S-wave superconductivity in superconducting BaTi₂Sb₂O revealed by 121^/123Sb-NMR/nuclear quadrupole resonance measurements

We report the 121^/123Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the superconductor BaTi₂Sb₂O with a two-dimensional Ti₂O square-net layer formed with Ti3⁺ (3d1). NQR measurements revealed that the in-plane four-fold symmetry is broken at the Sb site below T_A∼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 T_A. The spin-lattice relaxation rate 1/T₁, measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature T_c, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below T_A. Furthermore, 1/T₁ of 121Sb-NQR shows a coherence peak just below T_c 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.