From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe ($T_{\mathrm{Curie}}\sim 2.5\mathrm{K}$ and $T_{\mathrm{SC}}\sim 0.6\mathrm{K}$), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the $c$ axis. We found that magnetic fields along the $c$ axis ($H\parallel c$) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by $H\parallel c$ induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.

Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor Na _x CoO ₂ · y H ₂ O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na _x CoO ₂ were examined, a SC bilayer-hydrate sample with T _c ∼4.7 K and a non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/ T ₁ in the SC sample, it was found that the coherence peak is absent just below T _c and that 1/ T ₁ is proportional to temperature far below T _c . These results, which are in qualitative agreement with the previous result by Fujimoto et al. , suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/ T ₁ T of the SC sample shows a gradual increase below 100 K down to T _c , whereas 1/ T ₁ T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/ T ₁ T and χ _bulk in the SC sample, the increase in 1/ T ₁ T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related to the unconventional superconductivity in this compound. The implication of this finding is discussed.

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 $\lambda$. 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 $2\mathrm{K}$, 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 $2\mathrm{K}$ 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 $2\mathrm{K}$ with the MHz frequency range. Below $0.5\mathrm{K}$, 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 $0.8\mathrm{K}$. 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 $0.8\mathrm{K}$.

We report a NMR and magnetometry study on the expanded intercalated fulleride Cs3C60 in both its A15 and face centered cubic structures. NMR allowed us to evidence that both exhibit a first-order Mott transition to a superconducting state, occurring at distinct critical pressures pc and temperatures Tc. Though the ground state magnetism of the Mott phases differs, their high T paramagnetic and superconducting properties are found similar, and the phase diagrams versus unit volume per C60 are superimposed. Thus, as expected for a strongly correlated system, the interball distance is the relevant parameter driving the electronic behavior and quantum transitions of these systems.

We have carried out direction-dependent ${}^{59}\mathrm{Co}$ NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The Knight-shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state.

A weak magnetic order was found in a nonsuperconducting bilayered-hydrate Na _x CoO ₂ · y H ₂ O sample by Co nuclear quadrupole resonance (NQR) measurement. The nuclear spin–lattice relaxation rate divided by temperature 1/ T ₁ T shows a prominent peak at 5.5 K, below which a Co NQR peak splits due to an internal field at the Co site. From the analyses of the Co NQR spectrum at 1.5 K, the internal field is evaluated to be ∼300 Oe and found to be in the a b -plane. The magnitude of the internal field suggests that the ordered moment is as small as ∼0.015 µ _B using the hyperfine coupling constant reported previously. It is shown that the NQR frequency ν _Q correlates with magnetic fluctuations from measurements of NQR spectra and 1/ T ₁ T in various samples. The higher-ν _Q sample has stronger magnetic fluctuations. A possible phase diagram for Na _x CoO ₂ · y H ₂ O is depicted using T _c and ν _Q , in which the crystal distortion along the c -axis of the tilted CoO ₂ octahedron is considered to be a physical parameter. Superconductivity with the highest T _c is seemingly observed in the vicinity of a magnetic phase, suggesting strongly that magnetic fluctuations play an important role in the occurrence of superconductivity.