New Definition for 2D superlubricity criterion using graphene.

Researchers have developed a quantitative framework to determine when two-dimensional materials, such as graphene, can achieve structural superlubricity. This framework offers new insights into minimizing friction at the nanoscale. Li Wang of Nanjing University and Yunjie Ye of Sichuan ZeroNestor Microelectronics Technology Co. introduce a thermodynamic pinning criterion that distinguishes between fully sliding and pinned phases, providing a measurable method to predict whether layered materials will move without resistance.

Structural superlubricity is typically linked to incommensurate atomic lattices, where mismatched crystal structures significantly reduce friction. However, the researchers demonstrate that incommensurability and elastic reconstruction alone are insufficient for frictionless sliding. The transition between sliding and pinned behavior is governed by thermodynamic limits defined through maximum and minimum depinning torques, establishing a more rigorous criterion for identifying superlubric states.

To evaluate reconstructed interfaces, the researchers examined the reconstruction susceptibility measure, Λ. Using a 15-harmonic Leven potential, they increased the largest tested value of Λ from 0.142 to 0.212, extending the range for analyzing structural reconstruction. They emphasize that a value of Λ equal to 1 represents a reconstruction scale rather than a universal threshold for a static phase transition, clarifying this widely discussed parameter.

The study also challenges conventional views of friction in moiré materials. Simulations using a diffusion quantum Monte Carlo potential showed that even a clean, smooth, infinite moiré continuum can undergo elastic structural reconstruction without developing equilibrium or metastable energy barriers that would prevent sliding. This finding indicates that structural reconstruction does not necessarily lead to pinning, reinforcing the distinction between reconstruction and frictional behavior.

Across the zero-temperature in-plane models examined, the researchers consistently identified an elastically relaxed sliding regime, demonstrating that fully reconstructed interfaces can remain superlubric under appropriate thermodynamic conditions. Rather than relying solely on geometric alignment or lattice mismatch, the proposed framework provides a quantitative method for predicting when two-dimensional materials will remain free to slide.

By establishing a thermodynamic criterion for structural superlubricity, this work offers a practical foundation for engineering ultralow-friction materials. The results could guide the design of next-generation nanoscale mechanical systems, layered electronic devices, and other technologies where reducing friction and wear is essential.