Scientists just pushed superconductors beyond their usual current limit

This happens because electrons form correlated pairs called Cooper pairs. Instead of moving independently, these pairs act collectively, somewhat like a wave traveling through the material. This unusual property makes superconductors promising for applications including powerful magnets, highly sensitive detectors, and quantum circuits.

But superconductivity cannot withstand unlimited electrical current. Once the current becomes too large, the superconducting state begins to fail. A material's critical current describes the maximum current it can carry before resistance and energy loss appear.

Why Superconductors Usually Break Down Early

In type-II superconductors, the critical current measured in experiments often does not reflect the true microscopic limit of superconductivity itself. Instead, failure is usually triggered by the movement of vortices, tiny regions where magnetic flux can pass through the material.

As the current rises, these vortices can begin to move. Their motion creates electrical resistance and generates heat, which can ultimately destroy the superconducting state.

Superconductors actually possess a higher fundamental limit known as 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," explains Eryin Wang, lead author of the study.

If this quantum state is twisted too far, it becomes unstable. At that point, the Cooper pairs responsible for superconductivity begin to break apart. Conventional direct-current (DC) transport measurements rarely reach this intrinsic limit because moving vortices and heating usually disrupt superconductivity first.

Outrunning the Vortices

The researchers developed a way to get around that problem by delivering extremely short bursts of electrical current.

"Our strategy was to outrun the vortex dynamics," says Eryin Wang.

Vortices can travel at speeds of tens of kilometers per second, but over the span of a picosecond they move only tens of nanometers. By applying current for an extremely short period, researchers can drive the current density to very high levels before the vortices have enough time to move significantly or heat the material.

This sharply reduces the energy loss caused by vortex motion and allows researchers to push the superconducting state much closer to its fundamental current limit.

Electrical Pulses Lasting Just Picoseconds

To produce these exceptionally short currents, the team used an ultrafast electrical transport platform developed at MPSD.

"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," says Guido Meier, co-author of the study.

The system uses photoconductive switches triggered by 300-femtosecond green laser pulses with a wavelength of 515 nanometers. Once activated, the switches produce electrical pulses lasting only a few picoseconds. These pulses travel along a coplanar waveguide and then pass through superconducting samples only micrometers in size.

Two Superconductors Respond Very Differently

The researchers tested two materials, NbN and YBCO, because they represent fundamentally different forms of superconductivity. NbN has a relatively uniform superconducting energy gap (s-wave), while YBCO has an energy gap that changes strongly depending on direction (d-wave).

Studying both materials allowed the researchers to investigate how the microscopic structure of the superconducting state influences what happens when extremely strong, ultrafast currents are applied.

NbN remained strongly superconducting until the current reached a distinct threshold that was far higher than its conventional DC critical current. Once the current passed this point, the material's response changed suddenly, a sign that its Cooper pairs were beginning to break apart.

YBCO behaved in a very different way. Instead of remaining stable and then failing abruptly, its superconducting state weakened progressively as the current increased.

The researchers attribute this difference to the distinct internal structures of the two materials. In NbN, the superconducting energy gap is almost equal in every direction. In YBCO, however, the gap varies considerably with direction and disappears entirely along certain directions. That structure allows superconductivity in YBCO to weaken gradually rather than collapsing at one sharply defined threshold.

Revealing Hidden Superconducting Physics

"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," says Andrea Cavalleri, who leads the research group.

Researchers will need to study a wider range of superconducting materials to determine how broadly this relationship applies.

More generally, the results show that extremely short electrical pulses can expose forms of superconducting transport that are normally concealed by slower effects such as vortex motion and heating. By working on timescales comparable to those of the superconducting state itself, scientists may gain new ways to investigate and control these quantum materials.

This newly accessible regime of ultra-high current superconductivity could also have implications for optoelectronics and potentially for magnetic devices.