On-chip superconductivity achieved with light manipulation

Researchers at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) have shown that a previously demonstrated ability to turn on superconductivity with a laser beam can be integrated on a chip, opening up a route toward opto-electronic applications. The research findings, published in Nature Communications, show that the electrical response of a 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 previous observations and provides new information on the physics of K₃C₆₀ thin films.

The optical manipulation of materials to produce superconductivity at high temperatures is a key research focus on the MPSD. So far, this strategy has proven successful in several quantum materials, including cup rates, 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, the researchers deployed on-chip non-linear THz spectroscopy to better understand picosecond transport measurements (a picosecond is a trillionth of a second). The researchers connected thin films of K₃C₆₀ to photo-conductive switches with co-planar waveguides and, using a visible laser pulse to trigger the switch, they sent a strong electrical current pulse lasting 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 information such as the characteristic electrical signatures of superconductivity.

By exposing the K₃C₆₀ films to mid-infrared light, the researchers were able to observe non-linear current changes in the optically excited material. This critical current behaviour and the Meissner effect are the two key features of superconductors; however, neither has been measured so far, making the demonstration of critical current behaviour in the excited solid quite significant. The researchers also found that the optically driven state of K₃C₆₀ resembles that of a granular superconductor, consisting of weakly connected superconducting islands.

Lead author Eryin Wang said the researchers developed a technique platform that was perfect for probing non-linear transport phenomena away from equilibrium, like the non-linear and anomalous Hall effects, or the Andreev reflection. The integration of non-equilibrium superconductivity into optoelectronic platforms may lead to new devices based on this effect. Andrea Cavalleri, who is leading the research group, said the research underscores the scientific and technological developments within the MPSD.

“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,” Cavalleri said.

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