Quantum geometric moment encodes stacking order of moir\'e matter

TL;DR

This paper demonstrates how the stacking order in moiré materials, specifically twisted double bilayer graphene, influences quantum geometric properties like Berry curvature and Berry curvature dipole, using nonlinear Hall effect measurements.

Contribution

It provides experimental evidence linking stacking order to quantum geometric quantities in moiré materials, enabling quantum geometry engineering and detection.

Findings

Berry curvature dipole varies with stacking order

Nonlinear Hall effect detects stacking-induced quantum geometry

Valley Chern numbers depend on stacking configuration

Abstract

Exploring the topological characteristics of electronic bands is essential in condensed matter physics. Moir\'e materials featuring flat bands provide a versatile platform for engineering band topology and correlation effects. In moir\'e materials that break either time-reversal symmetry or inversion symmetry or both, electronic bands exhibit Berry curvature hotspots. Different stacking orders in these materials result in varied Berry curvature distributions within the flat bands, even when the band dispersion remains similar. However, experimental studies probing the impact of stacking order on the quantum geometric quantities are lacking. 1.4$^\circ$ twisted double bilayer graphene (TDBG) facilitates two distinct stacking orders (AB-AB, AB-BA) and forms an inversion broken \moire superlattice with electrically tunable flat bands. The valley Chern numbers of the flat bands depend on the stacking order, and the nonlinear Hall (NLH) effect distinguishes the differences in Berry curvature dipole (BCD), the first moment of Berry curvature. The BCD exhibits antisymmetric behavior, flipping its sign with the polarity of the perpendicular electric field in AB-AB TDBG, while it displays a symmetric behavior, maintaining the same sign regardless of the electric field's polarity in AB-BA TDBG. This approach electronically detects stacking-induced quantum geometry, while opening a pathway to quantum geometry engineering and detection.