Science 347, 743–746 (2015)

Ionic liquids used as gate dielectrics in transistors enable large carrier densities in the transport layer. Large charge accumulation can cause electronic phase transitions. Although ionic liquid gating allows superconductivity to be induced in different materials, the ionic motion is frozen below a certain temperature, making the ionic liquid insensitive to variations of the gate voltage and preventing the superconducting phase from being switched on and off at a fixed temperature. Masayuki Suda and colleagues now report an alternative approach to modulate the superconductivity in an organic crystal. As a dielectric they use a self-assembled monolayer of molecules that undergo photoisomerization from a non-ionic to an ionic state when ultraviolet light is applied, and from the ionic to the non-ionic state when visible light is used. In addition to demonstrating reversible switching from an insulating to a superconducting phase using light, the technique could also broaden the range of carrier doping densities achievable in transistors.