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Investigating the pressure dependence of the superconducting (SC) transition temperature $T_{\rm c}$ is crucial for understanding the SC mechanism. Herein, we report on the pressure dependence of $T_{\rm c}$ in the nonmagnetic topological line-nodal material CaSb$_2$, based on measurements of electric resistance and alternating current magnetic susceptibility. $T_{\rm c}$ initially increases with increasing pressure and peaks at $\sim$ 3.1~GPa. With a further increase in pressure, $T_{\rm c}$ decreases and finally becomes undetectable at 5.9~GPa. Because no signs of phase transition or Lifshitz transition are observed in the normal state, the peculiar peak structure of $T_{\rm c}$ suggests that CaSb$_2$ has an unconventional SC character., Comment: 5 pages, 5 figures

To investigate spin susceptibility in a superconducting (SC) state, we measured the $^{125}\mathrm{Te}$-NMR Knight shifts at magnetic fields ($H$) up to 6.5 T along the $b$ and $c$ axes of single-crystal ${\mathrm{UTe}}_{2}$, a promising candidate for a spin-triplet superconductor. In the SC state, the Knight shifts along the $b$ and $c$ axes (${K}_{b}$ and ${K}_{c}$, respectively) decreased slightly, and the decrease in ${K}_{b}$ was almost constant up to 6.5 T. The reduction in ${K}_{c}$ decreased with increasing $H$, and ${K}_{c}$ was unchanged through the SC transition temperature at 5.5 T, excluding the possibility of spin-singlet pairing. Our results indicate that spin susceptibilities along the $b$ and $c$ axes slightly decrease in the SC state in low $H$, and the $H$ response of SC spin susceptibility is anisotropic on the $bc$ plane. We discuss the possible $\mathbit{d}$-vector state within the spin-triplet scenario and suggest that the dominant $\mathbit{d}$-vector component for the case of $H\ensuremath{\parallel}b$ changes above 13 T, where ${T}_{\mathrm{c}}$ increases with increasing $H$.

We conducted$^{195}$Pt-nuclear magnetic resonance measurements on various-diameter Pt nanoparticles coated with polyvinylpyrrolidone in order to detect the quantum size effect and the discrete energy levels in the electron density of states, both of which were predicted by Kubo more than 50 years ago. We succeeded in separating the signals arising from the surface and interior regions and found that the nuclear spin-lattice relaxation rates in both regions show the metallic behavior at high temperatures. Surprisingly, the magnetic fluctuations in both regions exhibited anomalous behavior below the same temperature $T^*$, which points to a clear size dependence and is well scaled with $\delta_\mathrm{Kubo}$. These results suggest that a size-tunable metal-insulator transition occurs in the Pt nanoparticles as a result of the Kubo effect., Comment: 11 pages, 10 figures, including supplemental materials, accepted for publication in Phys. Rev. B as a Rapid Communication

To identify the key parameter for optimal superconductivity in iron pnictides, we measured the $^{31}\mathrm{P}$-NMR relaxation rate on ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}\phantom{\rule{4pt}{0ex}}(x=0.22$ and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For $x=0.22$, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4 GPa, whereas the superconducting transition temperature ${T}_{\mathrm{c}}$ increases to twice its former value. In contrast, for $x=0.28$ near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and ${T}_{\mathrm{c}}$ reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the ${T}_{\mathrm{c}}$ dome as a function of Weiss temperature $\ensuremath{\theta}$, which measures the distance to the QCP. Moreover, the ${T}_{\mathrm{c}}\text{\ensuremath{-}}\ensuremath{\theta}$ curve under pressure precisely coincides with that with a chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$.

In this Rapid Communication, a set of $^{209}\mathrm{Bi}$-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) ${\mathrm{BaTi}}_{2}{\mathrm{Bi}}_{2}\mathrm{O}$ from a microscopic point of view. The NMR and NQR spectra at 5 K can be reproduced with a nonzero in-plane anisotropic parameter $\ensuremath{\eta}$, indicating the breaking of the in-plane fourfold symmetry at the Bi site without any magnetic order, i.e., ``the electronic nematic state.'' In the SC state, the nuclear spin-lattice relaxation rate divided by temperature, $1/{T}_{1}T$, does not change even below ${T}_{\mathrm{c}}$, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two dimensionality in ${\mathrm{BaTi}}_{2}{\mathrm{Bi}}_{2}\mathrm{O}$. Comparing the NMR/NQR results among ${\mathrm{BaTi}}_{2}P{n}_{2}\mathrm{O}$ ($Pn=\text{As}$, Sb, and Bi), it was found that the normal and SC properties of ${\mathrm{BaTi}}_{2}{\mathrm{Bi}}_{2}\mathrm{O}$ were considerably different from those of ${\mathrm{BaTi}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$ and ${\mathrm{BaTi}}_{2}{\mathrm{As}}_{2}\mathrm{O}$, which might explain the two-dome structure of ${T}_{\mathrm{c}}$ in this system.

We have performed $^{119}\mathrm{Sn}$-NMR measurements on the antiperovskite oxide superconductor ${\mathrm{Sr}}_{3\ensuremath{-}x}\mathrm{SnO}$ to investigate how its normal state changes with the Sr deficiency. A two-peak structure was observed in the NMR spectra of all the measured samples. This suggests that the phase separation tends to occur between the nearly stoichiometric and heavily Sr-deficient ${\mathrm{Sr}}_{3\ensuremath{-}x}\mathrm{SnO}$ phases. The measurement of the nuclear spin-lattice relaxation rate $1/{T}_{1}$ indicates that the Sr-deficient phase shows a conventional metallic behavior due to the heavy hole doping. In contrast, the nearly stoichiometric phase exhibits unusual temperature dependence of $1/{T}_{1}$, attributable to the presence of a Dirac-electron band.

$^{73}$Ge and $^{59}$Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed on a $^{73}$Ge-enriched single-crystalline sample of the ferromagnetic superconductor UCoGe in the paramagnetic state. The $^{73}$Ge NQR parameters deduced from NQR and NMR are close to those of another isostructural ferromagnetic superconductor URhGe. The Knight shifts of the Ge and Co sites are well scaled to each other when the magnetic field is parallel to the $b$ or $c$ axis. The hyperfine coupling constants of Ge are estimated to be close to those of Co. The large difference of spin susceptibilities between the $a$ and $b$ axes could lead to the different response of the superconductivity and ferromagnetism with the field parallel to these directions. The temperature dependence of the nuclear spin-lattice relaxation rates $1/T_1$ at the two sites is similar to each other above 5 K. These results indicate that the itinerant U-$5f$ electrons are responsible for the ferromagnetism in this compound, consistent with previous studies. The similarities and differences in the three ferromagnetic superconductors are discussed.

We have performed $^{63}\mathrm{Cu}$ nuclear-magnetic-resonance/nuclear-quadrupole-resonance measurements to investigate the magnetic and superconducting (SC) properties on a ``superconductivity dominant'' ($S$-type) single crystal of ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$. Although the development of antiferromagnetic (AFM) fluctuations down to 1 K indicated that the AFM criticality was close, Korringa behavior was observed below 0.8 K, and no magnetic anomaly was observed above ${T}_{\mathrm{c}}\ensuremath{\sim}0.6$ K. These behaviors were expected in $S$-type ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$. The temperature dependence of the nuclear spin-lattice relaxation rate $1/{T}_{1}$ at zero field was almost identical to that in the previous polycrystalline samples down to 130 mK, but the temperature dependence deviated downward below 120 mK. In fact, $1/{T}_{1}$ in the SC state could be fitted with the two-gap ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave model rather than the two-gap ${s}_{++}$-wave model down to 90 mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below ${T}_{\mathrm{c}}$, which is indicative of the formation of spin-singlet pairing. The residual part of the spin susceptibility was understood by the field-induced residual density of states evaluated from $1/{T}_{1}T$, which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field ${H}_{c2}$, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations.

We performed $^{17}\mathrm{O}$ nuclear magnetic resonance measurements on superconducting (SC) ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ under in-plane magnetic fields. We found that no new signal appears in the SC state and that the $^{17}\mathrm{O}$ Knight shifts obtained from the double-site measurements remain constant across the first-order phase-transition line, as well as across the second-order phase-transition line as already reported. The present results indicate that the SC spin susceptibility does not decrease in the high-field region, although a magnetization jump in the SC state was reported at low temperatures. Because the spin susceptibility is unchanged in the SC state in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, we suggest that the first-order phase transition across the upper critical field should be interpreted as a depairing mechanism other than the conventional Pauli-paramagnetic effect.

We carried out $^{101}\mathrm{Ru}$ nuclear quadrupole resonance (NQR) measurement on superconducting (SC) ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ under zero magnetic field $(H=0)$ and found that the nuclear spin-spin relaxation rate $1/{T}_{2}$ is enhanced in the SC state. The $1/{T}_{2}$ measurement in the SC state under $H=0$ is effective for detecting slow magnetic fluctuations parallel to the quantized axis of the nuclear spin. Our results indicate that low-energy magnetic fluctuations perpendicular to the ${\mathrm{RuO}}_{2}$ plane emerge when the superconductivity sets in, which is consistent with the previous $^{17}\mathrm{O}\text{-NQR}$ result that the nuclear spin-lattice relaxation rate $1/{T}_{1}$ of the in-plane O site exhibits anomalous behavior in the SC state. The enhancement of the magnetic fluctuations in the SC state is unusual and suggests that the fluctuations are related to the unconventional SC pairing. We suggest that this phenomenon is a consequence of the spin degrees of freedom of the spin-triplet pairing.

We report that the nuclear spin-lattice relaxation rate divided by temperature, ${1/T}_{1}T,$ has a broad maximum around ${T}^{*}\ensuremath{\sim}210\mathrm{K}$ and \ensuremath{\sim}100 K in underdoped and overdoped single crystals of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) with ${T}_{c}=79$ and 77.3 K, respectively, showing the normal-state spin-gap behavior. One result is that the steep decrease in the ${}^{63}$Cu Knight shift, $K(T),$ has been observed below ${T}_{K}^{*}\ensuremath{\sim}200\mathrm{K}$ and \ensuremath{\sim}100 K, suggesting the normal-state pseudogap behavior in quasiparticle density of states (DOS), which is consistent with the pseudogap behavior below ${T}_{\mathrm{ARPES}}^{*}\ensuremath{\sim}170\mathrm{K}$ revealed on underdoped Bi2212 from studies of angle-resolved photoemission spectroscopy. We propose that the spin-gap behavior in ${1/T}_{1}T$ has the same origin as the pseudogap in the quasiparticle DOS.

Ru NMR and nuclear quadrupole resonance studies are reported on single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ ${(T}_{c}=$0.7 K) with the same layered perovskite structure as ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$. The Pauli spin susceptibility deduced from the Ru Knight shift is found to be largely enhanced by a factor of $\ensuremath{\sim}$5.4 as compared with the value from the band calculation. In the superconducting state, the nuclear spin-lattice relaxation rate ${1/T}_{1}$ exhibits a sharp decrease with no coherence peak just below ${T}_{c}$ and the ${T}_{1}T$=constant behavior well below ${T}_{c},$ suggesting that the anisotropic pairing state is realized as in heavy-fermion and high-${T}_{c}$ superconductors.

In order to determine the superconducting paring state in the ferromagnetic superconductor UCoGe, ^{59}Co NMR Knight shift, which is directly related to the microscopic spin susceptibility, was measured in the superconducting state under magnetic fields perpendicular to spontaneous magnetization axis: ^{59}K^{a, b}. ^{59}K^{a, b} shows to be constant, but does not decrease below a superconducting transition. These behaviors as well as the invariance of the internal field at the Co site in the superconducting state exclude the spin-singlet pairing, and can be interpreted with the equal-spin pairing state with a large exchange field along the c axis, which was studied by Mineev [Phys. Rev. B 81, 180504 (2010)]., 5 pages, 4 figures, to be appear in PRB

We performed nuclear-magnetic-resonance measurements on itinerant-electron metamagnet UCoAl to investigate the critical behavior of the magnetism near a metamagnetic (MM) critical endpoint (CEP). We derived $c$-axis magnetization ${M}_{c}$ and its fluctuation ${S}_{c}$ from the measurements of Knight shift and nuclear spin-lattice relaxation rate $1/{T}_{1}$ as a function of the $c$-axis external field (${H}_{c}$) and temperature ($T$). We developed contour plots of ${M}_{c}$ and ${S}_{c}$ on the ${H}_{c}$-$T$ phase diagram, and observed the strong divergence of ${S}_{c}$ at the CEP. The critical exponents of ${M}_{c}$ and ${S}_{c}$ near the CEP are estimated and found to be close to the universal properties of a three-dimensional Ising model. We indicate that the critical phenomena at the itinerant-electron MM CEP in UCoAl have a common feature as a gas--liquid transition.

Static and dynamic magnetic properties of lightly P-substituted BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ were systematically investigated by ${}^{31}$P NMR. The averaged internal magnetic field at the P site in the zero-temperature limit evaluated from the broadening of NMR spectra in the antiferromagnetic (AFM) phase is gradually suppressed toward $x\ensuremath{\sim}0.35$ with increasing $x$, which provides definitive evidence for the existence of an AFM quantum critical point (QCP) at $x\ensuremath{\sim}0.35$. The location of the AFM QCP is consistent with the previous estimation from temperature dependence of spin dynamics in the normal state, and the superconducting transition temperature ${T}_{\mathrm{c}}$ takes the maximum around the QCP. Our experiments, revealing a signature of a QCP extending up to room temperature, establish BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ as one of the most accessible systems for unraveling the nature of quantum criticality and the relationship between AFM quantum criticality and unconventional superconductivity.

$^{75}$As and $^{139}$La NMR results of LaFeAs(O$_{1-x}$F$_x$) ($x$=0, 0.025, and 0.04) were reported. Upon F-doping, the tetragonal-to-orthorhombic structural phase transition temperature $T_S$, antiferromagnetic transition temperature $T_N$ and internal magnetic field $\mu_0H_{\rm int}$ are gradually reduced for $x

Zero-field muon spin relaxation experiments have been carried out in the Pr(Os${}_{1\ensuremath{-}x}$Ru${}_{x}$)${}_{4}$Sb${}_{12}$ and Pr${}_{1\ensuremath{-}y}$La${}_{y}$Os${}_{4}$Sb${}_{12}$ alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field ${B}_{s}$. In both alloy series ${B}_{s}$ initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping, with a $~$50% faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration $x\ensuremath{\gtrsim}0.6$ but persists for essentially all La concentrations. Our data support a crystal-field excitonic Cooper pairing mechanism for TRS-breaking superconductivity.

Magnetic measurements on optimally doped single crystals of ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ $(x\ensuremath{\approx}0.35)$ with magnetic fields applied along different crystallographic axes were performed under pressure, enabling the pressure evolution of coherence lengths and the anisotropy factor to be followed. Despite a decrease in the superconducting critical temperature, our studies reveal that the superconducting properties become more anisotropic under pressure. With appropriate scaling, we directly compare these properties with the values obtained for ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ as a function of phosphorus content.

$^{31}\text{P}$ and $^{75}\text{A}\text{s}$ NMR measurements were performed in superconducting ${\text{BaFe}}_{2}{({\text{As}}_{0.67}{\text{P}}_{0.33})}_{2}$ with ${T}_{c}=30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The nuclear-spin-lattice relaxation rate ${T\phantom{\rule{0.1em}{0ex}}}_{1}^{\ensuremath{-}1}$ and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and ${T\phantom{\rule{0.1em}{0ex}}}_{1}^{\ensuremath{-}1}$ in the superconducting (SC) state decreases without a coherence peak just below ${T}_{c}$, as observed in $({\text{Ba}}_{1\ensuremath{-}x}{\text{K}}_{x}){\text{Fe}}_{2}{\text{As}}_{2}$. In contrast to other iron arsenide superconductors, the ${T\phantom{\rule{0.1em}{0ex}}}_{1}^{\ensuremath{-}1}\ensuremath{\propto}T$ behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ despite a comparable ${T}_{c}$ value, an analogous phase diagram, and similar Fermi surfaces to $({\text{Ba}}_{1\ensuremath{-}x}{\text{K}}_{x}){\text{Fe}}_{2}{\text{As}}_{2}$.

One of the remaining issues concerning the spin-triplet superconductivity of Sr2RuO4 is the strong limit of the in-plane upper critical field Hc2 at low temperatures. In this study, we clarified the dependence of Hc2 on the angle theta between the magnetic field and the ab plane at various temperatures, by precisely and accurately controlling the magnetic field direction. We revealed that, although the temperature dependence of Hc2 for |theta| > 5 is well explained by the orbital pair-breaking effect, Hc2(T) for |theta| < 5 is clearly limited at low temperatures. We also revealed that the Hc2 limit for |theta| < 5 is present not only at low temperatures, but also at temperatures close to Tc. These features may provide additional hints for clarifying the origin of the Hc2 limit. Interestingly, if the anisotropic ratio in Sr2RuO4 is assumed to depend on temperature, the observed angular dependence of Hc2 is reproduced better at lower temperature with an effective-mass model for an anisotropic three-dimensional superconductor. We discuss the observed behavior of Hc2 based on existing theories.

Polycrystalline samples of ${\text{NiGa}}_{2}{\text{S}}_{4+\ensuremath{\delta}}$ $(\ensuremath{-}0.2\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}+0.2)$ have been synthesized by solid-state reaction. X-ray powder-diffraction measurements indicate that these samples are single phase with a trigonal lattice with a nominal concentration $\ensuremath{\delta}$ at least between $\ensuremath{-}0.12$ and $+0.16$. Rietveld analyses for time-of-flight neutron-diffraction data have revealed that there is no site mixing between Ni and Ga atoms, indicating that the triangular lattice is only formed by Ni atoms. Our susceptibility measurements have clarified that the freezing temperature and the magnitude of hysteresis systematically change with nominal sulfur concentration and form a respective peak and minimum at the nominal ${\text{NiGa}}_{2}{\text{S}}_{4.04}$. These results together with our chemical analyses indicate that the actual content of sulfur for the nominal ${\text{NiGa}}_{2}{\text{S}}_{4.04}$ should be stoichiometric, namely, 4.00. On the contrary, the Ga nuclear quadrupole resonance (NQR) has suggested the presence of two different Ga sites from the Ga-NQR spectra in both samples with the nominal concentration of ${\text{NiGa}}_{2}{\text{S}}_{4.00}$ and ${\text{NiGa}}_{2}{\text{S}}_{4.04}$, despite the fact that there is a single crystallographic site for Ga in the unit cell. High-resolution transmission electron microscopy scans conducted over a wide region of a ${\text{NiGa}}_{2}{\text{S}}_{4.04}$ single crystal found a clean defect-free in-plane structure. Diffraction patterns of the selected area indicate the partial presence of a superlatticelike stacking fault. This suggests that the two different Ga sites are not generated by the sulfur deficiency but by the superlatticelike stacking fault.

Muon spin rotation (muSR) experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet NiGa2S4. Long-lived disordered Ni-spin freezing (correlation time > 10-6 s at 2 K) sets in below T_f = 8.5 +- 0.5 K with a mean-field-like temperature dependence. The observed exponential temperature dependence of the muon spin relaxation above T_f is strong evidence for 2D critical spin fluctuations. Slow Ni spin fluctuations coexist with quasistatic magnetism at low temperatures but are rapidly suppressed for fields > 10 mT, in marked contrast with the field-independent specific heat. The muSR and bulk susceptibility data indicate a well-defined 2D phase transition at T_f, below which NiGa2S4 is neither a conventional magnet nor a singlet spin liquid.

The microscopic details of flux-line lattice state studied by muon spin rotation are reported in an electron-doped high-${T}_{c}$ cuprate superconductor, ${\text{Sr}}_{1\ensuremath{-}x}{\text{La}}_{x}{\text{CuO}}_{2}$ (SLCO) $(x=0.10--0.15)$. A clear sign of phase separation between magnetic and nonmagnetic phases is observed, where the effective magnetic penetration depth $[\ensuremath{\lambda}\ensuremath{\equiv}\ensuremath{\lambda}(T,H)]$ is determined selectively for the latter phase. The extremely small value of $\ensuremath{\lambda}(0,0)$ and corresponding large superfluid density $({n}_{s}\ensuremath{\propto}{\ensuremath{\lambda}}^{\ensuremath{-}2})$ is consistent with the presence of a large Fermi surface with carrier density of $1+x$, which suggests the breakdown of the ``doped Mott insulator'' even at the ``optimal doping'' in SLCO. Moreover, a relatively weak anisotropy in the superconducting order parameter is suggested by the field dependence of $\ensuremath{\lambda}(0,H)$. These observations strongly suggest that the superconductivity in SLCO is of a different class from hole-doped cuprates.

We have performed $^{69,71}\mathrm{Ga}$ nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and muon spin rotation and resonance on the quasi-two-dimensional antiferromagnet $\mathrm{Ni}{\mathrm{Ga}}_{2}{\mathrm{S}}_{4}$, in order to investigate its spin dynamics and magnetic state at low temperatures. Although there exists only one crystallographic site for Ga in $\mathrm{Ni}{\mathrm{Ga}}_{2}{\mathrm{S}}_{4}$, we found two distinct Ga signals by NMR and NQR. The origin of the two Ga signals is not fully understood, but possibly due to stacking faults along the $c$ axis which induce additional broad Ga NMR and NQR signals with different local symmetries. We found the spin freezing occurring at ${T}_{\mathrm{f}}$, at which the specific heat shows a maximum, from a clear divergent behavior of the nuclear spin-lattice relaxation rate $1∕{T}_{1}$ and nuclear spin-spin relaxation rate $1∕{T}_{2}$ measured by Ga-NQR as well as the muon spin relaxation rate $\ensuremath{\lambda}$. The main sharp NQR peaks exhibit a stronger tendency of divergence, compared with the weak broader spectral peaks, indicating that the spin freezing is intrinsic in $\mathrm{Ni}{\mathrm{Ga}}_{2}{\mathrm{S}}_{4}$. The behavior of these relaxation rates strongly suggests that the Ni spin fluctuations slow down towards ${T}_{\mathrm{f}}$, and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multicomponent ${T}_{1}$ were observed below $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating that a static magnetic state with incommensurate magnetic correlations or inhomogeneously distributed moments is realized at low temperatures. However, the wide temperature region between $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{f}}$, where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below ${T}_{\mathrm{f}}$, but keep fluctuating down to $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the MHz frequency range. Below $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, all components of $1∕{T}_{1}$ follow a ${T}^{3}$ behavior. We also found that $1∕{T}_{1}$ and $1∕{T}_{2}$ show the same temperature dependence above ${T}_{\mathrm{f}}$ but different temperature dependence below $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These results suggest that the spin dynamics is isotropic above ${T}_{\mathrm{f}}$, which is characteristic of the Heisenberg spin system, and becomes anisotropic below $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

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.

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.

The superconducting property of sodium cobalt oxyhydrate, ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}1.3{\mathrm{H}}_{2}\mathrm{O}\phantom{\rule{0.2em}{0ex}}({T}_{c}=4.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, has been studied by means of muon spin rotation/relaxation down to $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. It was found that the zero-field muon spin relaxation rate is independent of the temperature, indicating that no static magnetism appears in this compound, at least above $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The result also provides evidence of no breaking of time-reversal symmetry for the superconductivity. Meanwhile, the muon Knight shift at $60\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, which is proportional to the local spin susceptibility, shows no obvious reduction below ${T}_{c}$. Considering these observations, possible unconventional superconductivity of ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}∙1.3{\mathrm{H}}_{2}\mathrm{O}$ is discussed.

Muon spin rotation and relaxation experiments have been carried out in single crystals of ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2},$ a compound that exhibits non-Fermi-liquid (NFL) behavior associated with a quantum critical point (QCP) at $T=0.$ The zero-field muon relaxation rate is found to be independent of temperature down to 100 mK but to increase below $\ensuremath{\sim}70\mathrm{mK},$ which suggests magnetic order at low temperatures. From the relation between the internal field at the ${\ensuremath{\mu}}^{+}$ stopping site and the hyperfine coupling constant the ordered ${\mathrm{Yb}}^{3+}$ moment is very small, $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\ensuremath{\mu}}_{B}.$ Muon spin rotation linewidths in a transverse field of 6 kOe indicate a homogeneous susceptibility down to 2 K, which is an order of magnitude lower than the characteristic (Kondo) temperature ${T}_{K}\ensuremath{\approx}25\mathrm{K}.$ This is evidence against the importance of disorder-driven NFL mechanisms in ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}.$ In longitudinal magnetic fields the muon spin-lattice relaxation function $G(t)$ is exponential, again indicative of a homogeneous system. The relaxation obeys the time-field scaling relation $G(t,H)=G(t/H),$ which suggests long-lived spin correlations at low temperatures. The ${\mathrm{Yb}}^{3+}$ spin dynamics derived from muon spin relaxation appear to be intimately related to critical magnetic fluctuations near the QCP.