Chirality Switching On Demand
Topological phases of matter have captivated physicists for several decades, promising exotic phenomena and new paradigms for electronic devices [1]. So-called Chern insulators—systems exhibiting quantized Hall conductance without an external magnetic field—are particularly enticing. These materials support dissipationless, one-way electron transport along their edges, which could enable robust low-power electronics or even form the backbone of future topological quantum-computing architectures [2]. Yet, the defining feature of a Chern insulator—its chirality, which determines the direction of the edge-state current—is set by material symmetry and is therefore notoriously rigid and difficult to manipulate dynamically [3–5]. Now Jing Ding at Westlake University in China and colleagues have demonstrated electric-field-induced chirality switching using a new class of Chern insulators based on rhombohedral stacking of layers of graphene [6]. By achieving such in situ control over the material’s topological properties, the researchers underscore the versatility of the rhombohedral-graphene platform and pave the way for more complex topological circuits.
In rhombohedral multilayer graphene, adjacent graphene layers are laterally displaced such that carbon atoms form the vertices of rhombohedrons. This material naturally hosts flat bands with large Berry curvature―a measure of the band-structure topology―that can be tuned using electric fields perpendicular to the graphene layers. Such systems allow the stabilization of a variety of Coulomb-interaction-driven topological phases, with recent experiments in rhombohedral-graphene-based moiré superlattices showing integer and fractional quantum anomalous Hall effects, for example [7–12].
Rhombohedral graphene can be made to exhibit the Chern insulator state by arranging the material with other graphene layers to form certain moiré superlattices and then combining the resulting multilayer with a material with strong spin-orbit coupling. Ding and colleagues used 7- and 10-layer rhombohedral graphene encapsulated in hexagonal boron nitride, with the necessary spin–orbit coupling induced by an adjacent layer of tungsten diselenide. When the researchers applied a displacement field across the stack, they observed a sequence of topological phase transitions, including a striking switch between two insulating states, C = −1 and C = +1, which correspond to the two possible edge-current directions. More remarkably, in the 10-layer device, they also stabilized a C = 2 state (Fig. 1). This state supports two copropagating chiral edge channels, offering richer edge-state dynamics and the potential for multichannel topological transport.
To uncover these topological phase transitions, the researchers performed magnetotransport measurements on dual-gated graphene devices. By tuning the vertical displacement field, they observed clear signs of quantized anomalous Hall states with different Chern numbers, each characterized by quantized Hall plateaus. These plateaus serve as the defining transport signature of Chern insulators, indicating the presence of robust, dissipationless chiral edge states and a gapped bulk [2]. Notably, the chirality of edge transport reverses across a critical field, as seen in the sign change of the Hall resistance. These observations are supported by mean-field calculations, which capture the displacement-field-driven reordering of spin- and valley-polarized ground states. Such calculations are essential in this strongly interacting flat-band regime, where electron–electron interactions play a dominant role in stabilizing competing topological phases.
The team’s achievement represents a milestone in the active control of topological edge-state properties. Unlike the previously used techniques of magnetic doping or twist-angle engineering [2], electric-field tuning offers a fast, reversible, and energy-efficient handle on chirality. In magnetically doped systems, the Chern number is fixed by the choice of dopant and magnetic configuration, while twist-angle moiré structures are defined during fabrication and cannot be dynamically altered. In contrast, electric displacement fields can be applied in situ, enabling real-time switching between distinct topological phases.
While this concept of tuning topology with an electric field may seem straightforward, achieving full control over the Chern number—including the reversal of chirality and access to higher-Chern-number states—has proven difficult. This difficulty is largely because such control requires a material system with flat bands, strong interactions, and multiple broken symmetries, criteria recently realized in rhombohedral-graphene multilayers. As a result, electric-field-tunable topological phases are only now becoming experimentally accessible, making this an exciting and still rapidly evolving frontier. The ability to switch between different Chern numbers―including higher-Chern-number phases like the C = 2 demonstrated in this case―could enable topological logic elements, memory devices, or reconfigurable nonreciprocal circuits, all integrated in a single platform.
Looking forward, the combination of flat bands, high mobility, and strong gate tunability exhibited by rhombohedral multilayer graphene opens the door to even more exotic states—perhaps fractional ones with controllable chirality or engineered junctions between distinct topological phases. For now, Ding and colleagues have shown that chirality in a Chern insulator is not just a fixed label—it can be turned on its head with the twist of a gate voltage.
References
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