Spin-singlet superconductivity with a full gap in locally noncentrosymmetric SrPtAs

We report ${}^{195}\mathrm{Pt-NMR}$ and ${}^{75}\mathrm{As}$ nuclear quadrupole resonance measurements for the locally noncentrosymmetric 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 a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-symmetry breaking is not large enough to bring about an exotic superconducting state, or the interlayer hopping interaction is larger than the spin-orbit coupling.

Figure 1

${}^{75}\mathrm{As}$-NQR spectrum of SrPtAs measured at $T=4.2$ K.

Figure 2

Temperature dependence of the ${}^{75}\mathrm{As}$ spin-lattice relaxation rate $1/T_1$  measured by NQR. The straight line above $T_c$ represents the $T_{1}T=\text{const}$ relation. The solid curve below $T_c$ is a calculation assuming the BCS function. The dotted curve is a calculation for a chiral $d$-wave state (see text for details).

Figure 3

The enlarged part of $1/T_1$ around $T_c$. The straight line above $T_c$ represents the $T_{1}T=\text{const}$ relation. The solid curve below $T_c$ is a calculation assuming the BCS function.

Figure 4

${}^{195}\mathrm{Pt-NMR}$ spectrum measured at a magnetic field of $H=0.0842$ T above and below $T_c\left(H\right)$, respectively.

Figure 5

(a) The temperature dependence of the full width at the half maximum (FWHM) for the ${}^{195}\mathrm{Pt-NMR}$ spectrum. (b) The temperature dependence of the penetration depth $\lambda$. The solid curve is a calculation by assuming a conventional $s$-wave superconducting state (see text for details).

Figure 6

The temperature dependence of the Knight shift for SrPtAs measured in $H=0.0842$ T. The solid curve below $T_c$ is a calculation assuming a spin-singlet pairing (see text for details).