S-Wave Superconductivity in Superconducting BaTi[2]Sb[2]O Revealed by [121/123]Sb-NMR/Nuclear Quadrupole Resonance Measurements

Abstract

We report the [121/123]Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the superconductor BaTi[2]Sb[2]O with a two-dimensional Ti[2]O square-net layer formed with Ti[3]+ (3d[1]). NQR measurements revealed that the in-plane four-fold symmetry is broken at the Sb site below TA∼40 K, without an internal field appearing at the Sb site. These exclude a spin-density wave (SDW)/ charge density wave (CDW) ordering with incommensurate correlations, but can be understood with the commensurate CDW ordering at TA. The spin-lattice relaxation rate 1/T[1], measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature Tc, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below TA. Furthermore, 1/T[1] of [121]Sb-NQR shows a coherence peak just below Tc and decreases exponentially at low temperatures. These results are in sharp contrast with those in cuprate and iron-based superconductors, and strongly suggest that its SC symmetry is classified to an ordinary s-wave state.