Twisted double bilayer graphene (tDBLG) moiré superlattices have been shown to exhibit electronic hysteresis and plasticity due to twist-angle disorder. The inversion symmetry breaking at moiré length scales results in a second-order nonlinear electrical response through disorder-mediated extrinsic mechanisms. By controlling carrier concentration and vertical displacement field, the sign and magnitude of this nonlinearity are tunable. These combined effects allow the realization of a second-order synaptic memory device, showing that complex functions can arise from symmetry-breaking physics in single-element materials.
Researchers from Spain’s CIC nanoGUNE and Japan’s National Institute for Materials Science have fabricated tDBLG devices with twist angles from 0.7 to 1.9 degrees and confirmed that hysteresis in resistance is linked to twist-induced strain—absent in control samples with larger twist angles. This hysteresis does not stem from traps or ferroelectric switching but from changes in the electronic band structure, retained even after the field is removed. Second-order nonlinear response was seen in the bulk, varying with field and carrier concentration.
By combining hysteresis and second-order nonlinearity, the team built a memory element with multiple stable levels, adjustable by electric fields and carrier density. These states remained stable and required no continuous power. The device performed full synaptic memory functions, such as potentiation and depression, with reliable retention and minimal signal decay.
The system’s energy efficiency and architecture suggest a practical path for neuromorphic computing using pure graphene moiré superlattices.