Scientists studying a promising quantum material have stumbled upon a surprise: within its crystal structure, the material naturally forms one of the world’s thinnest semiconductor junctions—a building block of most modern electronics. The junction is just 3.3 nanometers thick, about 25,000 times thinner than a sheet of paper.

“This was a big surprise,” said Asst. Prof. Shuolong Yang. “We weren’t trying to make this junction, but the material made one on its own, and it’s one of the thinnest we’ve ever seen.”

The discovery offers a way to build ultra-miniaturized electronic components, and also provides insight into how electrons behave in materials designed for quantum applications.

The paper, titled “Spectroscopic evidence of intra-unit-cell charge redistribution in a charge-neutral magnetic topological insulator,” is published in the journal Nanoscale.

Uneven electrons

Researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) and Pennsylvania State University were studying the electronic properties of MnBi₆Te₁₀, a type of topological substance known for its unusual properties—like letting electricity flow freely along its edges without any resistance. Scientists hope that this class of topological material could someday be used in quantum computers or ultra-efficient electronic devices.

But to work properly, materials like MnBi₆Te₁₀ need to have carefully balanced and distributed electrons. The team thought they had achieved the right balance by tweaking the material’s chemical makeup and infusing MnBi₆Te₁₀ with antimony. Regular electrical tests confirmed the material was, overall, neutral.

Then Yang’s team looked closer, using a technique called time- and angle-resolved photoemission spectroscopy (trARPES) that uses ultrafast laser pulses to observe how electrons are distributed and how their energy levels shift in real time. The scientists saw something unexpected. Within each repeating layer of the crystal—just a few atoms thick—the electrons were not evenly spread. Instead, they were clumping up in some parts and leaving other parts with fewer electrons. This created tiny built-in electric fields within the material.

“In an ideal quantum material, you want a really uniform distribution of charges,” said UChicago PME graduate student Khanh Duy Nguyen, the first author of the new work. “Seeing this uneven distribution suggests that we may not enable quantum applications in the originally planned fashion, but reveals this other really useful phenomenon.”

These tiny regions acted like p-n junctions, a type of semiconductor junction that contains internal electric fields and is used to build diodes—similar to the ones found in everyday electronics like phones and computers. But unlike manufactured p-n junctions, these ones form naturally as part of the crystal itself.

A boon for quantum and electronic applications

Because the new, naturally forming p-n junction is also highly responsive to light, it could be useful for next-generation electronics, including spintronics—a type of technology that stores and manipulates data using an electron’s magnetic spin rather than its charge.

By modeling what was occurring within the crystal structure of MnBi₆Te₁₀, Nguyen, Yang and their colleagues were able to form a hypothesis about how it formed the p-n junctions. Introducing antimony into MnBi₆Te₁₀, they suspect, leads to swapping between manganese atoms and antimony, causing charge differences throughout the material.

While the finding adds complexity to efforts to use the material for certain types of quantum effects, it opens up new applications in electronics. It also paves the way toward further engineering of MnBi₆Te₁₀ so that it does maintain evenly distributed electrons—and can be useful in quantum engineering.

The UChicago PME team is fine-tuning ways of fabricating thin films of the material—rather than large, three-dimensional crystals. This could let them more precisely control the behavior of electrons, either to boost quantum properties, or to boost the yield of tiny p-n junctions.

“This once again demonstrates the value of pursuing basic, fundamental scientific research and being open about where it leads,” said Yang. “We set out with one goal and found a surprise that led us in another, really exciting direction.”