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We report a novel phenomenon intimately related to the spin-triplet superconductivity. It is well known that the spin susceptibility decreases below the superconducting transition temperature in almost all superconductors because of spin-singlet pair formation, while it may remain unchanged in a handful of spin-triplet exceptions. Here we report the observation in Sr2RuO4 with nuclear magnetic resonance (NMR) that the spin susceptibility originating from the Ru-4d electron slightly increases by ∼ 2% of total and becomes inhomogeneous in the superconducting state. These are reasonably explained if the electron pairs form the equal-spin-pairing (ESP) in the mixed state. A similar phenomenon was predicted for superfluid 3 He forty years ago, but had never been demonstrated in any superconductor. Ru [5,6] nuclei are unchanged in the SC state. The unchanged spin susceptibility, which was supported also from the polarized neutron scattering measurements [7], strongly suggests that the equal-spin-pairing (ESP) state of the spin-triplet superconductivity is realized in SRO, in which the SC pairs consist of the up-up (| ↑↑ ) or down-down (| ↓↓ ) pairs. In addition, the µSR [8] and Kerr-effect [9] measurements in the SC state suggest the broken time-reversal symmetry. Unlike heavy-fermion systems, the effect of spin-orbit coupling should be relatively weak, the pairing state in SRO would be closely analogous to the case of superfluid 3 He. By taking the experimental results into account, the chiral p-wave spin-triplet state, which is analogous to the A-phase of superfluid 3 He [10], has been considered as the most promising candidate for the SC pairing state [11,12].However, toward the establishment of this pairing state, there still remains a controversy since the superconductivity is strongly suppressed with a first-order transition under the in-plane magnetic fields near the SC critical field H c2 as shown in Fig. 1 (a) [13,14]. In addition, sharp magnetization jump with the hysteresis at H c2 was observed when the magnetic field is exactly parallel to the ab plane at 0.1 K [15]. These are phenomena usually expected in a spin-singlet superconductor [16,17]. Furthermore, non-detection of some of the behaviors expected for the chiral p-wave state (e.g. chiral edge current [18][19][20] and splitting of T c by in-plane magnetic fields of any magnitude [21]) casts some doubts on this pairing state. Therefore, convincing evidence for establishing the SC pairing state in SRO, particularly finding a new phenomenon specific to the pairing state, has been desired.In this paper, we report, from the "double-site" Knight-shift measurement in the SC state, which is the reliable method to subtract the Meissner effect from the observed Knight shift, that the spin susceptibility of SRO becomes inhomogeneous and its average slightly increases in the SC state. The small increase of spin susceptibility cannot be explained with a singlet-pairing state but consistently interpreted with the ESP state in triplet superconductivity. As far as ...

A 59 Co nuclear quadrupole resonance (NQR) was performed on a single-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The FM phase vanished at a critical pressure P c , and the NQR spectrum just below P c showed phase separation of the FM and paramagnetic (PM) phases below Curie temperature T Curie , suggesting first-order FM quantum phase transition (QPT). We found that the internal field was absent above P c , but the superconductivity is almost unchanged. This result suggests the existence of the nonunitary to unitary transition of the superconductivity around P c . Nuclear spin-lattice relaxation rate 1/T 1 showed the FM critical fluctuations around P c , which persist above P c and are clearly related to superconductivity in the PM phase. This FM QPT is understood to be a weak first order with critical fluctuations. 1/T 1 sharply decreased in the superconducting (SC) state above P c with a single component, in contrast to the two-component 1/T 1 in the FM SC state, indicating that the inhomogeneous SC state is a characteristic feature of the FM SC state in UCoGe.

We report resistivity measurements under pressure for Kondo-lattice ferromagnet CeRh 6 Ge 4 , and present that a quantum ferromagnetic (FM) phase transition is easily achieved. In most clean metallic ferromagnets, a quantum critical point (QCP) at zero field is avoided by changing the FM transition to a discontinuous transition or to an antiferromagnetic transition. In CeRh 6 Ge 4 , to the contrary, the Curie temperature of 2.5 K decreases continuously as increasing pressure without any clear signature that the transition changes to first order. The obvious non Fermi liquid behavior is observed in the vicinity of the quantum FM phase transition. The experimental data do not contradict a picture in which CeRh 6 Ge 4 shows the FM QCP at zero field. Band structure calculation suggests the unusual electronic state of CeRh 6 Ge 4 among Ce-based Kondo lattices. CeRh 6 Ge 4 deserves further investigations and will be a key material to understand the matter of the FM QCP.

We have performed 63 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 CeCu2Si2. 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 Tc ∼ 0.6 K. These behaviors were expected in S-type CeCu2Si2. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 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/T1 in the SC state could be fitted with the two-gap s±-wave rather than the two-gap s++-wave model down to 90 mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below Tc, 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/T1T , which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field Hc2, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations.PACS numbers: