Nonlinear Hall quantum oscillations to probe topological Brown-Zak fermions in graphene moir\'e systems

TL;DR

This paper introduces a new type of nonlinear Hall effect quantum oscillations in graphene moiré systems, enabling detection of topological Brown-Zak fermions and revealing their quantum geometric properties.

Contribution

It proposes and experimentally demonstrates a novel nonlinear Hall quantum oscillation technique to probe topological quasiparticles in moiré systems.

Findings

Detected Brown-Zak fermions at low magnetic fields as low as 0.5 T.

Established the first experimental evidence of the topological nature of Brown-Zak fermions.

Unveiled quantum geometric contributions to nonlinear transport under specific conditions.

Abstract

Due to the deep connection with the quantum geometry of electronic Bloch wavefunctions, the second-order nonlinear Hall effect (NLHE) has been an attractive topic since its proposal. However, studies on NLHE under a magnetic field have been lacking. Given that quantum oscillations in the linear response regime have been proven to be useful tools in investigating electronic systems, searching for quantum oscillations in NLHE is of great interest and is expected to provide new avenues to unveil rich quantum geometric properties of novel quasiparticles. Here, we propose a new type of NLHE quantum oscillations and experimentally probe it in graphene moir\'e systems. It stems from the alternation of the dominant NLHE mechanisms with recurring Bloch states under magnetic field, which enables sensitive detection of Brown-Zak fermions, giving an onset field as low as 0.5 T. Most importantly, when the commensurability condition is satisfied, the nonlinear transport of Brown-Zak fermions is mainly governed by quantum geometric contributions. Our findings not only establish a new type of quantum oscillations, but also demonstrate the first experimental detection of the topological nature of Brown-Zak fermions, shedding light on the exploration of novel topological quasiparticles.