Scientists have known since the 1960s that a superconductor — a material that has zero electrical resistance — can be created by increasing the density of electrons in solids to high levels. Chemical doping is one route to achieve this, but the process is irreversible. A beautifully simple technique for turning superconductivity on and off by applying a voltage has now been presented by scientists at Tohoku University in Japan.1

Solid dielectrics — electrically insulating materials that become polarized under an electric field — have a tendency to break down in strong electric fields, which makes it difficult to create the high electron densities required for superconductivity. Jianting Ye and his co-workers have now taken an electrochemical approach to overcome this problem. They immersed the potential superconductor in a liquid that behaves in a similar way to the gate dielectrics in transistors. When a voltage was applied, ions moved to the solid–liquid interface and created a thin double layer of charge with a density high enough to induce a phase change.

The Tohoku University research group previously demonstrated electric-field-induced superconductivity in the undoped insulator strontium titanate using a polymer electrolyte as the gate medium. Strontium titanate was a good starting point because it becomes a superconductor at the lowest electron density for any known material. The scientists have now shown that the same approach can be applied to materials with more typical superconductor properties if an ionic liquid is used as the gate dielectric. “It is a promising indication that the method can be used in the search for new superconductors,” says Ye.

Fig. 1: The high charge density required to induce superconductivity in a thin flake of ZrNCl is provided by applying a voltage across an ionic liquid so that ions form a thin layer on the surface.© J. T. Ye

The scientists obtained thin and perfectly flat layers of the nitride compound ZrNCl by peeling away flakes using sticky tape. They then mounted the layers on a substrate and immersed the assembly in an ionic liquid (Fig. 1). To induce the transition to the superconducting state in the ZrNCl film, a voltage of 4.5 V was applied to the system at 220 K, and then the temperature was lowered until the resistance abruptly dropped to zero, which in this case occurred at 15.2 K. For comparison, the strontium titanate device needed to be cooled to 0.4 K to induce the superconducting transition.

The present device is still far from being usable in practical applications, as the switching action only works at above 220 K, whereas the sample must be cooled to very low temperatures to obtain the superconducting state. Ye explains, “Real applications require a superconductor that works at higher temperatures and an ionic liquid in which the ions aren’t frozen at that temperature. Such materials are available; integrating them is the next step.”