Upper Critical Field Based on a Width of ΔH = ΔB region in a Superconductor

We studied a method of measuring upper critical field (H c 2 ) of a superconductor based on a width of ΔH = ΔB region, which appears in a superconductor that volume defects are many and dominant. Here we show basic concepts and details of the method. Although H c 2 of a superconductor is fixed according to a kind of superconductor, it is difficult to measure H c 2 experimentally. Thus, results are different depending on experimental conditions. H c 2 was otained by a theory on a width of ΔH = ΔB region, which is that pinned fluxes at volume defects are picked out and move into an inside of the superconductor when the distance between pinned fluxes is the same as that at H c 2 of the superconductor. H c 2 of MgB 2 obtained by the method was 65.4 Tesla at 0 K, which is quite same as that of Ginzburg-Landau theory. The reason that H c 2 obtained by the method is closer to ultimate H c 2 is based on that F p i n n i n g /F p i c k o u t is more than 4 when pinned fluxes at volume defects of 163 nm radius are depinned, which means that the H c 2 is less sensitive to fluctuation. The method will help to find the ultimate H c 2 of volume defect-dominating superconductors.