Summary
A study used roughly 2-ps electrical pulses to probe intrinsic current limits in thin films of NbN and YBa₂Cu₃O₇. NbN showed a sharp depairing onset at about 2.2 times its conventional critical current density, while YBa₂Cu₃O₇ lost superconductivity gradually as current increased.
Researchers have used electrical pulses lasting about 2 picoseconds to probe how much current two type-II superconductors can carry before their Cooper pairs break apart. In the thin-film material NbN, the response showed a sharp change at a current density about 2.2 times its conventional critical current density. In YBa₂Cu₃O₇, superconductivity weakened more gradually as current rose. The results, reported in Nature Physics, show how ultrafast measurements can reveal differences associated with superconductors’ gap symmetry.
Why a brief pulse can reveal a higher current limit
A conventional critical current density is the point at which a superconductor’s current-carrying state becomes dissipative. In type-II superconductors, magnetic vortices can move through the material under current, producing resistance and heat. The measured critical current is therefore influenced by vortex motion and by defects that pin vortices in place.
A distinct, intrinsic limit is the depairing current density: the point at which the current disrupts Cooper pairs, the paired electrons that carry superconducting current. The study’s authors used pulses short enough that, in their account, vortices could move only a small distance before the measurement ended. That reduced the influence of vortex motion and heating, allowing them to investigate the material’s response closer to the depairing limit.
The team generated pulses with a full width at half maximum of about 2 ps using laser-triggered photoconductive switches, then measured the electrical signals transmitted through and reflected from the samples. The experiments were performed on thin films of NbN, an s-wave superconductor, and YBa₂Cu₃O₇, a d-wave superconductor.
NbN shows a sharp onset; YBa₂Cu₃O₇ changes gradually
For NbN at 7 K, the conventional critical current density was about 100 GA m⁻². The transmitted pulse response changed sharply at around 2.2 times that value, or about 220 GA m⁻². Above the transition, the response moved towards that of the material in its normal, non-superconducting state. The authors interpret this abrupt change as rapid depairing of Cooper pairs.
The YBa₂Cu₃O₇ measurements showed a different pattern. At 50 K, below its reported critical temperature of 85 K, the transmitted signal declined continuously as current increased. The researchers did not observe the sharp onset seen in NbN. They interpret this gradual response as consistent with the d-wave superconducting gap, which varies with direction and reaches zero at certain points, rather than having the isotropic gap associated with s-wave superconductors.
The authors also compared the NbN results with calculations based on microscopic Bardeen–Cooper–Schrieffer theory. The measured depairing currents were close to the calculated temperature-dependent values. Their analysis found that, as NbN’s temperature approached 0.8 times its critical temperature, its conventional critical current density was nearly an order of magnitude lower than its depairing current density.
What the measurements could make possible
The experiment offers a way to study superconducting properties that ordinary direct-current transport measurements can obscure. Comparing the different responses in NbN and YBa₂Cu₃O₇ also suggests that ultrafast transport could help investigate superconducting gap symmetry in materials that are difficult to probe spectroscopically.
The work points to possible applications in superconducting electronics and in generating ultrashort, strong magnetic-field pulses. Those are prospective uses: the study measured brief responses in laboratory thin-film samples, rather than demonstrating a device operating continuously at the intrinsic current limit.