We report the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements for non-centrosymmetric superconductors Re 7 B 3 , LaBiPt, and BiPd containing heavy elements. For all three compounds, the spin-lattice relaxation rate 1/T 1 shows a coherence peak just below T c and decreases exponentially at low temperatures, which indicates that an isotropic superconducting gap is dominant in these compounds. In BiPd, the height of the coherence peak just below T c is much suppressed, which suggests that there exists a substantial component of gap with nodes in this compound. Our results indicate that heavy element is not the only factor, but the extent of inversion symmetry breaking is also important to induce a large spin-orbit coupling and an unconventional superconducting state. Japan.J. Phys. Soc. Jpn. FULL PAPERSinduced and a parity-mixed superconducting state is allowed. The extent of the parity mixing is determined by the strength of the ASOC. 1-3 Moreover, the existence of topologically protected zero-energy surface-or edge-states has been pointed out recently in noncentrosymmetric superconductors. 4,5 Since the discovery of the non-centrosymmetric compound CePt 3 Si, 6 many superconductors without inversion symmetry have been reported. They can be categorized into two types.Namely, the strongly-correlated electron systems such as UIr 7 and CeRh(Ir)Si 3 , 8, 9 and the weakly correlated electron systems that include Li 2 Pd 3 B, Li 2 Pt 3 B and Mg 10 Ir 19 B. [10][11][12] In the former class of materials, the electron correlations seem to play an important role in determining the superconducting properties. The latter class is therefore more suitable for studying the pure effects of inversion-symmetry breaking and ASOC interaction.The isostructural Li 2 Pd 3 B and Li 2 Pt 3 B show considerable differences. [13][14][15] The 11 B spinlattice relaxation rate (1/T 1 ) in Li 2 Pd 3 B shows a coherence peak just below T c and decreases exponentially at low temperatures. 13 On the contrary, 1/T 1 of Li 2 Pt 3 B decreases below T c without a coherence peak and follows T 3 variation. 14 Also, the Knight shift (K) changes below T c in Li 2 Pd 3 B but does not change across T c in the case of Li 2 Pt 3 B. These results suggest isotropic gap, spin singlet superconductivity in Li 2 Pd 3 B, but nodal gap, spin triplet superconductivity in Li 2 Pt 3 B. In this case, a strikingly different ASOC was believed to be

We report 195 Pt-NMR and 75 As-NQR measurements for the locally non-centrosymmetric superconductor SrPtAs where the As-Pt layer breaks inversion symmetry while globally the compound is centrosymmetric. The nuclear spin lattice relaxation rate 1/T 1 shows a well-defined coherence peak below T c and decreases exponentially at low temperatures. The spin susceptibility measured by the Knight shift also decreases below T c down to T < T c /6. These data together with the penetration depth obtained from the NMR spectra can be consistently explained by assuming a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-breaking is not large enough to bring about an exotic superconducting state, or the inter-layer hopping interaction is larger than the spin-orbit coupling.

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We report the 207 Pb nuclear magnetic resonance (NMR) measurements on polycrystalline sample of PbTaSe 2 with noncentrosymmetric crystal structure and topological electronic band. The nuclear spin-lattice relaxation rate 1/T 1 shows a suppressed coherence peak below the superconducting transition temperature T c = 4.05 K and decreases as an exponential function of temperature. The penetration depth derived from the NMR spectrum is almost temperature independent below T = 0.7 T c . The Knight shift K decreases below T c . These results suggest spin-singlet superconductivity with a fully-opened gap 2∆ = 3.5 k B T c in PbTaSe 2 . arXiv:1805.03416v2 [cond-mat.supr-con]

Non-centrosymmetric superconductor Re 6 Zr has attracted much interest, for its possible unconventional superconducting state with time reversal symmetry broken. Here we report the 185/187 Re nuclear quadrupole resonance (NQR) measurements on Re 6 Zr (T c = 6.72 K) and the isostructural compounds Re 27 Zr 5 (T c = 6.53 K) and Re 24 Zr 5 (T c = 5.00 K). The nuclear spin-lattice relaxation rate 1/T 1 shows a coherence peak below T c and decreases exponentially at low temperatures in all three samples. The superconducting gap ∆ derived from the 1/T 1 data is 2∆ = 3.58 k B T c , 3.55 k B T c , and 3.51 k B T c for Re 6 Zr ,Re 27 Zr 5 , and Re 24 Zr 5 , respectively, which is close to the value of 3.53 k B T c expected for weak-coupling superconductivity. These data suggest conventional s-wave superconductivity with a fully-opened gap in this series of compounds.