Evidence of Directional Structural Superlubricity and Lévy Flights in a van der Waals Heterostructure.

Structural superlubricity is a special frictionless contact in which two crystals are in incommensurate arrangement such that relative in-plane translation is associated with vanishing energy barrier crossing. So far, it has been realized in multilayer graphene and other van der Waals (2D crystals with hexagonal or triangular crystalline symmetries, leading to isotropic frictionless contacts. Directional structural superlubricity, to date unrealized in 2D systems, is possible when the reciprocal lattices of the two crystals coincide in one direction only. Here, directional structural superlubricity a α-bismuthene/graphite van der Waals system is evidenced, manifested by spontaneous hopping of the islands over hundreds of nanometers at room temperature, resolved by low-energy electron microscopy and supported by registry simulations. Statistical analysis of individual and collective α-bismuthene islands populations reveal a heavy-tailed distribution of the hopping lengths and sticking times indicative of Lévy flight dynamics, largely unobserved in condensed-matter systems.