Chicago Pritzker team develops new quantum materials

Layer-by-layer materials engineering makes possible exotic quantum phenomena such as interfacial superconductivity and quantum anomalous Hall effect. At the same time, deciphering electronic states layer by layer remains a fundamental scientific challenge due to the difficulty of understanding the layer origins of topological electronic states in magnetic topological insulators, which are key to understanding and controlling topological quantum phases.

Recently, researchers at the University of Chicago Pritzker School of Molecular Engineering have developed a new tool to help explain the origin of electronic states in designed materials, which means a further step toward using materials for future applications in quantum technology. The tool will help researchers better understand magnetic topological insulators, materials with special surface features that could make them an integral part of quantum information science and technology.

The results were published March 23 in Nature Physics ("Layer-by-layer separation of Bloch states by frequency-domain photoelectric emission").

Understanding layered materials is important, and many materials scientists today design and create materials at the atomic level in a layer-by-layer process - combining two or more materials together to create materials with new properties for future technologies.

Through a technique called layer-encoded frequency-domain photoelectron emission, researchers send two laser pulses into a layered material. The resulting vibrations, combined with energy measurements, allowed the researchers to put together a "movie" showing how electrons move through each layer.

"In our daily lives, when we want to understand a material better - to know its composition or whether it's hollow - we knock on it," said paper author Professor Shuolong Yang said. "This is a similar approach at the microscopic level. Our new technique allows us to 'knock and listen' to layered materials, and it allows us to show that a particular magnetic topological insulator works differently from what theory predicts."

When the research team made the bilayer magnetic topological insulator (MnBi2Te4) by combining magnetic and non-magnetic materials (Bi2Te3), they developed a material with exotic quantum properties. Electrons move around the surface perimeter while maintaining their energy and quantum properties. In future quantum computers, this supercurrent may be used to transfer information stored in quantum bits.

Because these layers are so thin - about a few nanometers - conventional material characterization tools, such as spectroscopy, are unable to distinguish between these layers. While ideally, electrons should move around the surface of magnetic materials, previous experiments conducted by other groups suggest that perhaps they move around non-magnetic materials.

The team used a layer-encoded frequency domain ARPES experiment on a magnetic topological insulator (MnBi2Te4) (Bi2Te3) to characterize the layer origin of the electronic states. To understand what happens in the two different layers, the new tool first sends a femtosecond (or trillionth of a second) infrared pulse. This short pulse causes the layers to vibrate differently depending on their composition. Then, the researchers send a second ultraviolet laser pulse that allows them to measure the energy and momentum of electrons in the material. Together, these two measurements can record the movement of the electrons over time.

"It's essentially a femtosecond time-scale movie," says Professor Yang. "It allows us to tell which electrons are coming from which layer."

When the researchers applied the technique to the material (MnBi2Te4) (Bi2Te3), they found that particular electronic states were not in the magnetic layer, which ran counter to theoretical predictions. But because the quantum properties of the material would be significantly improved if this supercurrent were located within the magnetic layer, Yang and his team inspired the entire research community to return to the drawing board to redesign the material.

The technique could also be used to better understand other special materials, such as topological superconductors and so-called doubletronics, layers of material that are somehow tilted together to produce different electronic behaviors, the researchers say.

"When you're creating new materials for future applications, it's important that you have a feedback loop between synthesis and characterization," said Professor Yang. "This will guide the next iteration of synthesis and will help us fill the technology gap."