• Physics 16, 122
A superconducting strip allows more superconducting current to flow in one direction than the other, resulting in a stronger diode effect than previous devices.
A. Varambally, Y. Hou and H. Chi/MIT
Semiconductor diodes conduct current in one direction but not the other, giving them a myriad of applications in electronics. Their unidirectional property is made possible by a difference in the conduction behavior of the two types of charge carriers: electrons and holes. Superconducting diodes could also be useful in sensors and other devices. But since supercurrents have only one type of carrier, electrons in so-called Cooper pairs, making a superconducting diode is more difficult. In 2020 researchers demonstrated a diode effect in a superconducting device made of a layered material that required precise stacking, strong spin-orbit coupling, and a unique form of Cooper coupling [1]. Now Jagadeesh Moodera of the Massachusetts Institute of Technology and collaborators have made a superconducting diode that is more effective, simpler in design, and independent of esoteric electronic effects. [2].
The team’s diode design consists of a thin strip of 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 supercurrent vortices that all rotate in the same direction. Moodera and colleagues applied such a field perpendicular to the surface of their device, inducing eddies within the strip, as well as supercurrents (called Meisner currents) along the edges of the strip. Seen from above, one edge current flowed to the right (in the “forward” direction), the other to the left (in the “reverse” direction). The researchers then sent an external current through the ends of the strip, both forward and in the opposite direction, and measured the net current for each case.
In principle, the counter-propagating edge currents are equal, so their contributions to the net current should cancel out. But in practice, manufacturing one strip inevitably leads to structural differences between the two edges. That accidental asymmetry, the MIT team found, was large enough to result in a diode efficiency of 20%, defined as the difference between the net forward and reverse currents, divided by the sum. The researchers found that they could increase the efficiency of the diode to 50% by deliberately adding notches to one of the edges. But they achieved an efficiency of 65%, the highest figure seen so far, by replacing the applied magnetic field with the intrinsic field of a top layer of a ferromagnetic insulator, europium sulfide.
Indeed, Moodera and his colleagues showed that in ordinary superconductors there is a giant diode effect that results from the breaking of a simple geometric symmetry. Such superconducting diodes could find immediate use in superconducting electronics and future use in superconducting circuits or topological qubits, Moodera says.
Philip Moll studies quantum materials at the Max Planck Institute for the Structure and Dynamics of Matter in Germany. He points out that the observation of a large diode effect in single element superconductors is significant because their simplicity will make applications easier and more scalable. “The beauty of Moodera and colleagues’ work is that they achieved record efficiencies without even trying,” he says. “Their facilities are far from optimized.”
Charles Day
Charles Day is Senior Editor for Physics magazine.
References
- F. Ando et al.Observation of the effect of the superconducting diode, Nature 584373 (2020).
- Y. Hou et al.Ubiquitous superconducting diode effect in superconducting thin films, Phys. Rev. Lett. 131027001 (2023).
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