Superlattice Potential Tuning Generates Giant Shift Current in Twisted Bilayer Graphene

Researchers are increasingly exploring how to engineer materials with tailored electronic properties, and a new study demonstrates a remarkably strong light-to-current conversion in a unique form of graphene. Nabil Atlam from Northeastern University, Swati Chaudhary from The University of Tokyo, and Arpit Raj from Northeastern University, along with colleagues, investigate graphene bilayers arranged in a superlattice pattern, created by either electrical tuning or twisting the layers. The team’s work reveals a ‘shift current’, an electrical current generated by light, that is significantly larger than predicted in similar materials, offering a promising pathway towards more efficient optoelectronic devices. By carefully controlling the superlattice pattern, researchers can optimise this nonlinear response, potentially leading to breakthroughs in light harvesting and conversion technologies.

Twisted Graphene and Moiré Band Engineering

This research focuses on the exciting field of twistronics, 2D materials, and related phenomena, particularly manipulating band structure and exploring novel optical and electronic properties. The core theme revolves around twisted bilayer and multilayer graphene, where the interplay of layers creates Moiré patterns. These patterns lead to the formation of flat bands, which enhance electron-electron interactions and can give rise to correlated insulating states and even superconductivity. A key area of investigation is the creation and control of these flat bands through twisting, strain, and the introduction of superlattices, which are periodic potential landscapes.

Researchers are also exploring the possibility of realizing topological phases, such as Chern insulators, within these engineered systems. These phases exhibit robust edge states and potentially fractionalized excitations. A significant portion of the research investigates the optical response of these materials, focusing on the shift current, a nonlinear optical effect that generates a DC current from light and offers potential for photovoltaic applications. Other areas of study include photoconductivity and various photogalvanic effects. Superlattices and van der Waals heterostructures, created by stacking different 2D materials, are crucial tools for modifying band structure and inducing topological phases.

Graphene Superlattices Boost Shift Current Generation

Recent research demonstrates a pathway to significantly enhance the generation of electrical current from light within layered graphene structures. By introducing a patterned energy landscape, known as a superlattice potential, into these materials, scientists have discovered a dramatic increase in a phenomenon called the shift current. This current arises from the asymmetry of electron movement induced by light, and the magnitude observed surpasses predictions for similar systems by several orders of magnitude. The team engineered this superlattice potential through precise control of the graphene layers, either by applying external voltages or by carefully twisting the layers relative to each other.

This level of control allows for optimization of the nonlinear optical response, potentially leading to more efficient light-harvesting and energy conversion technologies. The key to this enhancement lies in the unique way the superlattice potential alters the movement of electrons when exposed to light. The patterned energy landscape causes electrons to shift slightly with each absorption of a photon, and these small shifts accumulate to create a measurable current. Importantly, the researchers found that the arrangement of the superlattice potential, specifically its symmetry, plays a crucial role in maximizing this effect.

Certain orientations break the symmetry of the material, unlocking a stronger shift current response than previously achievable. The magnitude of the observed shift current is particularly noteworthy, significantly exceeding that predicted by existing theoretical models and surpassing the performance of similar twisted bilayer and trilayer graphene systems. This suggests that the engineered superlattice potential provides a highly effective mechanism for enhancing light-to-current conversion, opening up possibilities for novel optoelectronic devices. The ability to manipulate the superlattice potential through external means also offers a pathway to dynamically control the generated current, paving the way for advanced light-based technologies.

Superlattice Potential Boosts Graphene Shift Current

This research demonstrates that bilayer graphene, when subjected to a superlattice potential, exhibits a significantly enhanced shift current response. The team investigated how manipulating the superlattice potential, through electrostatic means or lattice twisting, influences this response, revealing a strong dependence on both gate voltage and the potential’s strength and phase. These findings provide insight into the nonlinear optical properties of materials and suggest pathways for optimising such responses through careful engineering of the superlattice potential. The study establishes a clear link between the superlattice potential and the resulting shift current, showing that specific configurations, determined by the potential’s phase, can dramatically alter the electronic band structure and, consequently, the material’s optical behaviour.

The researchers note that their model aligns with experimental setups utilising gating techniques to induce superlattice potentials, offering a versatile method for tuning material properties. While the current work focuses on specific parameter ranges and a fixed value for a key parameter defining interlayer coupling, it lays the groundwork for exploring a wider range of configurations and materials. Future research could investigate the impact of varying this parameter and extending the analysis to include additional layers, potentially leading to even more pronounced nonlinear optical effects.