Critical Fields and Critical Currents in MgB2

TL;DR

This paper reviews measurements of critical magnetic fields in MgB2, discusses its superconducting properties, and suggests modifications to enhance its high-current application potential.

Contribution

It provides a detailed analysis of MgB2's critical fields, anisotropies, and proposes strategies to improve its high-field performance.

Findings

MgB2 is in the clean limit with low Hc2 (~3 T in c-direction)

Enhancing Hc2 requires introducing strong electron scattering

Pinning defects of a few nanometers can increase critical current density

Abstract

We review recent measurements of upper (Hc2) and lower (Hc1) critical fields in clean single crystals of MgB2, and their anisotropies between the two principal crystallographic directions. Such crystals are far into the "clean limit" of Type II superconductivity, and indeed for fields applied in the c-direction, the Ginzburg-Landau parameter k is only about 3, just large enough for Type II behaviour. Because m0Hc2 is so low, about 3 T for fields in the c-direction, MgB2 has to be modified for it to become useful for high-current applications. It should be possible to increase Hc2 by the introduction of strong electron scattering (but because of the electronic structure and the double gap that results, the scatterers will have to be chosen carefully). In addition, pinning defects on a scale of a few nm will have to be engineered in order to enhance the critical current density at high fields.