Conventional type-II superconductivity in locally noncentrosymmetric LaRh2As2 single crystals

We report on the observation of superconductivity in $\mathrm{La}{\mathrm{Rh}}₂{\mathrm{As}}₂$, which is the analog without $f$ electrons of the heavy-fermion system with two superconducting phases $\mathrm{Ce}{\mathrm{Rh}}₂{\mathrm{As}}₂$ [S. Khim et al., Science 373, 1012 (2021)]. A zero-resistivity transition, a specific-heat jump, and a drop in magnetic ac susceptibility consistently point to a superconducting transition at a temperature of ${T}_c=0.28$ K. The magnetic-field temperature superconducting phase diagrams determined from field-dependent ac-susceptibility measurements reveal small upper critical fields ${\ensuremath{\mu}}₀{H}_c2\ensuremath{\approx}12$ mT for $H\ensuremath{\parallel}ab$ and ${\ensuremath{\mu}}₀{H}_c2\ensuremath{\approx}9$ mT for $H\ensuremath{\parallel}c$. The observed ${H}_c2$ is larger than the estimated thermodynamic critical-field ${H}_c$ derived from the heat-capacity data, suggesting that $\mathrm{La}{\mathrm{Rh}}₂{\mathrm{As}}₂$ is a type-II superconductor with Ginzburg-Landau parameters ${\ensuremath{\kappa}}_GL^ab\ensuremath{\approx}1.9$ and ${\ensuremath{\kappa}}_GL^c\ensuremath{\approx}2.7$. The microscopic Eliashberg theory indicates superconductivity to be in the weak-coupling regime with an electron-phonon-coupling constant ${\ensuremath{\lambda}}_{e\ensuremath{-}ph}\ensuremath{\approx}0.4$. Despite a similar ${T}_c$ and the same crystal structure as the Ce compound, $\mathrm{La}{\mathrm{Rh}}₂{\mathrm{As}}₂$ displays conventional superconductivity, corroborating the substantial role of the $4f$ electrons for the extraordinary superconducting state in $\mathrm{Ce}{\mathrm{Rh}}₂{\mathrm{As}}₂$.