Elettra | Nanospectroscopy Friction without resistance: Directional superlubricity and Lévy flights in 2D nanostructures Friction is a universal phenomenon — present in every mechanical contact, from car engines to nanodevices. While essential for grip and braking, friction also causes massive energy loss. It is estimated that roughly 23% of global energy consumption is related to overcoming friction or repairing its effects. Finding ways to reduce it — especially at the microscopic scale — has become a key goal for both fundamental science and advanced technologies. One of the most promising directions is structural superlubricity: a special regime in which two crystalline materials, when misaligned just right, can slide past each other with almost no resistance — no lubricants required. Until now, this phenomenon was limited to a handful of homobilayer 2D material systems, such as two layers of graphene, hBN, or MoS₂. In all these cases, superlubricity was observed to be nearly isotropic, occurring equally in every direction except when the layers are in high-symmetry alignment. But recent work by researchers from Poland, Italy, New Zealand, and the USA demonstrates something fundamentally new: a directional form of superlubricity in a van der Waals heterostructure composed of α-bismuthene islands on graphite. In this system, the islands — atomically-thin crystalline nanostructures of bismuth — were seen to spontaneously hop across the graphite surface, over distances of hundreds nanometers, at room temperature and without external force. What makes this system unique is the rotational mismatch between the atomic lattices of α-bismuthene and graphite. Unlike previously studied combinations with symmetric (triangular or hexagonal) lattices, α-bismuthene introduces an asymmetry that results in partial atomic registry: the lattices align well in one direction, but not in others. This creates an energy landscape where the energy barriers vanish along one crystallographic axis — allowing resistance-free sliding — but remain high in other directions. This anisotropic energy profile was confirmed through registry index simulations, a technique that maps how interfacial atoms at the contact junction interact. As a result, the α-bismuthene islands move preferentially along graphitic zigzag directions, tracing out invisible “nanohighways” where friction is virtually zero — a nanoscale realization of directional structural superlubricity never before observed in such a clean, atomically-resolved system. Beyond the frictionless motion itself, the way the islands move is equally intriguing. By capturing thousands of displacement events using low-energy electron microscopy (LEEM) in real time, the researchers noticed that the island dynamics don’t follow a simple Brownian pattern. Instead, both the hopping lengths and the waiting times between hops follow heavy-tailed distributions — a signature of Lévy flights. Lévy flights are a form of random motion characterized by rare but very long steps, interspersed with periods of relative stillness. They’re found in models of animal foraging, human travel, and financial markets — but are almost unheard of in condensed matter physics, especially in systems as large and structured as the α-bismuthene islands. The appearance of Lévy statistics in a nanoscale solid-state system suggests that thermal fluctuations, combined with a directionally corrugated energy landscape, can induce complex and efficient forms of surface transport — with possible implications for nanoscale diffusion, selforganization, and device design. The implications of this work are profound. For one, it extends the frontier of superlubricity from symmetric to asymmetric interfaces, opening a path toward engineered nanotribology, where friction can be finely tuned, directed, or even switched by controlling lattice orientation. Secondly, it introduces Lévy dynamics into the vocabulary of solid-state motion, offering a new lens through which to understand transport at interfaces. This study not only advances our understanding of 2D material interactions but also points to novel design principles for low-friction mechanical systems at the nanoscale — where energy efficiency, precision, and durability are paramount. The findings were made possible through an interdisciplinary effort involving surface physicists, material scientists, and theorists from across four continents. From synthesizing atomically-thin bismuth films to visualizing their motion at sub-second intervals and modeling their interfacial potential energy, this collaboration highlights the power of international partnerships in advancing frontier nanoscience. As research continues into exotic 2D materials and their interactions, systems like α-bismuthene on graphite may hold the key to a future where friction is no longer a limitation, but a controllable parameter — one that could revolutionize nanomechanics, sensing, and actuation. Elettra Highlights 2024–2025 124
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