New discovery in flat electronic bands opens doors to advanced quantum materials

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Researchers at Rice University, led by Qimiao Si, have made a groundbreaking discovery in quantum materials.

Their study, published in Nature Communications, predicts the existence of flat electronic bands at the Fermi level.

This discovery could pave the way for new forms of quantum computing and advanced electronic devices.

Quantum materials are governed by quantum mechanics, where electrons exist in unique energy states.

These states form a ladder, with the highest rung called the Fermi energy. Electrons, being charged, repel each other and move in coordinated ways. Si’s team discovered that these interactions can create new flat bands at the Fermi level, enhancing their significance.

“Most flat bands are located far from the Fermi energy, limiting their impact on the material’s properties,” explained Si, the Harry C. and Olga K. Wiess Professor of Physics and Astronomy at Rice.

In typical conditions, a particle’s energy changes with its momentum. However, in quantum mechanics, electrons can exhibit quantum interference, maintaining a flat energy level even as their momentum changes. These are known as flat bands.

“Flat electronic bands can enhance electron interactions, potentially creating new quantum phases and unusual low-energy behaviors,” Si said.

These bands are especially sought after in materials known as d-electron materials, which contain transition metal ions with specific crystal lattices. These materials often exhibit unique properties due to their structure.

The team’s findings suggest new ways to design these materials, which could inspire applications in quantum bits (qubits) and spintronics. Their research demonstrates that electron interactions can link immobile and mobile electron states, creating new opportunities for quantum computing.

Using a theoretical model, the researchers showed that these interactions could create a new type of Kondo effect. This effect describes how immobile particles gain mobility by interacting with mobile electrons at the Fermi energy. The Kondo effect involves the scattering of conduction electrons in a metal due to magnetic impurities, which changes the metal’s electrical resistivity with temperature.

“Quantum interference can enable the Kondo effect, allowing us to make significant progress,” said Lei Chen, a Ph.D. student at Rice.

A key attribute of the flat bands is their topology. “The flat bands pinned to the Fermi energy provide a means to realize new quantum states of matter,” Chen said. This includes anyons and Weyl fermions, which are massless quasiparticles and fermions that carry an electric charge. Anyons are promising for qubits, and materials hosting Weyl fermions could be used in spin-based electronics.

The study also highlights the potential for these materials to respond to external signals and achieve advanced quantum control. The flat bands could lead to strongly correlated topological semimetals, potentially operating at high temperatures or even room temperature.

“Our work provides the theoretical foundation for utilizing flat bands in strongly interacting settings to design and control novel quantum materials that operate beyond the realm of low temperatures,” Si said.

This discovery opens new possibilities for developing advanced quantum materials and technologies, bringing us closer to practical quantum computing and other innovative applications.