Visualization of unconventional interfacial ferroelectricity in parallel-stacked twisted double bilayer graphene

Interfacial ferroelectricity in moiré-engineered van der Waals systems offers a promising route toward electrically tunable polarization at the atomic scale. The emergence of interfacial ferroelectricity generally requires the breaking of spatial symmetries, as demonstrated in parallel-stacked transition metal dichalcogenides (TMDs) and hexagonal boron nitride (h-BN) systems. In twisted double bilayer graphene (TDBG), previous studies have primarily focused on the anti-parallel stacking configuration (180° + δ twist), where transport signatures of ferroelectricity have been observed. In contrast, in TDBG with parallel stacking (0° + δ twist), however, the system preserves inversion symmetry, thus precluding intrinsic interfacial ferroelectricity. Here, by introducing an h-BN substrate to break these spatial symmetries, we report the first direct observation of interfacial ferroelectricity in parallel stacked TDBG. Using piezoresponse force microscopy (PFM), we visualize gate-tunable moiré domain expanding and shrinking evolution and reversible interlayer stacking transitions. Our findings reveal that symmetry tuning in TDBG constitutes a promising strategy for investigating interfacial ferroelectricity.