S wave superconductivity in newly discovered superconductor BaTi₂Sb₂O revealed by ¹²¹/¹²³Sb-NMR/Nuclear Quadrupole Resonance measurements

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We report the ¹²¹/¹²³Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the newly-discovered 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.