Effect of pressure on the reentrant superconductor-normal-metal phase boundaries of the (La1-nCen)Al2 and (La1-xThx)1-nCen Kondo systems

Measurements of the superconducting critical temperature Tc(n,P) as a function of Ce concentration n and pressure P≤20 kbar are reported for the superconducting Kondo systems (La1-nCen)Al2 and (La1-xThx)1-nCen (0≤x≤0.25). Data for the (La1-nCen)Al2 system are partially consistent with the theory of Müller-Hartmann and Zittartz (MHZ), assuming an increase of TK/Tc0 with P, where TK is the Kondo temperature and Tc0 is the critical temperature of the pure host superconductor. However, the pressure evolution of the reentrant superconducting-normal phase boundaries for the (La0.8Th0.2)1-nCen 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 (La0.8Th0.2)1-nCen or (La1-nCen)Al2 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.