Specific heat measurements on superconducting Ce-La alloys

Abstract

In Ce x La 1− x alloys one can study the co-existence of the Kondo effect and superconductivity. The first electron tunneling measurements by Edelstein on such a system were made on this alloy system. These measurements on alloy films showed that these alloys were gapless at the concentrations investigated. The number of states at the Fermi surface in the concentration range 0.002 ⩽ x ⩽ 0.005 was proportional to the impurity concentration. If the first Born approximation were applicable, there would not have been any states. Hence, unlike the Kondo effect these measurements provided direct microscopic evidence for the failure of the first Born approximation. Specific heat measurements on bulk samples indicate that the tunneling results were indicative of bulk samples. The low temperature (0.5 K ⩽ T ⩽ 3.0 K) specific heat contains a γ T term. Such a term implies the existence of states at and around the Fermi surface. For x = 0.001, 0.002, 0.005, and 0.0075 values of γ = 0.11±0.08, 0.90±0.05, 2.40±0.20, and 4.66±0.04 mJ/ mole K 2 were obtained. These specific heat results are in agreement with the tunneling results. The fact that there is a critical Ce concentration below which there are no states at the Fermi level has important theoretical implications.