Graphene Flakes Demonstrate Strain-Controlled Boundary States and Phase Transitions

Strain, a well-established method for manipulating material properties, now reveals a surprising ability to control the fundamental behaviour of electrons within graphene-like structures. Yongsheng Liang from The MOE, alongside Shiqi Xia and Daohong Song, and colleagues demonstrate that carefully applied strain can create or eliminate edge states, special electronic pathways confined to the boundaries of these flakes. This discovery, made using a photonic platform mimicking graphene, shows that the existence and location of these edge states directly correlate with the underlying topological properties of the material. Importantly, the team finds that maximum strain induces not only edge states, but also highly localized corner states, effectively creating a simplified model for a higher-order topological insulator, a state of matter with potential applications in robust and efficient electronic devices.

Topological Photonics And Engineered Light States

This research focuses on harnessing topological phenomena, like protected edge and corner states, within engineered photonic structures. Scientists aim to mimic the behavior of topological insulators and semimetals, materials with unique electronic properties, but using light instead of electrons. By carefully designing photonic structures, including lattices, crystals, and metamaterials, they seek to control and manipulate light in novel ways. This field explores how these engineered structures can support robust waveguiding and signal transmission, offering potential for advanced optical devices.

Researchers are particularly interested in edge and corner states, which are protected from scattering, and higher-order topology, where states localize at corners or hinges. Strain engineering, which uses mechanical stress to modify material properties, also plays a crucial role. The ultimate goal is to develop improved photonic devices, such as robust waveguides, single-photon emitters, and innovative optical sensors.

Strain Control of Photonic Graphene Edge States

Scientists have demonstrated precise control over edge states in engineered photonic graphene flakes by applying uniaxial strain, revealing a pathway to create or eliminate these states depending on the direction of applied force. The study investigates two distinct flake structures, featuring pairs of twig and zigzag edges, and armchair and bearded edges, to understand how strain influences topological properties. Applying compression along a specific direction shifts key points in momentum space, enabling the creation of edge states in one flake type while simultaneously destroying them in the other. Beyond a certain threshold, strain opens a full band gap, driving the system into an insulating state, where one flake type hosts a complete flatband of edge states, maximizing their number, while edge states vanish entirely in the other. Measurements of bulk polarization reveal distinct values for the two flake types after the gap opens, demonstrating the crucial role of uniaxial strain in manipulating edge states and realizing a minimal-model higher-order topological insulator based on strained graphene flakes.

Strain Controls Edge States in Photonic Graphene

Scientists have demonstrated precise control over edge states in engineered photonic graphene flakes by applying uniaxial strain. This work centers on flakes with custom boundaries, specifically those featuring pairs of twig and zigzag edges, and armchair and bearded edges, allowing for detailed investigation of topological properties. Experiments show that the existence and positioning of edge states accurately correspond to predicted theoretical models. Applying strain along one direction alters the interactions between atoms and shifts key points in momentum space, enabling the creation of edge states in one flake type while simultaneously destroying them in the other.

In the insulating regime, one flake type hosts a complete flatband of edge states, maximizing their number, while edge states vanish in the other. Experimental validation involved fabricating photonic graphene flakes using laser-writing techniques, revealing that edge states are preserved only in the insulating regime. These findings demonstrate the ability to manipulate topological properties through strain, opening possibilities for advanced photonic devices and materials with tailored electronic properties.

Strain Controls Graphene’s Topological Edge States

Researchers have demonstrated that applying uniaxial strain to graphene flakes with specifically designed boundaries allows for the creation or destruction of edge states, and enables transitions between different topological phases. Through both theoretical analysis and experimental observation using laser-written photonic graphene, the team confirmed that the direction of applied strain, combined with the flake’s boundary conditions, directly influences the presence and characteristics of these edge states. Importantly, the research reveals that under certain strain conditions, these strained flakes exhibit characteristics of a higher-order topological insulator, giving rise to localized corner states. This work establishes strained graphene flakes as a versatile platform for engineering boundary phenomena, potentially opening new avenues for applications in areas such as topological lasers and quantum emitters, and for exploring fundamental physics in photonics and related fields.