From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (TCurie ∼ 2.5 K and TSC ∼ 0.6 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 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 c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.PACS numbers: 71.27.+a 74.25.nj, 75.30.Gw The discovery of superconductivity in ferromagnetic (FM) UGe 2 opened up a new paradigm of superconductivity [1,2], since most unconventional superconductivity has been discovered in the vicinity of an antiferromagnetic (AFM) phase [3]. From the theoretical point of view, in an itinerant FM superconductor with the presence of a large energy splitting between the majority and minority spin Fermi surfaces, exotic spintriplet superconductivity is anticipated, in which pairing is between parallel spins within each spin Fermi surface. In addition, it has been argued that critical FM fluctuations near a quantum phase transition could mediate spin-triplet superconductivity [4]. However, there have been no experimental results indicating a relationship between FM fluctuations and superconductivity.Among the FM superconductors discovered so far, UCoGe is one of the most readily explored experimentally, because of its high superconducting (SC) transition temperature (T SC ) and low Curie temperature (T Curie ) at ambient pressure [5]. Microscopic measurements have shown that superconductivity occurs within the FM region, resulting in microscopic coexistence of ferromagnetism and superconductivity [6,7]. Studies of the SC upper critical field (H c2 ) and its angle dependence along each crystalline axis have reported remarkable enigmatic behavior [8,9]: superconductivity survives far beyond the Pauli-limiting field along the a and b axes, whereas H c2 for fields along the c direction (H c c2 ) is as small as 0.5 T. Colossal H c2 for fields along the a and b axes seems to suggest spin triplet pairing. In addition, a steep angle dependence of H c2 was reported when the field was tilted slightly from the a axis toward the c axis [9]. The observed characteristic H c2 behavior is one of mysterious features of SC UCoGe and its origin can be related to the mechanism of the superconductivity.Unlike the three dimensional crystal structure, magnetic properties are strongly anisotropic [8]. The magnetization has Ising-like anisotropy with the c axis as a magnetic easy axis, and direction-dependent nuclear-spin lattice relaxation rate (1/T 1 ) measurements on a single crystalline sample have revealed the magnetic fluctuations in UCoGe to be Ising-...

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 p{c} and temperatures T{c}. 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.

Co NQR studies were performed in recently discovered superconductor NaxCoO2·yH2O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same NaxCoO2 were examined, SC bilayer-hydrate sample with Tc ∼ 4.7 K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/T1 in the SC sample, it was found that the coherence peak is absent just below Tc and that 1/T1 is proportional to temperature far below Tc. These results, which are in qualitatively agreement with the previous result by Fujimoto et al., suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/T1T of the SC sample shows gradual increase below 100K down to Tc, whereas 1/T1T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/T1T and χ bulk in the SC sample, the increase of 1/T1T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related with the unconventional superconductivity in this compound. The implication of this finding is discussed.