Depairing current and superconducting transition of YBCO at intense pulsed currents

Abstract

The critical current density of a superconductor as a function of temperature and magnetic field is a key quantity for the design of superconductor applications. Tremendous efforts are currently undertaken to enhance the critical current in technical materials by improving the pinning forces. The ultimate thermodynamic limit for carrying a superconducting current is named the depairing current and usually cannot be measured directly. Our measurements are carried out using short-pulse currents with about 50 ns duration in order to avoid thermal heating and eventually destruction of the samples that result inevitably from such high dissipation. In addition, ultrathin films of YBa 2Cu3O7−x are used, patterned to microbridges to allow for an effective heat transfer into the MgO substrate. The depairing current is directly measured and is in good accordance with the values predicted by Ginzburg-Landau theory. In intense currents the critical temperature is not only shifted to lower temperatures, as seen in conventional d.c. measurements, but the shape of the transition changes significantly. The latter effect is attributed to a reduction of the fluctuation paraconductivity with increasing electric field and is compared to our recent theoretical work.

Non-ohmic Electrical Transport Properties Above the Critical Temperature in Optimally and Underdoped Superconducting YBa2Cu3O6+x

“…A satisfactory agreement with the theoretical models for layered superconductors based on a microscopic approach [11] or on the time-dependent Ginzburg-Landau theory [12,13] has been found only when the inherent self-heating of the sample was minimized by using a pulsed current technique and additionally very thin films [8,10].…”

“…In the high-temperature cuprate superconductors (HTSC) several investigations of a non-ohmic in-plane conductivity above T c have been reported and attributed to the paraconductivity suppression in high electric fields [5][6][7][8][9][10]. A satisfactory agreement with the theoretical models for layered superconductors based on a microscopic approach [11] or on the time-dependent Ginzburg-Landau theory [12,13] has been found only when the inherent self-heating of the sample was minimized by using a pulsed current technique and additionally very thin films [8,10].…”

Critical current enhancement in YBa2Cu3O7−δ towards the intrinsic depairing value in short current pulses

“…In thin films it is possible to extend such measurements over the entire superconducting transition if j < 1 MA cm À2 . Such curves exhibit a very steep transition, shifted to lower temperatures for increasing current density [4]. This is a clear signature of a thermal run-away effect and, thus, not useful for a determination of j d .…”

“…Applying high current densities on HTSC is however not an easy task, since the dissipated power density would attain several GW cm À3 in the normal state. Only few experiments have explored the supercritical current region far above j c in HTSC [1][2][3][4].…”

“…We present resistivity data from two epitaxial YBa 2 -Cu 3 O 7Àd (YBCO) films, with 12 and 22 nm electrically active layer thickness [4], respectively, fabricated by pulsed-laser deposition on MgO substrates. Data were collected at current densities up to about 7 MA cm À2 using a pulsed-current technique, described in detail elsewhere [2].…”