Hecto-Ångstrom modulation and peak effect in superconducting (Hg,Pb)(Ba,Sr)-1223

To elucidate the origin of the “peak effect” (PE) (or the so-called “fishtail” phenomenon) in the magnetization hysteresis loop of (Hg,Pb)(Ba,Sr) 2Ca 2Cu 3O 8+ δ superconductors, the nanoscopic structure was studied for two different samples: one was prepared by an encapsulation technique, exhibiting PE and the other by a high-pressure technique, showing no PE. By a combination of transmission electron microscopy, electron diffraction and local compositional analysis, a hecto-ångstrom-scale structural distortion accompanied by compositional modulation of both Hg and Pb along the 〈110〉 direction was found for the sample obtained by the encapsulation technique. The spatial inhomogeneity was considered to be a weak vortex pinning site, which may be responsible for PE. In contrast, no such inhomogeneity was detected in the high-pressure synthesized sample. The origin of the compositional modulation might be attributed to spinodal decomposition.