Single-Cooper-Pair Transistor in an Electrostatically Controllable Superconducting KTaO₃ Constriction

Several complex oxides are known to form two-dimensional electron gases (2DEGs) at their interfaces. Some of these 2DEGs become superconducting at relatively low electron densities. While the nature of the superconductivity in these films is an interesting question in itself, the ability to lithographically pattern nanoscale devices offers the opportunity to study properties beyond those found in the 2DEG. Here, we present micron-scale constrictions etched in a  KTaO₃-based 2DEG. The coplanar side gates are created by the same etching step, thereby forming superconducting FETs. With such a device, we are able to efficiently tune the electron density in the constrictions to observe a gate-modulated periodic supercurrent characteristic of single-Cooper-pair transistors. Through the local gating, we directly influence the Josephson energy, selectively passing between regimes dominated by either the Josephson effect or charging effects.