Measurements of the superconducting critical temperature ${\mathit{T}}_{\mathit{c}}$(n,P) as a function of Ce concentration n and pressure P≤20 kbar are reported for the superconducting Kondo systems (${\mathrm{La}}_{1\mathrm{-}\mathit{n}}$${\mathrm{Ce}}_{\mathit{n}}$)${\mathrm{Al}}_2$ and (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Th}}_{\mathit{x}}$${)}_{1\mathrm{-}\mathit{n}}$${\mathrm{Ce}}_{\mathit{n}}$ (0≤x≤0.25). Data for the (${\mathrm{La}}_{1\mathrm{-}\mathit{n}}$${\mathrm{Ce}}_{\mathit{n}}$)${\mathrm{Al}}_2$ system are partially consistent with the theory of Müller-Hartmann and Zittartz (MHZ), assuming an increase of ${\mathit{T}}_{\mathit{K}}$/${\mathit{T}}_{\mathit{c}0}$ with P, where ${\mathit{T}}_{\mathit{K}}$ is the Kondo temperature and ${\mathit{T}}_{\mathit{c}0}$ is the critical temperature of the pure host superconductor. However, the pressure evolution of the reentrant superconducting-normal phase boundaries for the (${\mathrm{La}}_{0.8}$${\mathrm{Th}}_{0.2}$${)}_{1\mathrm{-}\mathit{n}}$${\mathrm{Ce}}_{\mathit{n}}$ system is unexpected, and inconsistent with the MHZ theory. We find no evidence for the existence of a third transition (predicted by MHZ) back to the superconducting state for either the (${\mathrm{La}}_{0.8}$${\mathrm{Th}}_{0.2}$${)}_{1\mathrm{-}\mathit{n}}$${\mathrm{Ce}}_{\mathit{n}}$ or (${\mathrm{La}}_{1\mathrm{-}\mathit{n}}$${\mathrm{Ce}}_{\mathit{n}}$)${\mathrm{Al}}_2$ systems. Recent Monte Carlo calculations by Jarrell may explain our data for both alloy systems, provided strong electron-phonon coupling effects are taken into account, although the effects of interimpurity interactions on the data remain uncertain.

• Received 10 September 1993

DOI:https://doi.org/10.1103/PhysRevB.49.3894