Vortex state in gradient composite superconductor prepared in mega-gravity field

Okayasu, Satoru  ; Ono, Masao  ; Nishio, Taichiro*; Iguchi, Yusuke*; Mashimo, Tsutomu

We prepared a gradient composite superconductor alloy BiPb under mega-gravity field at 130 C for 100 hours. The prepared sample is separated into four parts along the gravity field direction, A) a Pb-rich phase with a almost constant molar proportion between Bi and Pb (3:7), B) a gradient composite mole ratio phase (the average mole ratio near by Bi:Pb=4:6), C) another gradient composite mole ratio phase with a different crystal structure to B), D) a bismuth-precipitated phase. We investigated the vortex state of this gradient superconducting sample with a scanning SQUID microscope. It is strange that no vortex can be found in the B)-phase, and the B)-phase act as a type-I superconductor. The differences among the superconducting phases can be explained by these of the pinning energies or by these of the superconducting condensation energies at individual phases.