SFB Extra Seminar

Materials crystallizing on the kagome lattice present an attractive venue to study topological and strongly correlated electronic states, owing to the Dirac and flat band electronic states, both native to this lattice structure in the two-dimensional (2D) limit. The intermetallic kagome family of FeSn is particularly interesting because the electronic, magnetic, and topological material properties can be tuned by the stoichiometry and choice of chemical elements. In this talk, I will present our most recent results from electric transport and scanning tunneling microscopy experiments on epitaxially grown thin films of the FeSn family.

Conducting electric transport measurements on the A-type antiferromagnet FeSn, we discover a new type of nonlinear anomalous Hall effect, which is based on magnetic point group symmetries. When a longitudinal a.c. drive current is applied, we observe a 3^rd order nonlinear anomalous Hall effect as a transverse voltage response at the third harmonic frequency at room temperature. A scaling analysis in terms of the charge carrier scattering time reveals the almost purely topological origin of 3^rd order anomalous Hall response. Conducting a symmetry analysis, model and ab-initio calculations, and magnetic Monte Carlo simulations, we demonstrate this intrinsic contribution can be attributed to the Berry curvature quadrupole moment, which arises in the spin-canted state of FeSn [1].

Destructive interference between electron wavefunctions on the two-dimensional (2D) kagome lattice induces an electronic flat band. Key to realize various proposals for emerging quantum states in a kagome flat band is to demonstrate the real space localization of kagome flat band electrons. It also remains unknown how the more complex lattice structure and orbital composition of realistic materials influence the localizing effect of destructive interference. We used scanning tunneling microscopy to visualize the non-trivial Wannier states of a kagome flat band at the surface of CoSn, a kagome metal. Our results show that the flat band electrons exhibit an extremely small localization length of two to three angstroms concomitant with a strongly renormalized quasiparticle velocity v ≈ 1 × 10⁴ m/s, a value comparable to that of moiré superlattices. These findings provide fundamental insight into the electronic properties of kagome metals and are a key step for future research on emergent many-body states in transition metal based kagome materials.

[1] arXiv:2303.03274 [cond-mat.mes-hall] (2023)