Pressure evolution of the normal- and superconducting-state properties of the line-nodal material ${\mathrm{CaSb}}_2$ revealed by ${}^{123}\mathrm{Sb}$ nuclear quadrupole resonance

${\mathrm{CaSb}}_2$ is the Dirac line-nodal material that exhibits a superconducting (SC) transition at 1.7 K. In spite of its conventional SC state at ambient pressure, the transition temperature $T_{\mathrm{c}}$ shows a peak structure against hydrostatic pressure. We performed ac magnetic susceptibility and ${}^{123}\mathrm{Sb}$ nuclear quadrupole resonance (NQR) measurements on single-crystalline ${\mathrm{CaSb}}_2$ under pressures up to 2.08 GPa. $T_{\mathrm{c}}$ monotonically increased in this pressure region, which is consistent with a previous study. We observed continuous broadening of the NQR spectrum against pressure, which is a sign of unique compression behavior of the lattice. In the normal state, the nuclear spin-lattice relaxation rate $1/T_1$ is proportional to temperature in all pressure values, typical of a metal. However, $1/T_{1}T$ in the normal state is independent of pressure, indicating that the density of states at the Fermi energy $N\left(E_{\mathrm{F}}\right)$, which is one of the parameters governing $T_{\mathrm{c}}$, is insensitive to pressure. From these results, we conclude that $N\left(E_{\mathrm{F}}\right)$ does not govern the origin of the enhancement in $T_{\mathrm{c}}$. This is unusual for a weak electron-phonon coupling superconductor. In the SC state, we revealed that the SC gap becomes larger and more isotropic under pressure.

Figure 1

(a) Crystal structure of ${\mathrm{CaSb}}_2$ with two nonequivalent Sb sites drawn by vesta [32]. (b) Temperature dependence of the ac susceptibility ${\chi }_{\mathrm{AC}}$ measured by a self-inductance method using a frequency range of 83.5–89.7 MHz under various pressure values. (c) Pressure dependence of $T_{\mathrm{c}}$. The dashed lines are visual guidance identical to those in Ref. [30].

Figure 2

(a) ${}^{123}\mathrm{Sb}\left(1\right)$ NQR spectrum of ${\mathrm{CaSb}}_2$ measured at 4.2 K under various pressure values. Inset: ${}^{123}\mathrm{Sb}\left(1\right)$ NQR spectrum at 0 GPa with that of the powder samples. (b) Pressure dependence of peak frequency and FWHM of the NQR spectrum.

Figure 3

(a) $T/T_{\mathrm{c}}$ dependence of $1/T_{1}T$ under various pressure values. (b) Pressure dependence of observed Korringa value (black squares) and calculated ones (red circles).

Figure 4

$T/T_{\mathrm{c}}$ dependence of normalized $1/T_{1}T$ in the low-temperature region at ambient pressure and 1.68 GPa. Dashed curves show the fitting results using the BCS model.