Plate Nº 94 · recorded October 10, 2026
PhysicsReported finding
Picosecond Pulses Push Superconductors Past Their Critical-Current Limit
Researchers at the Max Planck Institute have pushed type-II superconductors past their critical-current limit by applying current for only a few picoseconds, reaching the higher depairing threshold.
By James Calloway3 min read627 words
In brief
- Researchers at MPSD pushed type-II superconductors past their critical-current limit using pulses lasting only a few picoseconds, reaching the higher depairing current.
- Vortices drift at tens of kilometers per second, covering only tens of nanometers during a single picosecond—short enough for the current pulse to outrun them.
- Photoconductive switches driven by 300-femtosecond green laser pulses at 515 nanometers generated the few-picosecond electrical transients.
- NbN (s-wave) collapsed at a single sharp threshold; YBCO (d-wave) weakened gradually, exposing differences tied to each material's gap symmetry.
- The study was published in Nature Physics in 2026 (DOI: 10.1038/s41567-026-03469-z).
Using electrical pulses lasting only a few picoseconds, researchers at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) have pushed type-II superconductors past their long-accepted critical-current limit and into a higher regime known as the depairing current. The work appeared in Nature Physics.
What is the critical current—and why does it cap performance?
Superconductors carry current with zero resistance, but only up to a ceiling. Below a characteristic transition temperature, electrons in these materials pair up to form Cooper pairs that travel collectively through the crystal like a wave. Raise the current high enough, though, and that state collapses. The maximum current a superconductor can handle before dissipation sets in is called the critical current, a key figure of merit for applications such as magnets, quantum circuits, and ultrasensitive detectors.
In type-II superconductors, the experimentally observed ceiling is usually set not by the Cooper pairs themselves, but by the movement of vortices—tiny channels of magnetic flux that thread the material. Once vortices start drifting under a strong current, they generate heat and resistance that quench superconductivity well before any intrinsic limit is reached.
What is the depairing current?
Beneath the vortex-limited ceiling sits a second, higher threshold: the depairing current. "One way to picture it is that the current 'twists' the phase of the coherent quantum state of the superconductor, rather like winding a spring," said lead author Eryin Wang. Wind the spring too tightly and the Cooper pairs break apart.
That intrinsic threshold almost never shows up in standard measurements because vortices arrive first.
How do picosecond pulses outrun vortex motion?
"Our strategy was to outrun the vortex dynamics," Wang said. Vortices drift at tens of kilometers per second, fast but slow enough to cover only tens of nanometers during a single picosecond. A current pulse that brief can climb to extreme density before vortices have time to move and dissipate energy.
To generate these transients, the team used an ultrafast electrical-transport platform developed at MPSD. Photoconductive switches triggered by 300-femtosecond green laser pulses at a wavelength of 515 nanometers produce electrical pulses lasting only a few picoseconds. Those pulses travel along a coplanar waveguide and through micrometer-scale superconducting samples.
Co-author Guido Meier noted: "To apply current to superconductors for only a few picoseconds, we used the ultrafast electrical-transport platform that we have been developing at our institute."
What did the two test materials reveal?
The team compared two superconductors with fundamentally different electronic structures:
- NbN, an s-wave superconductor with a nearly uniform energy gap
- YBCO, a d-wave high-temperature superconductor whose energy gap vanishes along certain crystal directions
In NbN, the superconducting state held firm until current hit a sharp threshold well above the conventional DC critical current, then collapsed abruptly—a signature of Cooper pairs breaking apart in unison.
YBCO behaved differently. Its superconducting state weakened gradually as current rose, with no single sharp breakdown. The researchers link this to YBCO's direction-dependent gap: with weaker protection along some crystal axes, superconductivity erodes progressively instead of failing at one point.
What could this approach enable?
The findings suggest picosecond transport can probe microscopic properties—including gap symmetry—that conventional DC measurements cannot directly access. Group leader Andrea Cavalleri said: "Our results suggest that picosecond transport can provide access to microscopic properties of superconductors, including their gap symmetry, that are not directly available from conventional DC transport."
The authors caution that measurements across a broader range of superconductors will be needed to confirm how generally the link between ultrafast response and gap symmetry holds. They also note that the work opens a new regime of ultrahigh-current superconductivity that could influence optoelectronics and, potentially, magnetic devices.
via Phys.org Physics (Source)
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