Matan Uzan - "de Haas-van Alphen spectroscopy and magnetic breakdown in moiré graphene"

(Followed by Prof. Berthold Jaeck)

Quantum oscillations(QO) originating from the quantization of the electron cyclotron orbits provide ultrasensitive diagnostics of electron bands and interactions in novel materials. We report on the first direct-space nanoscale imaging of the thermodynamic magnetization oscillations due to the de Haas-van Alphen effect in moiré graphene. Scanning by SQUID-on-tip in Bernal bilayer graphene crystal axis aligned to hBN reveals the unique structure of the QO frequencies with high sensitivity to the superlattice filling fraction. The oscillations allow us to reconstruct the complex band structure in exquisite detail, revealing moiré bands with multiple overlapping Fermi surfaces separated by unusually small momentum gaps. We identify distinct sets of oscillations that violate the textbook Onsager Fermi surface sum rule, signaling formation of exotic broad-band particle-hole superposition states induced by coherent magnetic breakdown.

Prof. Berthold Jaeck - "Visualizing many body phases in a partially filled kagome flat band"

(Following Matan Uzan)

Caiyun Chen¹, JiangChang Zheng¹, Soumya Sankar¹, Yi-Hsin Lin¹, Ruo-Peng Yu¹, Hoi Chun Po¹, Berthold Jaeck¹

¹ HKUST, Department of Physics, Clear Water Bay, Hongkong

The kagome lattice with spin-orbit coupling exhibits a topologically non-trivial flat band in which the effect of Coulomb interactions between the localized charge carriers is believed to be strong. Hence, material realizations of the kagome lattice provide a promising platform to search for new quantum phases of matter at the confluence of topology and strong electronic correlations. We previously showed that the kagome metal CoSn exhibits a quasi-two-dimensional flat band whose occupied electronic states are strongly localized in real space [1]. Here, we study the low energy density of states of Co_1-xFe_xSn in which partial flat band fillings are realized by hole-doping with Fe.

We will present results from temperature-dependent scanning tunneling microscopy measurements on Co_1-xFe_xSn. Combining high-resolution spectroscopy with spectroscopic imaging on samples with different doping levels x, we observe a rich sequence of states appearing at the Fermi energy that cannot be explained within a single-particle picture. We will discuss our findings in the context of electron-electron interaction induced many body states at partial flat band fillings.

We gratefully acknowledge support by the Hong Kong RGC and the Croucher Foundation.

[1] C. Chen et al., Phys. Rev. Research 5, 043269 (2023)