Comparison of the low-temperature specific heat of Fe- and Co-doped Bi1.8Pb0.2Sr2Ca(Cu1-xMx)2O8 (M=Fe or Co): Anomolously enhanced electronic contribution due to Fe doping.

Abstract

Specific-heat data of Fe-doped ${\mathrm{Bi}}_{1.8}$${\mathrm{Pb}}_{0.2}$${\mathrm{Sr}}_{2}$Ca(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$${)}_{2}$${\mathrm{O}}_{8}$ in the range 2--20 K are presented for x=1, 2, 4, 6, and 8 %. The data are compared with our previous measurements on Co-doped bismuth-strontium-calcium-copper oxide superconductors of nominal composition ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ (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 \ensuremath{\gamma} 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 ${\ensuremath{\gamma}}_{0}$=72 mJ/mole ${\mathrm{K}}^{2}$ (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. \textcopyright{} 1996 The American Physical Society.