An international group of researchers led by Qimiao Si of Rice University predicts the presence of flat electronic bands at the Fermi level, which was published in the journal Nature Communications. This discovery could lead to the development of new types of electronic devices and quantum computing.
A team of scientists led by Qimiao Si predicts the existence of flat electronic bands at the Fermi level. Image Credit: Jeff Fitlow/Rice University
The laws of quantum mechanics apply to quantum materials, where electrons have distinct energy states. These states are arranged on a ladder, with the Fermi energy at the top; as they are charged, electrons travel in correlated patterns and repel one another. Si's group discovered that new flat bands can be formed at the Fermi level through electron interactions, which increases their significance.
“Most flat bands are located far from the Fermi energy, which limits their impact on the material’s properties,” said Si, the Harry C. and Olga K. Wiess Professor of Physics and Astronomy at Rice.
A particle's energy usually varies with its momentum. However, electrons are capable of exhibiting quantum interference in quantum mechanics, in which case their energy stays constant despite changes in momentum, called flat bands.
Flat electronic bands can enhance electron interactions, potentially creating new quantum phases and unusual low-energy behaviors.
Qimiao Si, Department of Physics and Astronomy, Rice University
According to Si, these bands are particularly sought after in transition metal ions known as d-electron materials with particular crystal lattices, where they frequently exhibit distinctive features.
The results of the team's research point to novel approaches to their creation, which may lead to new uses of these materials in spintronics, quantum bits, and qubits. Their findings demonstrate that immobile and mobile electron states can be connected by electron interactions.
The researchers used a theoretical model to show that these interactions can produce a novel kind of Kondo effect, in which immobile particles become mobile through interactions with mobile electrons at the Fermi energy. The dispersion of conduction electrons in a metal by magnetic impurities, which causes a distinctive temperature-dependent change in electrical resistivity, is known as the Kondo effect.
Quantum interference can enable the Kondo effect, allowing us to make significant progress.
Lei Chen, Ph.D. Student, Rice University
A key attribute of the flat bands is their topology, Chen said. “The flat bands pinned to the Fermi energy provide a means to realize new quantum states of matter,” he said.
This includes massless quasiparticles and fermions with an electric charge, known as anyons and Weyl fermions, according to the team's findings. The researchers discovered that anyons show promise as qubit agents, while materials hosting Weyl fermions may have applications in spin-based electronics.
The paper also emphasizes how these materials may be able to achieve sophisticated quantum control and be extremely sensitive to outside inputs. The findings suggest that at relatively low temperatures, the flat bands may give rise to strongly correlated topological semimetals that may operate at room temperature or even higher temperatures.
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.
Qimiao Si, Department of Physics and Astronomy, Rice University
Co-authors include Fang Xie and Shouvik Sur, Rice Postdoctoral Associates of Physics and Astronomy; Haoyu Hu, Rice alumnus and Postdoctoral Fellow at Donostia International Physics Center; Silke Paschen, a Physicist at the Vienna University of Technology; and Jennifer Cano, a Theoretical Physicist at Stony Brook University and the Flatiron Institute.
The research was funded by the U.S. DOE, BES; Air Force Office of Scientific Research; Robert A. Welch Foundation, and the Vannevar Bush Faculty Fellowship.
Journal Reference:
Chen, L., et al. (2024) Emergent flat band and topological Kondo semimetal driven by orbital-selective correlations. Nature Communications. https://doi.org/10.1038/s41467-024-49306-w.