Physicists find electronic agents that govern flat band quantum materials

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Physicists have directly visualized the fundamental electronic building blocks of flat-band quantum materials, a class of systems in which electron motion is effectively quenched and strong interactions give rise to emergent phases of matter. In a study published in Nature Physics, Qimiao Si's group at Rice University, in collaboration with researchers at the Weizmann Institute of Science, identified compact molecular orbitals that act as the key electronic agents governing the exotic behavior of these materials.

"In flat band materials, electron motion experiences destructive interference," said Si, the Harry C. and Olga K. Wiess Professor of Physics and Astronomy and director of Rice's Extreme Quantum Materials Alliance.

These flat band materials are also topological with properties that are preserved as the material continuously bends or stretches in any symmetry-preserving way.

"The electron motion is subject to a global effect described by the mathematical notion of topology," said Mounica Mahankali, a graduate student and co-first author on this paper. "The electronic states are configured such that when one goes through the space of electron states and returns to the starting point, a nonzero winding number has been acquired."

When Si developed a theory that allowed him to ask how the topology affects correlation physics, or the interactions of electrons that determine how electrons are organized in the system, he said he was excited about the new questions it opened into the interplay between topology and correlation physics. This theory, previously published in Science Advances, centers on the quantum critical point, a point of transition in a quantum material which Si believed could be interrogated through compact molecular orbitals, the agents that represent the flat bands in flat band materials.

"Think of it like a highway with the right lane experiencing stopped, heavy traffic and the left lane experiencing free-flowing, fast-moving traffic," Si said. In this scenario, drivers will change lanes, moving to the right in order to prepare for an exit, or moving to the left to try to avoid traffic. The right lane is a solid, stuck, ordered state; the left lane is fast-moving and liquid. As the cars change lanes, however, the state of the lanes changes too. Eventually, there will be a critical point where each lane could enter into either a traffic-jammed state or a free-moving state, depending on the movement of the cars. By examining the compact molecular orbitals, or the traffic-jammed lane, at this quantum critical point, Si theorized he could learn about the free-moving state.

"As appealing as our theory is, it remains a hypothesis until it's proven by experiment," Si said.

Si met Haim Beidenkopf, a professor at the Weizmann Institute in Israel, during their joint stay at the Kavli Institute of Theoretical Physics at the University of California, Santa Barbara. Beidenkopf, a quantum experimentalist, specializes in imaging quantum materials using atomic resolution spectrometers. He was already running an experiment on a flat band material, but the conversation made it clear that his experiment is uniquely suited to test whether the hypothesized compact molecular orbitals actually underlie its exotic physical properties.

In the experiment, Beidenkopf used an atomic resolution spectrometer to study a highly correlated metal—a material with highly agitated electrons—called Ni3In. Ni3In was selected for its potential practical application, as resolving the mechanism for its unusual electronic properties could provide insights into high temperature superconductivity.

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"In this study, we combined atomic-scale spectroscopy with material-specific analytical modeling to probe the spatial profile of the current that goes in and out of the kagome metal Ni3In," said Beidenkopf, the corresponding author on this study. "By doing so, we have revealed the kagome flat-band origin of the unusual quantum critical behavior in this compound and demonstrate the exquisite spatial profile expected from the compact molecular orbitals that leads to it."

The experimental data both confirmed the existence of compact molecular orbitals and, through the application of Si's theory, allowed the researchers to identify the kagome structure that was responsible for the quantum critical state.

"This collaboration showed, experimentally, that compact molecular orbitals serve as the agents that underlie the highly agitated quantum critical state of matter," Si said. "This provides new insight into high temperature superconductivity and opens the door for new quantum applications."

Publication details

Jean C. Souza et al, Origin of strange metallicity in a d-orbital kagome metal, Nature Physics (2026). DOI: 10.1038/s41567-026-03216-4

Journal information: Science Advances , Nature Physics

Key concepts

Electronic structurePhase transitionsSuperconductivityTopological phases of matterQuantum many-body systemsStrongly correlated systemsTopological materials

Provided by Rice University