Current-Induced Pair Breaking in Magnesium Diboride
The transport of electrical current through a superconductor falls into three broad regimes: non-dissipative, dissipative but superconducting, and normal or non-superconducting. These regimes are demarkated by two definitions of critical current: one is the threshold current above which the superconductor enters a dissipative (resistive) state; the other is the thermodynamic threshold above which the superconductivity itself is destroyed and the superconducting order parameter vanishes. The first threshold defines the conventional critical current density Jc and the second defines the depairing (or pair-breaking) current Jd. Type II superconductors in the mixed state have quantized flux vortices, which tend to move when acted upon by the Lorentz driving force of an applied transport current. In such a mixed state the resistance vanishes only when vortices are pinned in place by defects and the applied current is below the threshold Jc required to overcome pinning and mobilize the vortices. Typically Jd >> Jc and a direct experimental measurement of Jd over the entire temperature range (0 < T < Tc) is prohibited by the enormous power dissipation densities (p ~ 10^10 -- 10^12 W/cm^3) needed to reach the normal state. In this work, intense pulsed signals were used to extend transport measurements to unprecedented power densities (p ~ 10^9 -- 10^10 W/cm^3). This together with MgB2's combination of low normal-state resistivity and high transition temperature have permitted a direct estimation of \jd over the entire temperature range. This review describes our experimental investigation of current-induced depairing in MgB2, and provides an introduction to the phenomenological theories of superconductivity and how the observations fit in their context.