Delta H = Delta B region in volume defect-dominating superconductor

Abstract

According to Ginzburg-Landau theory, it has been generally accepted that the diamagnetic property decreases after the lower critical field. However, we found that (Fe, Ti) particle doped MgB2 specimens reveal the Delta H = Delta B section in the magnetization curves, which are not following the theory. We present whether this phenomenon appears to be only confined to (Fe, Ti) particle doped Magnesium diborid superconductor, whether there is a theoretical basis and why it does not appear in other superconductors. We have understood that the cause of the Delta H = Delta B section is the pinning phenomenon of defects in the superconductor and it only occurs in volume defect dominating superconductors. The width of the Delta H = Delta B section along the number of defects and Hc2 was estimated assuming that defects are in the ideal state, and compared with experimental results. We hypothesized that pinned fluxes have to be picked out from the defect and move into an inside of a superconductor regardless free energy depth of the defect if the distance between fluxes pinned at the defect is equal to the one of upper critical field. It is considered that the reason that this phenomenon has not been reported yet is the flux jump of the volume defect dominating superconductor. The section means that the fluxes that have penetrated into a inside of a superconductor in which volume defects exist are preferentially pinned on them over the entire specimen before Ginzburg-Landau behavior. If the size of volume defects is uniform in some extent, the influence of the planar and line defects is small and the flux jump does not occur, we believe that the section must be observed in any superconductor. It is because this is one of the basic natures of pinning phenomenon in the volume defect dominating superconductor.