Structural origin of the low superconducting anisotropy of Bi1.7Pb0.4Sr2Ca0.9Cu2O8 crystals

Abstract

The structure of ${\mathrm{Bi}}_{1.7}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{0.9}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$ $({T}_{c}=93\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ was refined from x-ray single crystal diffraction data in space group $A2aa$, $a=5.3852(9)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $b=5.4286(9)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $c=30.997(6)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. For this composition, the crystal structure is free from the structural modulation connected with the presence of additional oxygen atoms in the BiO layers. The $c$-axis lattice parameter is slightly larger in (Bi,Pb)-2212 crystals than in Pb-free Bi-2212 crystals, but the complete structural refinement revealed an inhomogeneous redistribution of the inter-planar distances. In particular, the distance between the two neighboring BiO layers has significantly decreased in the modulation-free phase as compared to the Pb-free phase. We believe that this is a key point in understanding the widely observed reduction of the anisotropy in Bi-2212 by Pb-doping. The irreversibility line as well as the onset of the second magnetization peak in modulation-free (Bi,Pb)-2212 crystals were studied. Because of the strongly reduced value of the anisotropy, the 2D-3D crossover field value, ${H}_{\mathit{cr}}$, is significantly higher in (Bi,Pb)-2212 crsystals than in Bi-2212 crystals. As a direct consequence of the structural modifications induced by the removal of the modulation, the nature of the inter-layer coupling changes from electromagnetic in modulated Bi-2212 to Josephson in modulation-free (Bi,Pb)-2212. Annealing the (Bi,Pb)-2212 crystals under oxygen reintroduced the structural modulation.