Conventional s-wave Superconductivity in LaRh$_2$As$_2$; the Analog without the 4 f Electrons of CeRh$_2$As$_2$

Superconductor LaRh$_2$As$_2$ has the same crystal structures as CeRh$_2$As$_2$, which exhibits superconducting (SC) multiphase in the c-axis magnetic field. Although the SC transition temperatures T$_c$ are similar, around 0.3 K, LaRh$_2$As$_2$ shows conventional type-II superconductivity with a small upper critical field H$_{c2}$ ∼ 10 mT. At present, the SC properties of LaRh$_2$As$_2$ have not been clarified yet. We performed $^{75}$As-nuclear quadrupole resonance (NQR) measurements on LaRh$_2$As$_2$ to investigate the SC properties and gap structure. 1/T$_1$ shows a clear coherence peak just below T$_c$ and an exponential decrease at lower temperatures, suggesting full-gap s-wave superconductivity. The numerical calculations based on an s-wave SC model reveal an SC gap size of Δ(0)/k$_B$T$_c$ ∼ 1.48, consistent with the weak-coupling s-wave superconductivity. These results suggest that the 4f electrons in CeRh2As2 not only enhance the orbital limiting field but also contribute to the formation of unconventional superconductivity with SC multiphase.