility of both types of ions are in agreement with that in the bulk normal and superfluid 3He.

We report on superconducting nature under pressure in CeIr(In1–xCdx)5 by In-Nuclear-Quadrupole-Resonance (NQR) studies. In CeIr(In0.925Cd0.075)5, the inhomogeneous antiferromagnetic order at TN ~ 2.3 K is induced by Cd-dopants and superconductivity disappears at ambient pressure. However, the measurements of a nuclear-spin-lattice-relaxation rate 1/T1 have revealed that the superconductivity suddenly occurs above 2.1 GPa. It is observed that the superconducting gap is enhanced in the Cd-doped sample, indicating that the Cd-doping induces the strong coupling superconductivity leading to the enhancement of Tc in the CeIrIn5 system. Furthermore, we found that the residual density of states at the Fermi level increases with increasing pressure, suggesting that the superconducting nature of CeIrIn5 is quite different from those of CeCoIn5 and CeRhIn5.

31P-NMR study on LaFe(As1–x P x )(O0.86F0.14) unraveled that antiferromagnetic spin fluctuations (AFMSFs) emerge significantly at x=0.4 where the Tc is markedly enhanced, indicating that the AFMSFs are one of the important factors for raising Tc . From extensive comparison over wide compositions for 0 ≤ x ≤1 and 0 ≤ y ≤ 0.14, we revealed that there are two different types in temperature evolution of the AFMSFs: One is enhanced particularly at low energies that evolves only at low temperatures, which mainly derives from the two orbitals of dxz/yz . The other is distributed broadly at finite energies that appears up to high temperatures, which derives from three orbitals of dxy and dxz/yz . The highest Tc (=27 K) state in the present compositions appears at (x, y)=(0.4, 0.1) where two characteristics of AFMSFs merge, suggesting the contribution of the AFMSFs over wide energies to the onset of SC. The nonmonotonic variation of Tc in LaFe(As1–x P x )(O1–y F y ) is attributed to the AFMSFs from degenarated multiple-3d-orbitals on iron-pnictide superconductors.

P/As substitution effects on the transport properties of polycrystalline ${\mathrm{LaFeP}}_{1\text{\ensuremath{-}}x}{\mathrm{As}}_{x}{\mathrm{O}}_{1\text{\ensuremath{-}}y}{\mathrm{F}}_{y}$ with $x$ = 0--1.0 and $y$ = 0--0.1 have been studied. In the F-free samples $(y$ = 0), a superconducting (SC) dome with a maximum ${T}_{c}$ of 12 K is observed around $x$ = 0--0.3. This is separated from another SC dome with ${T}_{c}$ $\ensuremath{\sim}$ 10 K at $x$ = 0.6--0.8 by an antiferromagnetic region $(x$ = 0.3--0.6), giving a two-dome feature in the ${T}_{c}\ensuremath{-}x$ phase diagram. As $y$ increases, the two SC domes merge together, changing to a double-peak structure at $y$ = 0.05 and a single dome at $y$ = 0.1. This proves the presence of two different Fermi surface states in this system.

We revealed novel phase deagram of Fe-pnictide high-Tc superconductor LaFe(As_{1-x}P_{x})O in wide doping level (0.3, Comment: 4 pages, 5 figures

that a hole density p(IP) (p(OP)) at the inner (IP) (outer (OP)) plane is significantly smaller (larger) than an optimal hole density p = 0:16, at which Tc exhibits a maximum. It differs significantly from the result on optimally doped Hg1223 with Tc=133 K, in which p(IP) and p(OP) are both rather close to p=0.16. Based on the accumulated results on multilayered cuprates(n= 3 5), we suggest that this large imbalance between p(IP) and p(OP) is one of the important factors for the suppression of the bulk Tc. We suggest experimentally that Tc might be enhanced up to around 160 K in cuprates if every layer in the multilayered structure(n= 3 5) could be optimally doped with p=0.16.

Systematic P-NMR studies on LaFe(As_{1-x}P_x)(O_{1-y}F_y) with y=0.05 and 0.1 have revealed that the antiferromagnetic spin fluctuations (AFMSFs) at low energies are markedly enhanced around x=0.6 and 0.4, respectively, and as a result, Tc exhibits respective peaks at 24 K and 27 K against the P-substitution for As. This result demonstrates that the AFMSFs are responsible for the increase in Tc for LaFe(As_{1-x}P_x)(O_{1-y}F_y) as a primary mediator of the Cooper pairing. From a systematic comparison of AFMSFs with a series of (La_{1-z}Y_z)FeAsO_{\delta} compounds in which Tc reaches 50 K for z=0.95, we remark that a moderate development of AFMSFs causes the Tc to increase up to 50 K under the condition that the local lattice parameters of FeAs tetrahedron approaches those of the regular tetrahedron. We propose that the T_c of Fe-pnictides exceeding 50 K is maximized under an intimate collaboration of the AFMSFs and other factors originating from the optimization of the local structure.

We present the In-NQR results under pressure ( P ) in CeIr ( In 1 - x Cd x ) 5 . In CeCoIn 5 and CeRhIn 5 , the occurrence of superconductivity (SC) is related with the antiferromagnetic spin fluctuations (AFM-SFs) originating from the antiferromagnetic quantum-critical point (AFM-QCP). The high- T c SC ( T cmax > 2 K ) is realized in both compounds. However, in CeIrIn 5 which is apart from the AFM-QCP, SC occurs even without AFM-SFs and the quite small value of T cmax (∼1 K) is observed around P = 3 GPa . The mechanism of SC in CeIrIn 5 may be different from that in CeCoIn 5 and CeRhIn 5 .

This paper introduces nuclear magnetic resonance works in the strongly correlated super-conductors: heavy-Fermion, high-T C superconductors and Sr2RuO4. The analyses strongly support the spin-fluctuation-mediated superconductivity model in a high-T C superconductor.

We have investigated a gap structure in the spin-triplet superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ through the measurement of the ${}^{101}\mathrm{Ru}$ nuclear spin-lattice relaxation rate ${}^{101}(1/{T}_{1})$ down to 0.09 K at zero magnetic field. In the superconducting state, $1/{T}_{1}$ in a high-quality sample with ${T}_{c}\ensuremath{\sim}1.5\mathrm{K}$ exhibits a sharp decrease without the coherence peak, followed by a ${T}^{3}$ behavior down to 0.15 K. This result is in marked contrast to the behavior observed below $\ensuremath{\sim}0.4\mathrm{K}$ in samples with lower ${T}_{c}$, where ${T}_{1}T$ is a constant. This behavior is demonstrated to be not intrinsic. We conclude that the gap structure in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ is significantly anisotropic, consistent with line-node-like models.

By means of Ru NMR, magnetic properties in three-dimensional (3D) perovskites ${\mathrm{SrRuO}}_{3}$ (ferromagnet) and ${\mathrm{CaRuO}}_{3}$ (exchange enhanced paramagnet) were investigated and compared with those in the two-dimensional (2D) layered perovskite ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ (superconductor), ${\mathrm{RuO}}_{2},$ and Ru metal (Pauli paramagnet). We found that $4d$-spin contributions in the Knight shift and ${}^{101}{(1/T}_{1})$ of Ru are predominant in ${\mathrm{SrRuO}}_{3},$ ${\mathrm{CaRuO}}_{3},$ and ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4},$ but not in nonmagnetic ${\mathrm{RuO}}_{2}$ and Ru metal. The experimental results that a Stoner factor for ${\mathrm{CaRuO}}_{3}$ is close to 1 and that a correlation factor estimated from modified-Korringa behavior of $\mathrm{Ru}\ensuremath{-}{T}_{1}$ is much less than 1 indicate that ${\mathrm{CaRuO}}_{3}$ is a nearly ferromagnetic (FM) metal dominated by low-frequency and long-wavelength components of the spin fluctuations (SF). Even though the Stoner factor for ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ also indicates the closeness to the ferromagnetism, the SF in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ are different from those in ${\mathrm{CaRuO}}_{3}.$ The results combined with the previous ${}^{17}\mathrm{O}$ NMR study in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ indicate that the in-plane low-frequency components of SF are exchange enhanced without any significant q dependence due to 2D electronic character (2D nearly FM), while the out-of-plane low-frequency component of SF shows the existence of antiferromagnetic (AFM) SF between layers at low temperature. We propose that this evolution from 3D to 2D nearly FM SF is relevant to the onset of spin-triplet superconductivity in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ The 2D nearly FM SF in the in-plane ${4d}_{\mathrm{xy}}\ensuremath{-}{p}_{\mathrm{\ensuremath{\pi}}\mathrm{}}$ band may play a significant role for the stabilization of parallel spin pairing state within the basal plane among various representations of spin-triplet order parameter in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$

We review recent NMR investigations on superconducting and magnetic properties in layered perovskite Sr 2 RuO 4 , isostructural to a high-T c cuprate. Measurements of the Knight shift and the nuclear spin-lattice relaxation rate 1/T 1 have identified its Cooper-pair spin state as a triplet and uncovered pronounced anisotropy in spin-fluctuation spectrum in Sr 2 RuO 4 The in-plane low-frequency components of the dynamical susceptibility are exchange-enhanced with a weak q dependence associated with the two-dimensional (2D) character in electronic structure. By contrast, its out-of-plane component is unexpectedly enhanced by AF spin fluctuations upon cooling below T * ∼130 K where the c-axis resistivity shows metallic behavior. We have found that the slightly-distorted cubic perovskite CaRuO 3 is a nearly ferromagnetic metal dominated by ferromagnetic spin fluctuations with a Stoner factor α=0.98. Contrasting behavior of spin fluctuations between Sr 2 RuO 4 and CaRuO 3 is due to their different dimensionality in electronic structure with the closeness to ferromagnetism. We remark that a simple ferromagnetic-spin-fluctuation-mediated mechanism with analogy to 3 He is not appropriate for an onset of spin-triplet p-wave superconductivity in Sr 2 RuO 4 . Altemately, we propose possible scenario that the in-plane local exchange interaction, which originates presumably from the Hund's coupling between the 4d and 4d xz,yz orbitals, may play a role for the occurrence of spin-triplet p-wave superconductivity.

Recent NMR measurements on high- T c and related material Sr 2 RuO 4 by the Osaka group are reviewed. The following results are presented: (1) Study of YBa 2 Cu 3 O 7 +Ni has revealed that the attractive force is spin in origin. (2) Analysis of T 1 , T 2 and Knight shift ( K ) in various high- T c materials supports the antiferromagnetic spin-fluctuation mediated superconducting model. (3) Temperatures at which a pseudogap starts to open obtained by 1/ T 1 T , d K /d T , photoemission and resistivity agree with each other in the Bi 2 Sr 2 CaCu 2 O 8+ δ system. (4) 17 O Knight shift measurements in the related material Sr 2 RuO 4 suggests that a triplet pairing is realized in this compound as in the heavy-fermion superconductor UPt 3 .

17O Knight shift measurements in Sr2RuO4 were performed over the wide range of magnetic field 3.2-11.4kOe parallel to the basal RuO2 planes. The spin susceptibility is totally unchanged through its Tc, evidencing that the spin-triplet superconducting state is realized in Sr2RuO4. The result indicates that the Cooper pairs consist of the parallel spin pairs | ↑↑> and | ↓↓> with their quantization axis perpendicular to the c-axis direction. The in-plane 2D nearly ferromagnetic spin fluctuations may play a role for the stabilization of this state among various representations of spin-triplet order parameter.

We report a 63 Cu-NMR study on a tri-layered high-temperature superconductor Ba 2 Ca 2 Cu 3 O 6 F 2 (0223F(2.0)), which is composed of two outer planes (OPs) and one inner plane (IP). Since all of the apical oxygen sites are substituted by fluorine, this cuprate has a nominal Cu 2 + valence on average and should be antiferromagnetic insulator, however it is a superconductor with T c = 76 K . The Cu-NMR study has demonstrated that IP is hole-doped with a doping level of 13–15% and hence each doping level at two OPs is estimated to be 6–8% by incorporating a total charge balance. Such the self-doping that electrons are transferred from IP to OPs causes the superconductivity in 0223F(2.0) as well as in four-layered Ba 2 Ca 3 Cu 4 O 8 F 2 (0234F(2.0)).

We report the 73Ge-NQR studies on the antiferromagnetic (AFM) heavy-fermion compound CeNiGe 3 which shows two domes like pressure-induced superconducting phases in the pressure (P) ranges of 1.7–3.5 GPa and 5.9–7.3 GPa denoted as SC1 and SC2, respectively [M. Nakashima, et al., J. Phys. Condens. Matter. 16 (2004) L255, H. Kotegawa, et al., J. Phys. Soc. Jpn. 75 (2006) 044713]. The NQR spectra have revealed a change from an incommensurate AFM structure at P = 0 and 2.0 GPa into a commensurate one at P = 2.8 GPa . The onset of the SC1 may be relevant to an intimate evolution from the incommensurate into commensurate AFM-spin structure as P increases.

We report on the pressure (P)-induced phase diagram of antiferromagnetism (AFM) and superconductivity (SC) which emerges at their border at zero magnetic field. The nuclear-quadrupole-resonance (NQR) measurements have revealed that AFM can take place in the superconducting state with T c ∼ 2.1 K at P = 2.05 GPa and the AFM quantum phase transition (QPT) occurs near P = 2.1 GPa . These results reveal the existence of the tetracritical point in the P–T phase diagram of AFM and SC in CeRnIn 5 . We also found the gapless nature in the uniformly coexisting phase of AFM and SC, indicating an intimate relationship between AFM and SC.

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 on the pressure ( P ) -induced phase diagram of antiferromagnetism (AFM) and superconductivity (SC) which emerges at their border at zero magnetic field. The nuclear-quadrupole-resonance (NQR) measurements have revealed that AFM can take place in the superconducting state with T c ∼ 2.1 K at P = 2.05 GPa . The present experiments have, for the first time, demonstrated that AFM phase, AFM + SC uniformly coexisting phase, SC phase and paramagnetic phase all contact with each other at T tetra ∼ 2.1 K and P tetra ∼ 2 GPa which evidence the existence of the tetracritical point in the P – T phase diagram of AFM and SC in CeRnIn 5 .

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

We report on pressure-induced evolutions of antiferromagnetic spin fluctuations (AFM-SFs) and unconventional superconductivity on the x = 0.6 sample in a series of compounds CeRh 1 - x Ir x In 5 via the 115In nuclear-quadrupole-resonance (NQR) experiment. The In-NQR 1 / T 1 measurements have revealed that the slight application of pressure makes T c = 0.9 K at P = 0 increase up to T c = 1.06 K at P = 0.47 GPa . Concomitantly, the character of AFM-SF evolves from an isotropic regime to an anisotropic one as P increases. These results suggest that AFM-SFs play vital role in mediating the strong-coupling superconductivity in CeRh 1 - x Ir x In 5 .

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