A superconducting diode free of esoteric electronic effects

A diode is an electronic device with two electrodes. In the obsolescent thermionic diode a heated cathode emits electrons, which flow across the intervening vacuum to the anode when a positive potential is applied to it. The device permits flow of current in one direction only as a negative potential applied to the anode repels the electrons. This property of diodes was made use of in the first thermoionic radios, in which the diode was used to demodulate the transmitted signal.

In the semiconductor diode, a p-n junction performs a similar function. The forward current increases with increasing potential difference, whereas the reverse current is negligible, giving them myriad applications in electronics. Their one-way property is made possible by a difference in the conducting behaviour of the two types of charge carriers—electrons and holes.

Similar to a conventional semiconductor diode, a superconductor with nonreciprocal current flow, a superconducting diode, may form building blocks for, e.g., dissipationless superconducting digital logic. The recent observation of such a superconducting diode effect in a complex thin film superconductor heterostructure subjected to an external magnetic field has stimulated vigorous activity towards understanding and replicating it. But because supercurrents have just one type of carrier—electrons in so-called Cooper pairs—realizing a superconducting diode is more difficult.

Several theoretical mechanisms have been proposed to explain the superconducting diode effects in superconductors and in Josephson junctions, with special emphasis on the potential role of the finite-momentum Cooper pairing. While this mechanism focuses on the intrinsic depairing current, it is known that nearly all the superconductor films fail to be governed by the critical pair breaking mechanism, which merely offers the theoretical maximum for a specially designed sample. Now, a team of researchers has made a superconducting diode that is more effective, simpler in design, and independent of rare Cooper-pairing effects. ¹

The team’s diode design ² consists of a thin strip of either niobium or vanadium. Unlike most single-element superconductors, niobium and vanadium are both type II superconductors—which means that an applied magnetic field of the right strength induces the formation of vortices of supercurrent that all rotate in the same sense.

The so-called Abrikosov vortices are normally pinned to defect centres or surfaces of a superconductor. The current flow, however, produces a Lorentz force on the vortices. A critical current is often measured when the pinning centres or the surface barrier cannot hold vortices any more and dissipation starts in the superconductor. This principle has been exploited to engineer superconducting vortices-based rectifiers. Theory predicted the existence and the possibility of engineering of a superconducting diode effect employing controlled edge disorder. This escaped experimental realization thus far and the present work accomplishes it.

The researchers show a robust control of the nonreciprocity with record high efficiency in conventional superconductors without requiring additional spin-orbit coupling and/or exchange fields. In other words, they have demonstrated that a giant diode effect is present in ordinary superconductors that results from the breaking of simple, geometric symmetry. Such superconducting diodes could find immediate use in superconducting electronics and future use in superconducting or topological qubit circuits.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.