Order-disorder transition inBi2.1Sr1.9CaCu2O8+δsingle crystals doped with Fe and Pb

Abstract

The magnetic field ${H}_{\mathrm{dis}}(T)$ 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 ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) single crystals doped with iron and lead. Comparative studies are made of the temperature dependences of the field ${H}_{\mathrm{peak}}(T),$ where the second peak occurs in $|M(H)|,$ and the fields ${H}_{\mathrm{min}}(T),$ and ${H}_{\mathrm{infl}}(T)$ where a minimum and an inflection point occur at the low-field side of this peak. It is proposed that ${H}_{\mathrm{dis}}(T)$ lies close to ${H}_{\mathrm{infl}}.$ In ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{Ca}}_{1.0}({\mathrm{Cu}}_{1\ensuremath{-}y}{\mathrm{Fe}}_{y}{)}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ 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 ${H}_{\mathrm{infl}}(T)$ is independent of temperature T. We relate this peak to the field ${H}_{\mathrm{dis}}(T),$ which separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. In heavily Pb-doped single Bi2212 crystals, ${H}_{\mathrm{infl}}(T)$ decreases with increasing T. For the same crystals, a minimum in the normalized relaxation rate $S(H)$ is observed at ${H}_{\mathrm{infl}},$ 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 ${H}_{\mathrm{dis}}(T)$ in heavily-Pb-doped Bi2212 crystals is related to the enhanced c axis conductivity caused by the Pb sitting between the ${\mathrm{CuO}}_{2}$ layers and causing three-dimensional vortex lines, while in Fe-doped Bi2212 crystals the Fe ions sit on the ${\mathrm{CuO}}_{2}$ planes and thus do not enhance the coupling between pancake vortices.