# Upper Critical Field Based on the Width of $\Delta$H = $\Delta$B region in a Superconductor

## TL;DR

This paper introduces a novel method to measure the upper critical field (H$_{c2}$) in superconductors with many volume defects by analyzing the width of a specific magnetic field region, providing more accurate H$_{c2}$ values.

## Contribution

The paper proposes a new technique based on flux pinning and movement to determine the ultimate H$_{c2}$ in defect-rich superconductors, improving measurement accuracy.

## Key findings

- H$_{c2}$ of MgB$_2$ was measured as 65.4 Tesla at 0 K.
- The method's accuracy is linked to flux pinning and pickout dynamics.
- The approach helps identify the true H$_{c2}$ in defect-dominant superconductors.

## Abstract

We studied a method of measuring upper critical field (H$_{c2}$) of a superconductor based on the width of $\Delta$H = $\Delta$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 $\Delta$F$_{pinning}$/$\Delta$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.