Order-disorder transition in Bi2.1Sr1.9CaCu2O8+delta single crystals doped with Fe and Pb - art. no. 224501

摘要

The magnetic field H-dis(T) where an order-disorder transition of the vortex lattice in high-T-c superconductors occurs, is investigated by measurements of the magnetization M(H) in Bi2.1Sr1.9CaCu2O8+delta (Bi2212) single crystals doped with iron and lead. Comparative studies are made of the temperature dependences of the field H-peak(T), where the second peak occurs in M(H), and the fields H-min(T), and H-infl(T) where a minimum and an inflection point occur at the low-field side of this peak. It is proposed that H-dis(T) lies close to H-infl. In Bi2.1Sr1.9Ca1.0(Cu1-yFey)(2)O8+delta single crystals with Fe concentration y=0, 0.005, 0.016, and 0.022, a pronounced peak in the derivative dM/dH is observed, whose position H-infl(T) is independent of temperature T. We relate this peak to the field H-dis(T), which separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. In heavily Pb-doped single Bi2212 crystals, H-infl(T) decreases with increasing T. For the same crystals, a minimum in the normalized relaxation rate S(H) is observed at H-infl, indicating two different flux-creep mechanisms above and below that field and two different solid vortex phases. It is argued that the negative slope of H-dis(T) in heavily-Pb-doped Bi2212 crystals is related to the enhanced c axis conductivity caused by the Pb sitting between the CuO2 layers and causing three-dimensional vortex lines, while in Fe-doped Bi2212 crystals the Fe ions sit on the CuO2 planes and thus do not enhance the coupling between pancake vortices. [References: 41]