Probing the superconducting pairing of the La 4 Be 33 Pt 16 alloy via muon-spin spectroscopy.

We report a study of the superconducting pairing of the noncentrosymmetric La 4 Be 33 Pt 16 alloy using muon-spin rotation and relaxation (μSR) technique. Below T c = 2.4 K, La 4 Be 33 Pt 16 exhibits bulk superconductivity (SC), here characterized by heat-capacity and magnetic-susceptibility measurements. The temperature dependence of the superfluid density ρ sc ( T ), extracted from the transverse-field μSR measurements, reveals a nodeless SC in La 4 Be 33 Pt 16 . The best fit of ρ sc ( T ) using an s -wave model yields a magnetic penetration depth λ 0 = 542 nm and a superconducting gap Δ 0 = 0.37 meV at zero Kelvin. The single-gapped superconducting state is further evidenced by the temperature-dependent electronic specific heat C e ( T )/ T and the linear field-dependent electronic specific-heat coefficient γ H ( H ). The zero-field μSR spectra collected in the normal- and superconducting states of La 4 Be 33 Pt 16 are almost identical, confirming the absence of an additional field-related relaxation and, thus, of spontaneous magnetic fields below T c . The nodeless SC combined with a preserved time-reversal symmetry in the superconducting state prove that the spin-singlet pairing is dominant in La 4 Be 33 Pt 16 . This material represents yet another example of a complex system showing only a conventional behavior, in spite of a noncentrosymmetric structure and a sizeable spin-orbit coupling.