This review recalled that the transport of electrical current through a superconductor fell into 3 broad regimes: non-dissipative, dissipative but superconducting, and normal or non-superconducting. These regimes were demarcated by 2 definitions of critical current. One was the threshold current above which the superconductor entered a dissipative (resistive) state. The other was the thermodynamic threshold, above which the superconductivity itself was destroyed and the superconducting order parameter Δ vanished. The first threshold defined the conventional critical current density JC and the second defined the de-pairing (or pair-breaking) current Jd. Type-II superconductors in the mixed state had quantized flux vortices, which tended to move when acted upon by the Lorentz driving force of an applied transport current. In such a mixed state the resistance vanished only when vortices were pinned in place by defects and the applied current was below the threshold jC required to overcome pinning and mobilize the vortices. Typically jd was much greater than jC and a direct experimental measurement of jd over the entire temperature range (0 to TC) was prohibited by the enormous power dissipation densities (1010 to 1012/cm3) needed to reach the normal state. Intense pulsed signals were used here to extend transport measurements to unprecedented power densities (109 to 1010W/cm3). This, together with the combination of low normal-state resistivity (ρn) and high TC in MgB2, permitted a direct estimation to be made of jd over the entire temperature range.

Current-Induced Pair Breaking in Magnesium Diboride. M.N.Kunchur: Journal of Physics - Condensed Matter, 2004, 16, R1183-204