MIT Researchers Develop Air-Stable Monolayer Superconductors for Quantum Devices

Quantum computing and communication devices require materials that conduct electricity without resistance, but the physical constraints of manufacturing them have long held back their potential. Two-dimensional superconducting materials are only a few atoms thick, a scale that promises to shrink bulky circuitry. Yet these thin films typically degrade almost immediately upon exposure to air, making it nearly impossible to fabricate them reliably across a large wafer.

A team of researchers from MIT and other institutions has cleared that roadblock. By growing an ultrathin superconductor underneath a layer of protective carbon-based graphene, the team manufactured smooth, air-stable material spanning more than an inch in size.

Overcoming Oxidation in Niobium Diselenide

The material at the center of the advance is niobium diselenide, an ultrathin superconductor made of a closely packed layer of niobium atoms sandwiched between single layers of selenium. Members of the research team previously noted that the material possesses high kinetic inductance, allowing it to store a great deal of inductive energy within a tiny footprint. In conventional quantum circuits, engineers must string together arrays of Josephson junctions to achieve large kinetic inductance. Thin films of niobium diselenide could replace those sprawling electronic junction arrays with a compact piece of material.

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However, fabrication has remained difficult. Scientists traditionally grow the material by depositing chemical precursors onto a silicon dioxide substrate, then placing a protective 2D layer on top afterward. That postgrowth protection method leaves a critical vulnerability. The process also demands a stringent inert environment and delicate handling.

“Typically, once we make the material and remove it from its inert environment, it immediately starts to oxidize and degrade, ultimately becoming damaged.”

Xudong Sheldon Zheng, graduate student in the MIT Department of Electrical Engineering and Computer Science

The Interfacial Growth Technique Beneath Graphene

To bypass postgrowth oxidation, the MIT researchers inverted the fabrication order. They placed the graphene layer directly on the silicon dioxide substrate first, then deposited the precursors so that the superconducting crystal formed in the microscopic gap between the two layers.

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The silicon dioxide substrate traps the chemical precursors long enough for crystal formation to begin, while the overlying graphene lets the precursors move freely and spread into a continuous monolayer. Adjusting the growth parameters ensures the material forms precisely as designed between the layers.

“It took a long time for us to understand how the growth could happen underneath the graphene. Through collaboration and discussion, we eventually uncovered the mechanism for growing the material at the interface, and this solves a lot of problems and allows us to simplify our fabrication steps.”

Xudong Sheldon Zheng, graduate student in the MIT Department of Electrical Engineering and Computer Science

Integration into Superconducting Microwave Circuits

With the wafer-scale fabrication hurdle cleared, the team integrated the air-stable superconductor into a superconducting microwave circuit to test its real-world viability. Laboratory tests showed that the material retained its superconducting capabilities while preserving high kinetic inductance.

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That combination of stability and high kinetic inductance points toward more compact superconducting quantum computing architectures. It also supports ultrasensitive quantum detectors utilized in communications and cosmology. Oliver of MIT, Joel Î-j.

“Emerging superconductors that are only a monolayer thick have a lot of potential. Thanks to our new process, they are no longer materials that can only be made at a very small scale. There are now exciting opportunities for scientists to study these materials, utilize them in circuits, and explore their practical applications.”

Xudong Sheldon Zheng, graduate student in the MIT Department of Electrical Engineering and Computer Science