Breakthrough in Kagome Superconductors: Active Flat Bands Discovered

In a groundbreaking study published on August 14, 2025, in Nature Communications, researchers from Rice University and collaborating institutions have provided the first experimental evidence of active flat electronic bands in the kagome superconductor CsCr₃Sb₅. This discovery could revolutionize the design of quantum materials, potentially leading to advancements in superconductors, topological insulators, and spin-based electronics.

Kagome metals are characterized by their two-dimensional lattices of corner-sharing triangles, a structure that can lead to unique electronic properties, including flat electronic bands and Dirac points. These features have been of significant interest in condensed matter physics due to their potential to host exotic quantum phenomena such as unconventional superconductivity and novel magnetic orders.

The study focuses on the chromium-based kagome metal CsCr₃Sb₅, which exhibits superconductivity under pressure. Unlike previous kagome materials where flat bands were too distant from the Fermi level to significantly impact electronic properties, CsCr₃Sb₅ features flat bands actively involved in its superconducting and magnetic behaviors. This finding opens new avenues for engineering quantum materials with tailored electronic properties.

"Our results confirm a surprising theoretical prediction and establish a pathway for engineering exotic superconductivity through chemical and structural control," said Pengcheng Dai, the Sam and Helen Worden Professor of Physics and Astronomy at Rice University.

The research team employed advanced synchrotron techniques to investigate the presence of active standing-wave electron modes. They used angle-resolved photoemission spectroscopy (ARPES) to map electrons emitted under synchrotron light, revealing distinct signatures associated with compact molecular orbitals. Resonant inelastic X-ray scattering (RIXS) measured magnetic excitations linked to these electronic modes.

"By identifying active flat bands, we've demonstrated a direct connection between lattice geometry and emergent quantum states," said Ming Yi, an associate professor of physics and astronomy at Rice University.

Theoretical support was provided by analyzing the effect of strong correlations starting from a custom-built electronic lattice model, which replicated the observed features and guided the interpretation of results. Fang Xie, a Rice Academy Junior Fellow and co-first author, led that portion of the study.

Obtaining such precise data required unusually large and pure crystals of CsCr₃Sb₅, synthesized using a refined method that produced samples 100 times larger than previous efforts, said Zehao Wang, a Rice graduate student and co-first author.

"This work was possible due to the collaboration that consisted of materials design, synthesis, electron and magnetic spectroscopy characterization, and theory," said Yucheng Guo, a Rice graduate student and co-first author who led the ARPES work.

The confirmation of active flat bands in CsCr₃Sb₅ has several significant implications:

• Design of Quantum Materials: Understanding the role of flat bands near the Fermi level provides a new pathway for designing materials with tailored electronic properties, potentially leading to the development of novel superconductors and topological insulators.

• Advancements in Electronics: The insights gained from this study could inform the creation of spin-based electronic devices, contributing to the advancement of quantum computing and other cutting-edge technologies.

• Exploration of Strongly Correlated Systems: CsCr₃Sb₅ serves as a model system for studying the interplay between lattice geometry, electron correlation, and topology, offering a platform to explore emergent phenomena in strongly correlated electron systems.

The experimental confirmation of active flat bands in the kagome superconductor CsCr₃Sb₅ marks a pivotal advancement in the field of quantum materials. This discovery not only enhances our understanding of the electronic properties of kagome lattices but also paves the way for the development of next-generation electronic devices leveraging the unique characteristics of these materials.