Rice University researchers and their collaborators have confirmed that a long-sought “flat band” sits right at the heart of a kagome superconductor’s low-energy physics — and that it actively shapes the material’s electronic and magnetic behavior. The finding, published on August 14, 2025, nails down a key ingredient theorists have argued could seed exotic phases such as unconventional superconductivity and correlated magnetism.
Why flat bands matter
In most crystals, electrons spread out in energy–momentum space, forming dispersive “bands.” Flat bands are different: their energy barely changes with momentum, creating an enormous density of available electronic states that amplifies interaction effects. Get a flat band close to the Fermi level — where electrons actually participate in low-temperature physics — and small nudges can trigger big collective phenomena. Kagome lattices, built from corner-sharing triangles, are a natural host for such bands because of quantum interference in their geometry.
The material: CsCr₃Sb₅
The team focused on the chromium-based kagome metal CsCr₃Sb₅, which becomes superconducting under pressure. Prior work hinted that this compound might harbor flat-band physics, but direct proof that the flat band is active — tuned to the Fermi level and intertwined with the material’s emergent order — was missing.
How they proved it
Researchers combined two complementary synchrotron techniques with theory:
- Angle-resolved photoemission spectroscopy (ARPES) mapped the electronic structure and revealed flat-band signatures pinned at the Fermi level.
- Resonant inelastic X-ray scattering (RIXS) detected low-energy spin excitations that evolve across a low-temperature transition, consistent with the flat band’s shift and its coupling to magnetism.
- Correlated-electron modeling reproduced the observations and clarified how lattice geometry and interactions conspire to activate the flat band.
Together, the measurements show the flat band is no passive bystander: it participates directly in shaping the magnetic and electronic landscape.
Craft and scale enabled the science
Achieving this clarity depended on unusually large, ultra-pure CsCr₃Sb₅ crystals — reportedly about 100× bigger than typical samples — grown via a refined synthesis route. The crystal quality was crucial for high-resolution ARPES and RIXS to disentangle the flat band from nearby features.
What this unlocks
By pinning an interacting flat band at the Fermi level in a bulk kagome superconductor, the work validates a central design rule for quantum materials.Uuse geometry to engineer band flatness, then use chemistry and structure to tune it into play. That blueprint opens pathways to stabilize unconventional superconductivity, topological states, and spin-driven orders — and to control them with pressure, strain, or composition.
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