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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 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 $T_{rm A} sim$ 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 $T_{rm A}$. The spin-lattice relaxation rate $1/T_1$, measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature $T_{rm c}$, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below $T_{rm A}$. Furthermore, $1/T_1$ of $^{121}$Sb-NQR shows a coherence peak just below $T_{rm c}$ 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.
We report $^{121/123}$Sb nuclear quadrupole resonance (NQR) and $^{51}$V nuclear magnetic resonance (NMR) measurements on kagome metal CsV$_3$Sb$_5$ with $T_{rm c}=2.5$ K. Both $^{51}$V NMR spectra and $^{121/123}$Sb NQR spectra split after a charge
In this Rapid Communication, a set of $^{209}$Bi-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) BaTi$_2$Bi$_2$O from a microscopic poi
$A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) is a novel kagome superconductor coexisting with the charge density wave (CDW) order. Identifying the structure of the CDW order is crucial for understanding the exotic normal state and superconductivity in this syste
We performed $^{185/187}$Re nuclear quadrupole resonance (NQR) measurements under pressure to investigate the superconducting properties of noncentrosymmetric superconductor Cd$_{2}$Re$_{2}$O$_{7}$ under various crystal structures. The pressure depen
Topological superconductors have long been predicted to host Majorana zero modes which obey non-Abelian statistics and have potential for realizing non-decoherence topological quantum computation. However, material realization of topological supercon