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

Abstract

We studied a method of measuring upper critical field (H c2 ) of a superconductor based on the width of ∆H = ∆B region, which appears in the superconductor that volume defects are many and dominant. Here we present the basic concept and details of the method. Although H c2 of a superconductor is fixed according to kind of the superconductor, it is difficult to measure H c2 experimentally, and the results are different depending on the experimental conditions. H c2 was calculated from the theory that pinned fluxes at volume defects are picked out and move into an inside of the superconductor when their arrangement is the same as that of H c2 state of the superconductor. H c2 of MgB 2 obtained by the method was 65.4 Tesla at 0 K. The reason that H c2 obtained by the method is closer to ultimate H c2 is based on that ∆F pinning /∆F pickout is more than 4 when pinned fluxes at volume defects of 163 nm radius are picked out. The method will help to find the ultimate H c2 of volume defect-dominating superconductors.

Quantum Bit Behavior of Pinned Fluxes on Volume Defects in a Superconductor

“…The amplitude of the oscillation (∆M) was calculated at approximately 0.5 emu/cm 3 . And the ∆M would oscillate 8000 times in ∆H=∆B region [22]. A width of ∆H=∆B region is constant although max-diamagnetic properties are different according to the radius of volume defects in the state that vol.% of volume defects is fixed.…”

“…where α is aL H c2 Φo √ P c , n 2 is the number of quantum fluxes pinned at a spherical volume defect of radius r, H c2 is upper critical field of the superconductor, Φ o is flux quantum which is 2.07×10 −7 G•cm 2 , c is the velocity of light, aL is average length of quantum fluxes which are pinned and bent between volume defects (a is an average bent constant which is 1 < a < 1.2 and L is the distance between volume defects in vertically packed state as shown in Fig. 2), and P is the filling rate which is π/4 when flux quanta are pinned at a volume defect in the form of square [22].…”

“…The pinned fluxes on volume defects would move as a bundle due to the tension of them when they are depinned [22]. Therefore, a difference in diamagnetic property occurs between flux-pinning state on volume defects and depinning state from the volume defects in ∆H=∆B…”

Flux-Pinning Effects and Mechanism of Water-Quenched 5 wt.% (Fe, Ti) Particle-Doped MgB2 Superconductor

“…4 (b). Since quantum fluxes of volume defect part are fixed on the volume defects, they can withstand until the distance between them is same as that of H c2 because F pinning (the pinning force) of 163 nm radius volume defect is much larger than F pickout (the pick-out depinning force) in ∆H=∆B region [17].…”

Flux-pinning behaviors and mechanism according to dopant level in (Fe, Ti) particle-doped $$\text {MgB}_2$$ superconductor

“…The shortest distance between pinned fluxes at volume defects would determine whether the pinned fluxes have to be pick-out depinned or not. Thus they could be pick-out depinned and move although F is smaller than F 17 . Pick-out depinning was named by the behavior of depinning that pinned fluxes are picked out together when they are depinned from the volume defect.…”