# Nematic transition and highly two-dimensional superconductivity in BaTi$_2$Bi$_2$O revealed by $^{209}$Bi-nuclear magnetic resonance/nuclear quadrupole resonance measurements

## TL;DR

This study uses $^{209}$Bi-NMR/NQR to reveal electronic nematicity and highly two-dimensional superconductivity in BaTi$_2$Bi$_2$O, highlighting differences from related compounds and implications for the superconducting phase diagram.

## Contribution

First microscopic investigation showing nematic electronic state and two-dimensional superconductivity in BaTi$_2$Bi$_2$O using NMR/NQR measurements.

## Key findings

- Detection of in-plane anisotropic parameter indicating nematic order.
- No change in $1/T_1T$ below $T_c$ despite superconducting transition.
- Distinct normal and superconducting properties compared to BaTi$_2$Sb$_2$O and BaTi$_2$As$_2$O.

## Abstract

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 point of view. The NMR and NQR spectra at 5~K can be reproduced with a non-zero in-plane anisotropic parameter $\eta$, indicating the breaking of the in-plane four-fold symmetry at the Bi site without any magnetic order, i.e., `the electronic nematic state'. In the SC state, the nuclear spin-lattice relaxation rate divided by temperature, $1/T_1T$, does not change even below $T_{\rm c}$, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two-dimensionality in BaTi$_2$Bi$_2$O. Comparing the NMR/NQR results among BaTi$_2$$Pn$$_2$O ($Pn$ = As, Sb, and Bi), it was found that the normal and SC properties of BaTi$_2$Bi$_2$O were considerably different from those of BaTi$_2$Sb$_2$O and BaTi$_2$As$_2$O, which might explain the two-dome structure of $T_{\rm c}$ in this system.