NMR and NQR studies in candidate triplet superconductor LaNiGa₂

LaNiGa₂ is a layered centrosymmetric superconductor with an orthorhombic crystal structure. μSR experiments have observed spontaneous magnetic fields at the onset of superconducting state, implying time-reversal symmetry breaking and suggesting the possibility of triplet pairing. Here, we report on the microscopic investigations in the normal and superconducting (SC) states of LaNiGa₂ using nuclear magnetic resonance (NMR) and quadrupole resonance (NQR). Single-crystal NMR measurements were used to determine the anisotropy of the electric field gradient (EFG) at the two non-equivalent ⁶⁹Ga sites and one ¹³⁹La site. The field orientation dependence of the spectra revealed a large asymmetry in the EFG tensor for the ⁶⁹Ga sites, whereas EFG had axial symmetry at the ¹³⁹La site. Band structure calculations based on density functional theory provided a good agreement with these results. Zero-field powder NQR measurements were performed in the SC state down to dilution-fridge temperature. We discuss the size and structure of the SC gap and the type of order parameter based on the temperature dependence of the spin-lattice relaxation rate 1/T₁ at ¹³⁹La. The sizes of SC critical fields and the penetration depth have been examined in relation to the grain size.