Breaking Symmetry Induces Strong Topological Insulating State in Bulk Iron Selenide

Iron-based superconductors hold immense promise for future technologies, and iron selenide (FeSe) remains a particularly active area of research due to its complex behaviour. Mikel Garcıa-Dıez from the Donostia International Physics Center, Jonas B. Profe from Goethe-Universität Frankfurt, and Augustin Davignon from Université de Sherbrooke, alongside their colleagues, now demonstrate a pathway to induce a special state called topological insulation directly within bulk FeSe. Their work reveals that deliberately disrupting the material’s symmetry, either through applying strain or allowing it to adopt a specific crystal structure, drives a change in its electronic band structure, resulting in topological properties. This is significant because topological materials exhibit robust surface states with potential applications in spintronics and quantum computing, and this research identifies a promising method for creating them within a well-studied superconductor, potentially paving the way for novel devices. The team’s calculations, which incorporate the effects of electron interactions, confirm that these topological characteristics are stable and could be realised in practice.

Topology and Exotic States in FeSe

Iron selenide (FeSe) has become a focal point for materials scientists, displaying a remarkable range of phenomena including unconventional superconductivity, nematic order, magnetism, and structural phase transitions. Understanding how topology and strong electron interactions combine in FeSe is crucial for advancing our knowledge of unconventional superconductivity and topological materials. This research investigates the potential for inducing and controlling topological phases in FeSe through external stimuli and material modifications, ultimately aiming to unlock new functionalities and applications for this promising material system.

Strained FeSe Crystal Growth and Characterisation

This section details the methods used to create and analyse versions of the FeSe crystal structure subjected to strain, crucial because even small amounts of strain strongly affect FeSe’s electronic and magnetic properties. The researchers aimed to connect theoretical calculations with experimental observations, creating realistic strained structures for verification by other researchers. The researchers began with theoretical calculations, using Density Functional Theory (DFT), to predict how the crystal structure of FeSe changes under different amounts of strain, relaxing these structures to allow atoms to settle into their lowest energy positions. A strain tensor was calculated to mathematically describe the deformation of the crystal lattice, then used to modify experimentally determined structures, creating a realistic model of FeSe under strain. A key consideration was the position of the selenium (Se) atoms, as DFT calculations sometimes predict inaccuracies. The researchers addressed this by rescaling the Se position to ensure model accuracy, creating sophisticated methodology that connects theory with experimental observations and represents an important step towards understanding FeSe’s properties.

Strain Induces Topology in Iron Selenide

Iron-based superconductors, and specifically iron selenide (FeSe), are attracting considerable attention due to their potential for novel electronic properties and applications. Researchers have long sought to engineer topological phases within these materials, which could unlock pathways to advanced quantum technologies. Recent investigations demonstrate a new route to induce topological characteristics within the bulk of FeSe by manipulating its crystal symmetry. The research team discovered that applying strain, or inducing a structural transition to a lower temperature phase, effectively breaks the inherent symmetry of the material.

This symmetry breaking drives FeSe into a strong topological insulating state, characterised by conducting surface states and insulating bulk behaviour. The process involves altering the arrangement of electronic bands within the material, creating a non-trivial band topology that defines its topological nature. The team employed advanced computational methods, including density functional theory and dynamical mean-field theory, to model the electronic structure of FeSe under various conditions, revealing that both uniaxial strain and the structural transition to an orthorhombic phase reliably induce the desired topological transition. Importantly, the calculations showed that the topological characteristics remain robust even when considering the strong electronic correlations present in FeSe.

This discovery provides a new and potentially more accessible pathway to realising topological phases in FeSe, unlike previous approaches that relied on chemical doping or extremely thin samples. This method utilises external stimuli, strain or temperature, to control the material’s properties, opening up possibilities for designing and fabricating novel electronic devices based on FeSe, potentially leading to advancements in areas such as quantum computing and spintronics. The ability to predictably engineer topological states establishes FeSe as a promising platform for exploring and harnessing the unique properties of topological materials.

Strain and Temperature Induce Topological States

This research demonstrates a pathway to induce topological properties within the bulk of iron selenide (FeSe), a material already known for its superconductivity and other unusual electronic behaviours. The team’s calculations reveal that breaking the material’s rotational symmetry, either through the application of strain or by allowing it to transition to a lower temperature structural phase, can drive it into a strong topological insulating state, exhibiting unique surface states characterised by robust electronic conduction. Importantly, the inclusion of electron correlations does not eliminate these topological characteristics, suggesting the topological state remains stable even when considering realistic material properties. The researchers identified Dirac cones on the surface of all three structures studied, with the strained and orthorhombic phases exhibiting topologically protected cones. The authors acknowledge that their calculations do not fully capture the complex Fermi surface reconstruction observed in FeSe, and further work is needed to refine the model. Future research could focus on experimentally verifying these predicted topological states and exploring their potential applications in novel electronic devices, suggesting that strain engineering represents a promising method for controlling and inducing topological behaviour in FeSe.