S-wave superconductivity in superconducting BaTi2Sb2O revealed by121/123Sb-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${}^{3+}$ (3${d}^{1}$). NQR measurements revealed that the in-plane four-fold symmetry is broken at the Sb site below ${T}_{\mathrm{A}}\ensuremath{\sim}40$ 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 ${T}_{\mathrm{A}}$. The spin-lattice relaxation rate $1/{T}_{1}$, measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature ${T}_{\mathrm{c}}$, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below ${T}_{\mathrm{A}}$. Furthermore, $1/{T}_{1}$ of ${}^{121}$Sb-NQR shows a coherence peak just below ${T}_{\mathrm{c}}$ 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.