01.10.2024, Hong Kong

Farewell To Yekta, Jason, and Justin

After a summer brimming with undergraduate research in our lab, we say goodbye to our last summer student of 2024. Thank you Yekta, Jason, and Demin for all your hard work and contributions to our group and research and best of luck for your future paths. It is always a pleasure to have undergraduates join us from around the world for a few months of research and (from what I hear as feedback) fun. Are you interested to join us for a summer research internship? Get in touch!

11.09.2024, Hong Kong

New Pre-Print Online: Phase Diagram of Strongly Correlated States in a Partially Filled Kagome Flat Band

Check our new paper on the arXiv (link). We present experimental evidence obtained from scanning tunnelling microscopy measurements for a cascade of strongly correlated states, such as a nematic order parameter and an orbital selective Mott state, appearing in the partially occupied kagome flat bands of Co1−xFexSn whose filling can be controlled by the Fe-doping level x. Our observations demonstrate that the electronic ground state of a kagome flat band depends on the complex interplay between strong Coulomb repulsion, 3d-orbital degeneracy, and flat band 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 flat bands and can be controlled by the filling fraction.

01.08.2024, Hong Kong

Farewell to Sophie and Faith

This summer, we bid our farewell to Sophie and Faith who took a leading role in getting our lab up and running. As a postdoctoral fellow, Sophie's contribution to the construction of our MBE-STM, the training of students, and numerous scientific projects cannot be overestimated. You will be missed, and we wish you all the best in the next step of your career at the Max-Planck-Institute for Solid State Research.

Faith joined our group in his first year of study as part of the Fab Four team of undergraduates that fearlessly set up the MBE systems, which keeps delivering samples until today. He was also a great soccer player, and we wish him all the best for his foray into quantum information technology in Shanghai!

Our Work on Berry Curvature Multipoles Published in PRX

We are delighted that our manuscript ‘Experimental Evidence for a Berry Curvature Quadrupole in an Antiferromagnet’ has been published in Physical Review X (link). Berry curvature multipoles appearing in topological quantum materials have recently attracted much attention. Their presence can manifest in novel phenomena, such as nonlinear anomalous Hall effects (NLAHE). The notion of Berry curvature multipoles extends our understanding of Berry curvature effects on the material properties. Hence, research on this subject is of fundamental importance and may also enable future applications in energy harvesting and high-frequency technology. We demonstrated a fundamentally new mechanism for Berry curvature multipoles in antiferromagnets that are supported by the underlying magnetic symmetries. Carrying out electric transport measurements on epitaxial thin films of the kagome antiferromagnet FeSn, we observe a third-order NLAHE, which appears as a transverse voltage response at the third harmonic frequency when a longitudinal ac drive is applied. Interestingly, this NLAHE is strongest at and above room temperature. At a practical level, our study establishes NLAHE as a sensitive probe of antiferromagnetic phase transitions in other materials—such as moiré superlattices, two-dimensional van der Waal magnets, and quantum spin liquid candidates, which remain poorly understood to date. More broadly, Berry curvature multipole effects are predicted to exist for 90 magnetic point groups. Hence, our work opens a new research area to study a variety of topological magnetic materials through nonlinear measurement protocols.

12.03.2024, Minneapolis

APS March Meeting 2024

We just returned from our APS March Meeting trip to Minneapolis, where we presented our work on the in-plane Hall effect in a Weyl ferromagnet (Soumya), microscopic evidence for a chiral flux phase (Andrew), and interacting many-body phases in a kagome flat band (Berthold). It was a fun and very productive meeting to discuss our ongoing work with our collaborators, develop new research ideas, and to see old friends and make new friends. Check out the ‘Photos’ section for some impressions from our trip. We look forward to next year’s meeting in Los Angeles!

We are excited to share the first piece of work (link) that comes out of our home-built MBE-STM system. In this work, we used spectroscopic mapping with the scanning tunnelling microscope (STM) to examine the real space localisation of the non-trivial electronic wave functions of a kagome Flat Band. Kagome metals host an electronic flat band in their electronic structure. Because the kinetic energy of electrons occupying these flat bands is quenched, quantum-mechanical interactions can emerge as the leading energy scale. Hence, the kagome materials are an attractive venue to explore the possible emergence of strongy-correlated many-body states that are predicted to exist in their flat bands. However, the extent to which the complex structure of realistic materials counteract the localizing effect of destructive interference is hitherto unknown and a detailed understanding of the real-space distribution of the electronic states of kagome flat bands has not been developed yet. We used scanning tunneling microscopy to visualize the electronic states of a kagome flat band at the surface of CoSn, a kagome metal. Consistent with results from model calculations, we find that the local density of states associated with the kagome flat bands exhibits a unique real-space distribution by which it can be distinguished from the local density of states of dispersive electron bands and trivially localized states, such as well-localized orbitals and surface resonances. Our findings provide fundamental insight into the electronic properties of kagome metals and present a key step for future research on emergent many-body states in these systems.