A significant scientific advancement has been made by researchers at Rice University and their collaborators, who have identified direct evidence of active flat electronic bands within the kagome superconductor CsCr3Sb5. This discovery, published on August 14, 2025, in Nature Communications, is poised to revolutionize the design of quantum materials, potentially powering the next generation of electronics and computing technologies.
The study centers on CsCr3Sb5, a chromium-based kagome metal that exhibits superconductivity under pressure. Kagome metals are characterized by their unique two-dimensional lattices, formed by interconnected triangles. Theoretical predictions have suggested that these structures can host compact molecular orbitals, essentially standing-wave patterns of electrons, which could facilitate unconventional superconductivity and novel magnetic orders. Crucially, while such flat bands in most materials remain energetically distant and thus inactive, in CsCr3Sb5, they are actively involved, directly influencing the material's properties.
To pinpoint these active flat bands, the research team employed a combination of advanced synchrotron techniques, including angle-resolved photoemission spectroscopy (ARPES) and resonant inelastic X-ray scattering (RIXS), alongside sophisticated theoretical modeling. ARPES allowed the researchers to map emitted electrons, revealing distinct signatures of compact molecular orbitals, while RIXS measured magnetic excitations linked to these electronic modes. The theoretical component involved analyzing the impact of strong correlations using a custom-built electronic lattice model, which successfully replicated the observed phenomena and guided the interpretation of the experimental data.
Achieving such precise measurements necessitated the synthesis of unusually large and pure crystals of CsCr3Sb5. A refined method allowed for the production of samples approximately 100 times larger than previously possible, a critical factor in obtaining high-quality data. This interdisciplinary effort, spanning materials design, synthesis, spectroscopy, and theory, was led by Pengcheng Dai, Ming Yi, and Qimiao Si from Rice's Department of Physics and Astronomy and Smalley-Curl Institute, in collaboration with Di-Jing Huang from Taiwan's National Synchrotron Radiation Research Center. This groundbreaking work provides the first experimental validation for concepts previously confined to theoretical models. It demonstrates how the intricate geometry of kagome lattices can serve as a powerful design tool for controlling electron behavior in solids. By identifying these active flat bands, the team has established a clear link between lattice geometry and emergent quantum states. This opens up new avenues for engineering exotic superconductivity and other quantum phenomena through precise chemical and structural control, with potential applications in superconductors, topological insulators, and spin-based electronics.