With superconducting diodes, scholars advance work toward ultra-efficient quantum electronic devices

PROVIDENCE, R.I. [Brown University] — Superconductors — materials that conduct electricity with zero loss of energy — have been well-understood since the development of what’s called the BCS theory in the mid-1950s. However, the recent development of superconducting diodes using twisted, multi-layer graphene has made understanding how unconventional superconductors function an important new topic of fundamental research.

Now, an international research group that includes Brown Assistant Professor of Physics Jia Li has reached a critical milestone: Using graphene, a material with unique properties, they’ve demonstrated a prominent superconducting diode effect in a single two-dimensional superconductor. They reported their findings in a study in Nature Physics.

A superconducting diode effect occurs when there is a magnitude of current in which a material behaves like a superconductor in one direction of electricity flow and like a resistor in the opposite. In contrast to a conventional diode, a superconducting diode exhibits zero resistance and thus no energy loss in the forward direction.

The researchers’ new development could form the basis for ultra-efficient lossless quantum electronic devices.

Originally, the concept of a superconducting diode was predicted with an external magnetic field, which has some fundamental limitations. In the new experiments carried out at Brown, Li’s team created an extremely strong diode effect without a magnetic field. When turning on an electric current in one direction, the system almost immediately becomes a resistor, while it remains a superconductor in the opposite direction.

The system creates a unique situation with a diode effect with no external magnetic field in a single superconductor. The results confirm a hypothesis theorized by study co-author Mathias Scheurer, a theoretical physicist at the University of Innsbruck in Australia — namely, that superconductivity and magnetism can coexist in a system consisting of three graphene layers twisted against each other. The system can generate its own internal magnetic field, creating a diode effect.

Moreover, the team managed to reverse the diode direction using a simple electrical field.

“The demonstration of the field-free diode effect in a homogeneous superconductor creates a wonderful opportunity to explore possible device application,” Li said. “And the extra experimental control we demonstrated adds more possibilities for designing a programmable network of dissipation-less diodes.”

A promising material

The diode effect was produced using graphene, a material consisting of a single layer of carbon atoms arranged in a honeycomb pattern. Stacking several layers of graphene leads to completely new properties, including the ability of three graphene layers twisted against each other to conduct an electric current without loss. This was demonstrated in previous experiments carried out by Li and his collaborators.

The fact that a superconducting diode effect can exist without an external magnetic field in this system has significant implications for the study of the complex physical behavior of twisted tri-layer graphene. It demonstrates the coexistence of superconductivity and magnetism.

“Superconductivity and ferromagnetism usually occupy opposite ends of the material spectrum,” said Jiang-Xiazi Lin, a postdoctoral researcher at Brown and one of the lead authors of the study. “Coexistence between these two quantum phases is rare, and it is almost synonymous with exciting physics.”

Phum Siriviboon, who conducted research in Li’s lab as an undergraduate and earned a bachelor’s degree from Brown in May 2022, was the study’s other lead author: “This is the first time that the coexistence between superconductivity and ferromagnetism has been observed in two-dimensional materials,” Siriviboon said. “Our result establishes a new method for studying the interplay between superconductivity and ferromagnetism.”

The team’s latest research demonstrates that the observed diode effect not only has technological relevance but also the potential to improve understanding of fundamental processes in many-body physics. Li and Scheurer, along with Harley D. Scammell of the University of New South Wales, published the theoretical basis of this phenomenon in a 2022 article. That theoretical work points to the next step in the team’s work, which is to examine the dependence of the superconducting diode effect when electric currents flow in different directions.

“Phum and Jiang-Xiazi formed a wonderful team working together,” Li said. “This type of collaboration provides an ideal model for involving undergraduate students in cutting-edge research and enabling them to make meaningful and critical contributions.”

This story was authored by Pete Bilderback and based on content provided by the University of Innsbruck.