Depairing Current of Superconducting Heterostructures

The critical current density of a superconductor, marking the onset of dissipation in a material still in its superconducting state, is one of the key quantities of interest for both fundamental investigations and design of superconductor applications. Tremendous efforts have been reported to enhance the critical current in high- temperature superconductors (HTSC) by improving the pinning forces, but are ultimately limited by the superconducting depairing current, that marks the complete loss of superconductivity and the transition in the normal state. The depairing current should not be probed like the critical current, by a low-voltage criterion that is very sensitive to dissipation caused by vortex motion, but at some point within the resistive transition that marks the change in the thermodynamic state, e.g. at the mid-point. This requires to record the entire superconducting transition up into the normal state, at very high current densities and dissipation levels. Only few experiments have explored the supercritical current region far above the critical current in HTSC, being severely limited by sample heating. The fundamental requirements to achieve an accurate estimation of the depairing current in a broader temperature range below the superconducting transition are a specific pulse measurement technique, which we refined over the past years, and the very small thickness of the conducting layers where the high power dissipation takes place. The latter condition is realized through the specific layout of the investigated superlattices and heterostructures, where the ultrathin superconducting layers alternate with isolating ones, so that a much shorter phonon escape time characterizing the heat removal is possible, subsequently halved by the possibility to remove the dissipated heat on both sides of the active layers. Various artificial superconducting heterostructures are investigated, based both on standard HTSC materials, as well as on infinite-layered blocks. The latter kind of heterostructures illustrates the minimal structural requirement for high-temperature superconductivity, namely the simultaneous presence of the infinite layer and the charge reservoir blocks. The resistivity investigations at high current densities, used to assess the depairing current in the ultrathin superstructures, are supplemented by magnetoresistivity and Hall effect measurements under simultaneous application of the magnetic and electric fields, at various magnitudes and mutual orientations. The high current densities allows to explore the free flux-flow regime by overcoming pinning and also modulate the superconducting fluctuation suppression. Very thin vicinal HTSC films are used in order to probe the non-ohmic conductance in the c-axis direction at high electric fields and thus to indirectly estimate the depairing current in the c-axis. A nonzero Lorentz force on the string (Josephson) part of the "kinked" vortices results from this vicinal film geometry. In a high current density, the free flux-flow channeling regime for the Josephson vortices can thus be explored.

The critical current density of a superconductor, marking the onset of dissipation in a material still in its superconducting state, is one of the key quantities of interest for both fundamental investigations and design of superconductor applications. The critical current is ultimately limited by the superconducting depairing current that marks the complete loss of superconductivity and the transition in the normal state. The depairing current should not be probed like the critical current, by a low-voltage criterion that is very sensitive to dissipation caused by vortex motion, but at some point within the resistive transition that marks the change in the thermodynamic state. This requires recording the entire superconducting transition up into the normal state, at very high current densities and dissipation levels. Only few experiments have explored the supercritical current region far above the critical current in high temperature superconductors (HTSC), being severely limited by sample heating. The fundamental requirements to achieve an accurate estimation of the depairing current in a broader temperature range below the superconducting transition are a specific pulse measurement technique and the very small thickness of the conducting layers where the high power dissipation takes place. The resistivity investigations at high current densities, used to assess the depairing current in the ultrathin layered superconductors, are supplemented by magnetoresistivity and Hall effect measurements under simultaneous application of the magnetic and electric fields, at various magnitudes and mutual orientations. The high current density allows to explore the free flux-flow regime by overcoming pinning and also modulate the superconducting fluctuation suppression. In underdoped HTSC, the temperature onset of the non-linear behavior is best seen in the Hall conductivity and found to increase significantly with respect to the optimally doped films, suggesting that critical behavior is different from that of superconducting amplitude fluctuations. Vicinal films offer in addition the possibility to apply a bias current nonparallel to the crystallographic planes. When the magnetic field is applied parallel to these layers, the geometry of the experiment allows for a finite Lorentz force on the Josephson vortices, inducing their sliding motion along the ab-planes (vortex channeling). Investigations of the anisotropic transport behavior by vortex motion in vicinal HTSC films are performed at higher levels of the Lorentz force and vortex velocity that is at current densities much higher than in the previously published measurements, and electric fields up to more than six orders of magnitude higher than the dissipation onset criterion for the critical current. The channeling signature is found to get enhanced with increasing electric field.