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Monday, March 10, 2025
4:00 PM - 5:00 PM
East Bridge 114

Quantum Matter Seminar

Cascade of strongly correlated quantum states in a partially filled kagome flat band
Professor Berthold Jaeck, Assistant Professor, Physics, Hong Kong University of Science and Technology,
Speaker's Bio:
Prof. Berthold Jaeck obtained his diploma degree with distinction from Wรผrzburg University, Germany in 2011 and was an undergraduate fellow of the German Academic Exchange Association at University of California at Berkeley, USA in 2008-2009. He completed his doctoral studies in Physics under the supervision of Prof. Klaus KERN at the Ecole Polytechnique Fรฉdรฉrale de Lausanne, Switzerland in 2015, before joining the laboratory of Prof. Ali YAZDANI at Princeton University, USA as a Postdoctoral Fellow of the Alexander-von-Humboldt foundation. Since January 2021, Prof. Berthold Jaeck has become an Assistant Professor in the Department of Physics at The Hong Kong University of Science and Technology, where he is leading the laboratory for Quantum Matter and Microscopy.

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 ๐‘ฅ [1, 2]. At elevated temperatures (๐‘‡โ‰ฅ16 K), we find evidence for a nematic electronic state across a broad doping range 0.05<๐‘ฅ<0.25 that serves as the parent phase of a strongly correlated phase diagram. Comparison with model calculations reveals that strong Coulomb interactions (๐‘ˆ>100 meV) blend the states of two 3๐‘‘ orbital-derived FBs and impart a nematic order parameter. At lower temperatures ๐‘‡<16 K, we find direct evidence for an orbital-selective Mott state enabled by the 3๐‘‘ orbital degeneracy of the Co atom. This state can only be detected in samples with ideal Fe doping (๐‘ฅ=0.17) and descends into pseudo-gap phases upon electron and hole doping. At ๐‘‡<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, 3๐‘‘ 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.

For more information, please contact Annika Keating by email at akeating@caltech.edu.