We report a $^{71}$Ga nuclear-quadrupole-resonance (NQR) study on the characteristics of superconductivity in noncentrosymmetric Ir$_2$Ga$_9$ at zero field (H=0). The $^{71}$Ga-NQR measurements have revealed that $1/T_1$ has the clear coherence peak
just below $T_{rm c}$, and decreases exponentially upon further cooling in Ir$_2$Ga$_9$. From these results, Ir$_2$Ga$_9$ is concluded to be the conventional s-wave superconductor. Despite the lack of spatial centrosymmetry, there are no evidence for unconventional superconducting state ascribed to ASOC in Ir$_2$Ga$_9$.
We report on novel superconducting characteristics of the heavy fermion (HF) superconductor CePt3Si without inversion symmetry through 195Pt-NMR study on a single crystal with T_c= 0.46 K that is lower than T_c= 0.75 K for polycrystals. We show that
the intrinsic superconducting characteristics inherent to CePt3Si can be understood in terms of the unconventional strong-coupling state with a line-node gap below T_c= 0.46 K. The mystery about the sample dependence of T_c is explained by the fact that more or less polycrystals and single crystals inevitably contain some disordered domains, which exhibit a conventional BCS s-wave superconductivity (SC) below 0.8 K. In contrast, the Neel temperature T_N= 2.2 K is present regardless of the quality of samples, revealing that the Fermi surface responsible for SC differ from that for the antiferromagnetic order. These unusual characteristics of CePt3Si can be also described by a multiband model; in the homogeneous domains, the coherent HF bands are responsible for the unconventional SC, whereas in the disordered domains the conduction bands existing commonly in LaPt3Si may be responsible for the conventional s-wave SC. We remark that some impurity scatterings in the disordered domains break up the 4f-electrons-derived coherent bands but not others. In this context, the small peak in 1/T_1 just below T_c reported in the previous paper (Yogi et al, 2004) is not due to a two-component order parameter composed of spin-singlet and spin-triplet Cooper pairing states, but due to the contamination of the disorder domains which are in the s-wave SC state.
We report a $^{73}$Ge nuclear-quadrupole-resonance (NQR) study on novel evolution of unconventional superconductivity in antiferromagnetic (AFM) CeNiGe$_3$. The measurements of the $^{73}$Ge-NQR spectrum and the nuclear spin-lattice relaxation rate (
$1/T_1$) have revealed that the unconventional superconductivity evolves inside a commensurate AFM phase around the pressure ($P$) where N{e}el temperature $T_{rm N}$ exhibits its maximum at 8.5 K. The superconducting transition temperature $T_{rm SC}$ has been found to be enhanced with increasing $T_{rm N}$, before reaching the quantum critical point at which the AFM order collapses. Above $T_{rm SC}$, the AFM structure transits from an incommensurate spin-density-wave order to a commensurate AFM order at $Tsim 2$ K, accompanied by a longitudinal spin-density fluctuation. With regard to heavy-fermion compounds, these novel phenomena have hitherto never been reported in the $P$-$T$ phase diagram.
We report that a novel type of superconducting order parameter has been realized in the ferromagnetic states in UGe$_2$ via $^{73}$Ge nuclear-quadrupole-resonance (NQR) experiments performed under pressure ($P$). Measurements of the nuclear spin-latt
ice relaxation rate $(1/T_1)$ have revealed an unconventional nature of superconductivity such that the up-spin band is gapped with line nodes, but the down-spin band remains gapless at the Fermi level. This result is consistent with that of a ferromagnetic spin-pairing model in which Cooper pairs are formed among ferromagnetically polarized electrons. The present experiment has shed new light on a possible origin of ferromagnetic superconductivity, which is mediated by ferromagnetic spin-density fluctuations relevant to the first-order transition inside the ferromagnetic states.
