Superconductivity is one of the most striking collective phenomena in quantum materials. When certain materials are cooled below a characteristic transition temperature, their electrical resistance vanishes and electric current can flow without dissipating energy as heat. This unusual behavior arises because electrons form correlated pairs, known as Cooper pairs, which move collectively through the material, like a wave. Superconductors are therefore attractive for technologies ranging from powerful magnets and sensitive detectors to quantum circuits. However, this dissipationless state has its limits: if the current becomes too large, superconductivity breaks down. The critical current is a key property of any superconductor, defining the maximum current it can carry before electrical dissipation appears.In type-II superconductors, however, this experimentally observed limit is often not determined directly by the microscopic properties of the superconducting state. Instead, it is typically governed by the motion of vortices—tiny regions through which magnetic flux penetrates the material. At sufficiently high current, the vortices start to move, generating resistance and heat that can push the material out of its superconducting state.
Yet a superconductor has a higher, intrinsic current limit: 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 the superconducting condensate is twisted too far, the superconducting state becomes unstable. At this threshold, Cooper pairs—the paired electrons responsible for superconductivity—begin to break apart. Because vortex motion and heating normally occur before the intrinsic phase is pushed too far, this intrinsic limit is difficult to reach using conventional direct-current (DC) transport measurements.
"Our strategy was to outrun the vortex dynamics," says Eryin Wang. Vortices typically move at velocities of tens of kilometres per second, corresponding to only tens of nanometres during a picosecond. A sufficiently short current pulse can therefore reach very high densities before vortices have time to move and heat the sample. This strongly reduces vortex-induced dissipation and allows the superconducting condensate itself to be pushed much closer to its intrinsic limit.
To generate such ultrashort currents, the researchers 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. In this platform, photoconductive switches are activated by 300-femtosecond green laser pulses with a wavelength of 515 nanometres. The switches generate electrical pulses lasting only a few picoseconds, which are guided along a coplanar waveguide and through micrometre-scale superconducting samples.
The researchers compared NbN and YBCO because their superconducting states are fundamentally different: NbN has a more uniform energy gap (s-wave), while YBCO has a strongly direction-dependent gap (d-wave). Comparing the two helps reveal how the microscopic structure of superconductivity affects its breakdown under ultrafast currents.
In NbN, the superconducting state remained robust until the current reached a well-defined threshold, much higher than the usual DC critical current. Beyond this point, the response changed abruptly, indicating that the Cooper pairs responsible for superconductivity were breaking apart.
YBCO responded very differently: its superconducting state weakened gradually as the current increased. The researchers link this contrast to the different internal structures of the two superconductors. In NbN, the superconducting energy gap is nearly the same in all directions, whereas in YBCO it varies strongly and vanishes along certain directions. As a result, superconductivity in YBCO can be weakened progressively rather than collapsing at a single sharp threshold.
"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. Measurements on a broader range of superconductors will be needed to determine how generally this connection applies. More broadly, this work demonstrates that ultrashort electrical pulses can access regimes of superconducting transport that are normally hidden by slower processes such as vortex motion and heating, opening new possibilities for probing and controlling superconductors on their intrinsic timescales. This new regime of ultra-high current superconductivity could also impact both optoelectronics and potentially magnetic devices.
