Researchers make air-stable, ultrathin superconductors, for more scalable quantum devices

Super-thin superconducting materials, which are only one or a few atoms thick, have unique properties scientists can leverage to produce more compact, scalable, and efficient quantum devices. But these fragile materials degrade so rapidly in air that they are difficult to study or manufacture.

Now, researchers from MIT and elsewhere have discovered and harnessed a method to generate a large, uniform area of ultrathin superconducting material that remains stable in air.

They “grow” the superconducting material, called niobium diselenide, underneath another atomically thin material, carbon-based graphene. The graphene layer protects the fragile superconductor from oxidation, while guiding it to grow in a smooth layer over a large wafer-scale area.

The researchers further integrated this air-stable superconductor into a superconducting microwave circuit. When tested, the material maintained its superconducting properties and exhibited high kinetic inductance, which is a resource for many quantum devices.

In the long run, this advance could help miniaturize superconducting quantum computing hardware, as well as technologies like ultrasensitive quantum detectors for communications or cosmology.

“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,” says co-lead author Xudong Sheldon Zheng, a graduate student in the MIT Department of Electrical Engineering and Computer Science (EECS).

He is joined on the paper by co-lead authors Sameia Zaman SM ’24, an EECS graduate student, and Kenan Zhang, a recent postdoc in the MIT Research Laboratory of Electronics (RLE); corresponding authors William D. Oliver, the Henry Ellis Warren (1894) Professor of EECS and professor of physics, director of the Center for Quantum Engineering, and associate director of RLE; Joel Î-j. Wang, an assistant professor at New York University; and Jing Kong, the Jerry Mcafee (1940) Professor in Engineering at MIT and a member of RLE; as well as others at MIT and Lincoln Laboratory, Rice University, Yale University, and Pohang University in South Korea. The research appears today in Nature.

Powerful properties

Superconductors are materials that can conduct electricity without resistance, and they are essential for some types of quantum devices.

Two-dimensional superconducting materials retain their superconducting properties despite being only a few atoms thick. These materials hold the promise to miniaturize superconducting circuitry.

Niobium diselenide, an ultrathin superconductor composed of a single, closely packed layer of niobium atoms sandwiched between a single layer of selenium atoms on either side, has a very high kinetic inductance, as members of the research team recently reported.

This enables the material to store a great deal of inductive energy in a very small area. Large kinetic inductance in a small form-factor is a desirable design element in many quantum devices.

One commonly used approach to realizing a large kinetic inductance is to string together an array of devices called Josephson junctions.

If scientists could incorporate materials such as thin niobium diselenide with sufficiently large kinetic inductance into a quantum circuit, they could replace the large area of electronic junctions with a tiny piece of thin-film material, making the circuit more compact. But because niobium diselenide degrades rapidly in air, scientists have not been able to reliably fabricate devices at the wafer scale. Instead, they rely on exfoliation techniques that yield small flakes. Furthermore, researchers have struggled to grow material with uniform monolayer thickness. Consequently, it has been challenging to fully probe its properties or test it in practical applications.

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

Scientists usually grow niobium diselenide by depositing chemical precursors onto a silicon dioxide substrate. Then they place another layer of two-dimensional material, like graphene or hexagonal boron nitride, on top to protect the fragile superconductor from air.

But such postgrowth protection presents a challenge. The superconductor begins to oxidize almost immediately after synthesis, degrading its properties before it is protected. Meanwhile, the protection process requires a stringent inert environment and delicate processing.

Mind the gap

The MIT researchers used a different tactic. They put the layer of graphene on top of the silicon dioxide substrate first. Then they deposited the precursors and grew the superconducting material in the tiny gap between the two 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,” Zheng says.

The silicon dioxide substrate helps trap the precursors long enough for the crystal to begin forming, while the graphene layer allows them to move around easily and spread into a continuous monolayer.

The researchers used this technique to generate a perfectly smooth layer of niobium diselenide more than an inch in size.

“By carefully tuning the growth conditions, we can ensure the material grows between the layers in exactly the way we’ve designed,” Zheng says.

Even though the graphene is placed on top of the silicon dioxide, the weak adhesion between these materials leaves a gap between them less than 1 nanometer thick. The niobium diselenide grows only within that gap. Then, since it is already encapsulated by graphene, the researchers can safely remove it into the ambient environment without causing degradation.

Careful connections

The researchers also designed an oxidation-free transfer technique to peel the graphene-niobium diselenide structure from its growth substrate, building on prior work by members of the team.

Then, they developed a method to integrate the thin film into a quantum circuit without hampering the fragile superconductor or its properties.

“It is challenging to make a good electrical connection between this very thin material, which is only about 1 nanometer in thickness, and our electrodes, which are a few hundred nanometers in thickness,” Zaman says.

They carefully etch the side walls of the thin-film superconductor in a vacuum chamber, which preserves the smooth edge of the material. When they integrate the prepared niobium-graphene structure into a conventional superconducting circuit, it forms a reliable electrical connection. Importantly, the material maintained its superconducting properties and exhibited high kinetic inductance after clean room fabrication and integration into the circuit. This makes it particularly attractive for fabricating compact superconducting quantum devices and other quantum technologies.

Furthermore, the growth strategy is not limited to monolayer niobium diselenide. The researchers demonstrated that it can be extended to a broad family of monolayer quantum materials with diverse and technologically important properties.

In the future, the researchers aim to integrate these ultrathin superconducting materials into functional device architectures to enable the exploration of fundamental physics and the prototyping of quantum devices and other advanced technologies.

“We’ve taken a very good step toward exploring both the physics and the application side of this thin, monolayer superconductor, which we can now grow in wafer scale or in even larger areas. There are a lot of directions we can go in the future,” Zaman says.

This research was funded, in part, by the U.S. Army Research Office, the U.S. National Science Foundation, the Schlumberger Foundation, the U.S. Department of Energy, the U.S. Air Force Office of Scientific Research, the Semiconductor Research Corporation Center, the MIT Institute for Soldier Nanotechnologies, and the National Research Foundation of Korea. This work was carried out, in part, using MIT.nano facilities.