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 ($\mu$SR) 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 ${\rho }_{\mathrm{s}\mathrm{c}}(T)$, extracted from the transverse-field {\textmu}SR measurements, reveals a nodeless SC in La${}_4$Be${}_{33}$Pt${}_{16}$. The best fit of ${\rho }_{\mathrm{s}\mathrm{c}}(T)$ using an $s$-wave model yields a magnetic penetration depth ${\lambda }_0=542$ nm and a superconducting gap ${\mathrm{\Delta }}_0=0.37$ meV at zero Kelvin. The single-gapped superconducting state is further evidenced by the temperature-dependent electronic specific heat $C_{\mathrm{e}}(T)\mathrm{/}T$ and the linear field-dependent electronic specific-heat coefficient ${\gamma }_{\mathrm{H}}(H)$. The zero-field $\mu$SR 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.