The emergence of exotic states of matter in atomically thin materials continues to drive innovation in modern physics, and recent research focuses on ‘electronic crystals’, where electrons themselves spontaneously arrange into ordered structures. You Zhou from the University of Maryland, Ilya Esterlis from the University of Wisconsin-Madison, and Tomasz Smoleński from the University of Basel, along with their colleagues, investigate these fascinating phases that arise from the interplay of electron interactions and carefully designed layered materials. Their work summarises the current understanding of electron crystallisation in these van der Waals heterostructures, detailing the experimental techniques used to observe and study these novel states, and importantly, highlights the remaining challenges and future opportunities in this rapidly evolving field. This research promises to unlock a deeper understanding of collective electron behaviour and potentially pave the way for new electronic devices based on these uniquely ordered states of matter.
Electronic Crystals in Layered Two-Dimensional Materials
Modern two-dimensional (2D) materials, including graphene-based systems and atomically-thin transition-metal dichalcogenides, exhibit fascinating electronic behaviour. Strong electronic interactions, tunable moiré superlattices, and unique band topology give rise to rich phase diagrams and collective phenomena. Researchers now observe electronic crystals, where electrons self-organize into periodic, spatially modulated charge density waves, differing from conventional charge density waves as they arise purely from electronic interactions without any distortion of the material’s lattice. This research investigates the formation and properties of these electronically-driven crystals in layered materials, aiming to understand the mechanisms governing their emergence and stability, and how external parameters, such as layer stacking and electric fields, control their properties. This work advances our understanding of correlated electron systems in 2D materials and opens new avenues for manipulating electronic states within them.
Moiré Patterns Drive Exotic Electron Crystals
Scientists are making remarkable progress in understanding electron crystallization within two-dimensional materials, revealing the formation of exotic states of matter where electrons spontaneously arrange themselves into ordered structures. Research demonstrates the successful creation of these “electronic crystals” in van der Waals heterostructures, utilizing materials like twisted WSe2/WS2 and graphene multilayers. Experiments reveal that these crystals form at specific rational fillings of moiré superlattices, exhibiting properties distinct from those formed without the influence of the moiré potential. Specifically, the team observed the formation of new crystal structures beyond simple triangular lattices, including stripe and honeycomb arrangements at fillings of 1/2 and 2/3 within a triangular moiré lattice.
Further investigations into graphene multilayers provide evidence for topological electron crystals forming at zero magnetic field, simultaneously exhibiting a quantized Hall conductance. Measurements confirm that the presence or absence of this quantized Hall effect can be tuned using external displacement and magnetic fields, demonstrating precise control over the system’s properties. Scientists are achieving significant reductions in defect densities, with flux growth methods yielding samples containing fewer than 10 10 charged defects per cm 2 and fewer than 10 11 isovalent defects per cm 2 . These high-quality materials exhibit charge disorder well below the critical densities required for electron crystal formation.
Experiments demonstrate that even small amounts of disorder can locally enhance the stability of the electron crystal, lowering the critical density required for formation and increasing its melting temperature. Scanning tunneling microscopy directly images these disordered electron crystals and their melting transitions, providing detailed insights into the interplay between disorder and electron correlations. These advancements promise to shed light on fundamental questions surrounding metal-insulator transitions in strongly correlated two-dimensional electron gases.
Electron Crystals in Two-Dimensional Materials
This research comprehensively reviews recent advances in understanding electron crystallization within layered two-dimensional materials. Scientists have demonstrated that these materials, including atomically thin semiconductors and transition-metal dichalcogenides, provide ideal platforms for observing and studying electronic crystals, states where electrons spontaneously arrange themselves into ordered patterns. The work details how unique properties of these materials, such as strong electronic interactions and tunable moiré superlattices, facilitate the formation and investigation of these unusual phases of matter. The team highlights key experimental techniques, including optical measurements and scanning tunneling microscopy, which allow direct observation of electron ordering and the identification of different crystal phases.
They demonstrate that by carefully controlling electron density and temperature, researchers can tune the system and explore the boundaries between different electronic states. Current investigations focus on understanding the influence of factors like disorder, magnetic fields, and band topology on the formation and properties of these electron crystals. Future research will likely focus on exploring the interplay between electron crystallization and other emergent phenomena, such as magnetism and topological effects, potentially leading to new discoveries in the field of quantum materials.