Controlling superconductivity by light

In the growing field of nanotechnology, there is an interest in developing hybrid organic–inorganic devices that have controllable electrical or magnetic properties.¹ Because of the nanoscale involved, the surface-to-volume ratio in these devices is large; hence, the devices can be controlled by varying their surface properties. Exerting control by using light is particularly attractive because making conventional hard electric contacts may be difficult due to size and material properties. The work of Suda et al.² presents a device in which superconductivity is controlled by light through the excitation of a gate made from spiropyran.³ Figure 1 schematically presents the device and its mode of operation (right panel) relative to that of a common field effect transistor (left panel). Spiropyran serves as the gate and is reversibly photoisomerized from a nonionic to a zwitterionic form. In its neutral form, no field is applied to the thin single crystal of κ-(BEDT-TTF)₂Cu[N(CN)₂]Br (κ-Br) (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene). Upon photoexcitation with UV light, zwitterions are formed in the spiropyran film, and as a result, holes are injected into the κ-Br, converting it to a superconductor at low temperatures. Irradiation with visible light returns the film to its neutral state.

This work presents several interesting new concepts. The ability to observe the photo-induced transition to a superconducting state at low temperature is related to the special properties of spiropyran, in which photoisomerization occurs even at low temperatures. The electric double layer that forms as a result of the photoisomerization generates an enormous electric field, which is difficult to achieve using common gate materials. This effect is due to the proximity of the gate layer to the electrically active κ-Br. The field transforms κ-Br from a Mott insulator into a superconductor by doping it with holes. This work demonstrates the importance of controlling interfaces in order to determine the electric and magnetic properties of nanostructures.⁴