Quantum Matter Seminar

Coulomb interactions among charge carriers that occupy an electronic flat band (FB) can give rise to captivating phenomena such as quantum criticality, Mott-Hubbard states, and unconventional superconductivity at different FB filling fractions. Consequently, the search for new FB materials with tuneable charge carrier filling and strong interactions is a central research theme. We present experimental evidence obtained from scanning tunnelling microscopy measurements for a cascade of strongly correlated states appearing in the partially occupied kagome FBs of Co1-xFexSn whose filling can be controlled by the Fe-doping level 𝑥 [1, 2]. At elevated temperatures (𝑇≥16 K), we find evidence for a nematic electronic state across a broad doping range 0.05<𝑥<0.25 that serves as the parent phase of a strongly correlated phase diagram. Comparison with model calculations reveals that strong Coulomb interactions (𝑈>100 meV) blend the states of two 3𝑑 orbital-derived FBs and impart a nematic order parameter. At lower temperatures 𝑇<16 K, we find direct evidence for an orbital-selective Mott state enabled by the 3𝑑 orbital degeneracy of the Co atom. This state can only be detected in samples with ideal Fe doping (𝑥=0.17) and descends into pseudo-gap phases upon electron and hole doping. At 𝑇<8 K, the pseudo-gap phase evolves into another nematic low-temperature state. Our observations demonstrate that the electronic ground state of the topological kagome FB depends on the complex interplay between strong Coulomb repulsion, 3𝑑 orbital degeneracy, and FB filling fraction at different temperatures. More broadly, our research establishes kagome materials as a unique platform to search for strongly correlated quantum states that arise in non-trivial FBs and can be controlled by the filling fraction.