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We report the $^{121/123}$Sb-NMR/nuclear quadrupole resonance (NQR)measurements on the newly-discovered superconductor BaTi$_2$Sb$_2$O with atwo-dimensional Ti$_2$O square-net layer formed with Ti$^{3+}$ (3$d^1$). NQRmeasurements revealed that the in-plane four-fold symmetry is broken at the Sbsite below $T_{\rm A} \sim$ 40 K, without an internal field appearing at the Sbsite. These exclude a spin-density wave (SDW)/ charge density wave (CDW)ordering with incommensurate correlations, but can be understood with thecommensurate CDW ordering at $T_{\rm A}$. The spin-lattice relaxation rate$1/T_1$, measured at the four-fold symmetry breaking site, decreases belowsuperconducting (SC) transition temperature $T_{\rm c}$, indicative of themicroscopic coexistence of superconductivity and the CDW/SDW phase below$T_{\rm A}$. Furthermore, $1/T_1$ of $^{121}$Sb-NQR shows a coherence peak justbelow $T_{\rm c}$ and decreases exponentially at low temperatures. Theseresults are in sharp contrast with those in cuprate and iron-basedsuperconductors, and strongly suggest that its SC symmetry is classified to anordinary s-wave state.

S wave superconductivity in newly discovered superconductor BaTi$_2$Sb$_2$O revealed by $^{121/123}$Sb-NMR/Nuclear Quadrupole Resonance measurements