In superconducting ceramics, the superconducting transition temperature, Tc, is determined by the density of electrons or ‘holes’. Chemical doping—for example by adding oxygen into the material—is the most common method for changing the charge density. The problem with this strategy is that it introduces structural defects and is often not reversible. Thus studying a material’s evolution from an insulator to a superconductor necessitates the tedious task of fabricating vast numbers of samples having different chemical compositions.

Now, Dr. Masashi Kawasaki of Tohoku University is exploring ways of inducing superconductivity in ceramics simply by applying an electric field. In collaboration with colleagues at Tohoku University and CREST in Tokyo1, this group report a device made from a crystal of insulating SrTiO3 in which they induced superconductivity with a moderate electric field.

Attempts like this are not completely new. The standard way to electrically dope a superconductor is to put it in what is called a field-effect transistor geometry: the film sits on top of another insulator that in turn sits on top of a metal electrode. When a voltage is applied across the middle insulator, it produces an electric field that draws charges into the film.

Fig. 1: A schematic of the electrochemical cell that Kawasaki and colleagues use to create a large electric field near SrTiO3. The surface of a crystal of SrTiO3 forms one electrode, while the platinum metal foil forms the other. When a voltage is applied between the two electrodes, charge accumulates near either electrode. The charge produces an electric field near the surface of the SrTiO3 that pulls additional charge carriers into a thin region (indicated by the orange lines) and causes this region to become superconducting.

This method works well for metals, but not for materials like undoped SrTiO3, which are insulators. This team decided to borrow an idea from electrochemistry, where the surface of the SrTiO3 crystal functioned as one electrode of an electrochemical cell, while a platinum foil served as the other. Both electrodes were immersed in an electrolyte. When a voltage was applied between the two electrodes, positive ions moved toward one electrode and negative ions toward the other (Fig. 1). When sufficient positive charges accumulated near the SrTiO3 to produce a sizable electric field, the resistance of the SrTiO3 dropped, indicating that SrTiO3 was becoming a metal. Then, as they cooled the SrTiO3 to 0.4 K under these conditions, the resistance effectively dropped to zero—the onset of superconductivity.

Kawasaki notes that, “The new route for electrical doping offers tremendous freedom for the choice of host materials that might be turned into superconductors.”