Nonunitary Triplet Pairing in the Centrosymmetric Superconductor ${\mathrm{LaNiGa}}_2$

Muon spin rotation and relaxation experiments on the centrosymmetric intermetallic superconductor ${\mathrm{LaNiGa}}_2$ are reported. The appearance of spontaneous magnetic fields coincides with the onset of superconductivity, implying that the superconducting state breaks time reversal symmetry, similarly to noncentrosymmetric ${\mathrm{LaNiC}}_2$. Only four triplet states are compatible with this observation, all of which are nonunitary triplets. This suggests that ${\mathrm{LaNiGa}}_2$ is the centrosymmetric analogue of ${\mathrm{LaNiC}}_2$. We argue that these materials are representatives of a new family of paramagnetic nonunitary superconductors.

Figure 1

The orthorhombic crystal structure of ${\mathrm{LaNiGa}}_2$. The red spheres (largest) are La, blue spheres (smallest) are Ni, and the black spheres (medium) are Ga.

Figure 2

The upper graph is a typical muon asymmetry spectra in ${\mathrm{LaNiGa}}_2$ taken in a transverse field of 40 mT at 0.05 K (shown in the rotating reference frame (RRF) of 6.0 MHz. The line is a fit to the data using Eqn. (1). For clarity, only one of the two virtual detectors have been shown. The lower graph is the zero field $\mu \mathrm{SR}$ spectra for ${\mathrm{LaNiGa}}_2$. The blue symbols are the data collected at 56 mK and the red symbols are the data collected at 3.0 K. The lines are a least squares fit to the data.

Figure 3

The field dependence of $\sigma$ at 50 mK, after being field cooled. The line is a fit to the data using Eqn. (2).

Figure 4

The left graph shows the temperature dependence of $\sigma$, for ${\mathrm{LaNiGa}}_2$ in zero-field, which clearly shows the spontaneous fields appearing at $T_c=2.1\mathrm{K}$ (shown has the vertical line). The line is fit to the data using an approximation [34] to the BCS order parameter for ${\sigma }_e$. The right graph shows the temperature dependence of the electronic relaxation rate, $\Lambda$, for ${\mathrm{LaNiGa}}_2$ in zero-field, which shows no temperature dependence.