Pressure evolution of the normal- and superconducting-state properties of the line-nodal material CaSb2 revealed by Sb123 nuclear quadrupole resonance

CaSb₂ 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 Tc shows a peak structure against hydrostatic pressure. We performed ac magnetic susceptibility and ¹²³Sb nuclear quadrupole resonance (NQR) measurements on single-crystalline CaSb₂ under pressures up to 2.08 GPa. Tc 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/T₁ is proportional to temperature in all pressure values, typical of a metal. However, 1/T₁T in the normal state is independent of pressure, indicating that the density of states at the Fermi energy N(EF), which is one of the parameters governing Tc, is insensitive to pressure. From these results, we conclude that N(EF) does not govern the origin of the enhancement in Tc. 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.