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

TL;DR

This study reveals that LaRh$_2$As$_2$ exhibits conventional s-wave superconductivity with a full gap, contrasting with the unconventional multiphase superconductivity of CeRh$_2$As$_2$, highlighting the role of 4$f$ electrons.

Contribution

The paper provides the first detailed investigation of LaRh$_2$As$_2$'s superconducting gap structure using NQR, demonstrating conventional s-wave pairing unlike CeRh$_2$As$_2$.

Findings

LaRh$_2$As$_2$ shows a coherence peak in 1/T1 below Tc.

The superconducting gap is consistent with weak-coupling s-wave.

LaRh$_2$As$_2$ lacks the multiphase superconductivity seen in CeRh$_2$As$_2$.

Abstract

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 $H_{c2}\sim$ 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 $\Delta(0)/k_{B}T_{c} \sim 1.48$, consistent with the weak-coupling $s$-wave superconductivity. These results suggest that the 4$f$ electrons in CeRh$_2$As$_2$ not only enhance the orbital limiting field but also contribute to the formation of unconventional superconductivity with SC multiphase.