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

In this Rapid Communication, a set of ${}^{209}\mathrm{Bi}$-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$ from a microscopic point of view. The NMR and NQR spectra at 5 K can be reproduced with a nonzero in-plane anisotropic parameter $\eta$, indicating the breaking of the in-plane fourfold 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, $1/T_{1}T$, does not change even below $T_{\mathrm{c}}$, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two dimensionality in ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$. Comparing the NMR/NQR results among ${\mathrm{BaTi}}_{2}P{n}_2\mathrm{O}$ ($Pn=\text{As}$, Sb, and Bi), it was found that the normal and SC properties of ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$ were considerably different from those of ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$ and ${\mathrm{BaTi}}_2{\mathrm{As}}_2\mathrm{O}$, which might explain the two-dome structure of $T_{\mathrm{c}}$ in this system.

Figure 1

(a) Crystal structure of ${\mathrm{BaTi}}_{2}P{n}_2\mathrm{O}$. The figure of the crystal structure was modeled on the three-dimensional visualization program vesta [11]. (b) Electronic phase diagram of ${\mathrm{BaTi}}_2{\left({\mathrm{As}}_{1-y}{\mathrm{Sb}}_y\right)}_2\mathrm{O}$ and ${\mathrm{BaTi}}_2{\left({\mathrm{Sb}}_{1-x}{\mathrm{Bi}}_x\right)}_2\mathrm{O}$ [7]. SC denotes the superconducting phases. The open squares represent the CDW transition temperature $T_{\mathrm{A}}$. The solid circles represent the superconducting temperature $T_{\mathrm{c}}$ determined by magnetization and electrical resistivity measurements.

Figure 2

${}^{209}\mathrm{Bi}$-NQR spectra obtained by the frequency-swept method at 5 K. From the observed ${}^{209}\mathrm{Bi}$-NQR spectra, the quadrupole parameters for Bi nuclei are evaluated as shown in the figure. The solid red curves are the simulation of the NQR spectra using the estimated quadrupole parameters.

Figure 3

Field-swept ${}^{209}\mathrm{Bi}$-NMR spectra at (a) 5 K and (b) 50 K at 68.41 MHz. In addition, simulations of the NMR spectra are shown. At 5 K, the same NQR parameters as those for the NQR measurements are used for the simulation. The arrows indicate the magnetic field at which $T_1$ is measured.

Figure 4

Temperature dependence of $\eta$ deduced from the first and second largest NQR peaks ($\sim 10$ and 14 MHz). Inset: Temperature dependence of NQR frequency.

Figure 5

Temperature dependence of $1/T_{1}T$ at $\sim 10$ T. The dashed arrow indicates the temperature at which the anomaly was observed in the NMR spectrum $T_{\mathrm{anom}}$. Inset: Temperature dependence of $1/T_{1}T$ and ac susceptibility at 0 T. The solid arrow indicates the SC transition temperature $T_{\mathrm{c}}$.

Figure 6

Temperature dependence of $1/T_{1}T1/\left(T_{1}T{\gamma }^2\right)$ in ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$, ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$ [2], and ${\mathrm{BaTi}}_2{\mathrm{As}}_2\mathrm{O}$ [15]. For ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$, we plot the data at 10 T above 5 K and at 0 T below 5 K. For ${\mathrm{BaTi}}_2{\mathrm{As}}_2\mathrm{O}$, we use the angle average of $1/T_{1}T$ for simplicity. The dashed arrows indicate $T_{\mathrm{anom}}$ or $T_{\mathrm{A}}$. The solid arrows indicate $T_{\mathrm{c}}$.