Abstract
Specific-heat data of Fe-doped Bi1.8 Pb0.2 Sr2Ca(Cu1x Fex )2 O8 in the range 2–20 K are presented for x=1, 2, 4, 6, and 8 %. The data are compared with our previous measurements on Co-doped bismuth-strontium-calcium-copper oxide superconductors of nominal composition Bi2 Sr2 CaCu2 O8 (BISCO 2212). Both Fe and Co are magnetic substitutions with effective moments close to their free-ion value. In the normal state the magnetic susceptibility increases by more than a factor 2 over the doping range due to effective-mass enhancement. In the superconducting state both ions act as magnetic pair breakers. For Co doping the normal-state linear term γ is observed, enhanced due to the effective-mass increase. For Fe doping we observe a large anomalous contribution to the electronic specific heat starting near 15 K and leading at the lowest temperature to a linear term near γ0=72 mJ/mole K2 (1 mole=1 formula unit). The anomalous term is typical of heavy fermion behavior. Comparison with specific-heat data of Co-doped BISCO 2212 suggests that hybridization between 3d electrons of the dopant and the planar carriers is more effective for Fe doping than for Co doping. © 1996 The American Physical Society.