Photo-induced superconductivity on a chip

Their work, now published in Nature Communications, also shows that the electrical response of photo-excited K₃C₆₀ is not linear, that is, the resistance of the sample depends on the applied current. This is a key feature of superconductivity, validates some of the previous observations and provides new information and perspectives on the physics of K₃C₆₀ thin films.

The optical manipulation of materials to produce superconductivity at high temperatures is a key research focus of the MPSD. So far, this strategy has proven successful in several quantum materials, including cuprates, k-(ET)₂-X and K₃C₆₀. Enhanced electrical coherence and vanishing resistance have been observed in previous studies on the optically driven states in these materials.

In this study, researchers from the Cavalleri group deployed on-chip non-linear THz spectroscopy to open up the realm of picosecond transport measurements (a picosecond is a trillionth of a second). They connected thin films of K₃C₆₀ to photo-conductive switches with co-planar waveguides. Using a visible laser pulse to trigger the switch, they sent a strong electrical current pulse lasting just one picosecond through the material. After travelling through the solid at around half the speed of light, the current pulse reached another switch which served as a detector to reveal important information, such as the characteristic electrical signatures of superconductivity.

By simultaneously exposing the K₃C₆₀ films to mid-infrared light, the researchers were able to observe non-linear current changes in the optically excited material. This so-called critical current behavior and the Meissner effect are the two key features of superconductors. However, neither has been measured so far -- making this demonstration of critical current behavior in the excited solid particularly significant. Moreover, the team discovered that the optically driven state of K₃C₆₀ resembled that of a so-called granular superconductor, consisting of weakly connected superconducting islands.

The MPSD is uniquely placed to carry out such measurements on the picosecond scale, with the on-chip set-up having been designed and built in-house. "We developed a technique platform which is perfect for probing non-linear transport phenomena away from equilibrium, like the non-linear and anomalous Hall effects, the Andreev reflection and others," says lead author Eryin Wang, a staff scientist in the Cavalleri group. In addition, the integration of non-equilibrium superconductivity into opto-electronic platforms may lead to new devices based on this effect.

Andrea Cavalleri, who has founded and is currently leading the research group, adds: "This work underscores the scientific and technological developments within the MPSD in Hamburg, where new experimental methods are constantly being developed to achieve new scientific understanding. We have been working on ultrafast electrical transport methods for nearly a decade and are now in a position to study so many new phenomena in non-equilibrium materials, and potentially to introduce lasting changes in technology."

The research underpinning these results was carried out in the laboratories of the MPSD at the Center for Free-Electron Laser Science (CFEL) in Hamburg, Germany.