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

TL;DR

This study uses $^{121/123}$Sb-NMR/NQR measurements to reveal that BaTi$_2$Sb$_2$O exhibits S wave superconductivity coexisting with a commensurate CDW phase, characterized by a coherence peak and exponential decay of relaxation rate below $T_c$.

Contribution

It provides the first microscopic evidence of S wave superconductivity in BaTi$_2$Sb$_2$O and clarifies its coexistence with a CDW phase, contrasting with cuprate and iron-based superconductors.

Findings

In-plane four-fold symmetry breaking at Sb site below 40 K.

Microscopic coexistence of superconductivity and CDW phase.

Observation of a coherence peak and exponential decay of $1/T_1$ below $T_c$.

Abstract

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.