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Researchers at the National University of Singapore (NUS) have achieved a significant breakthrough by successfully creating and directly visualising electron-hole crystals. The discovery marks a major advancement in the understanding of exotic quantum materials and their potential applications in computing technologies.
Electron-hole crystals form when electrons in a material align with lattice sites, creating a structured pattern known as an electron crystal. When both electrons and their positive counterparts, known as holes, coexist, they can generate even more complex quantum states with unique properties.
One such property is counterflow superfluidity, where electrons and holes move in opposite directions without resistance or energy dissipation. This phenomenon is intriguing for quantum simulations but maintaining these crystals without recombination has been challenging.
Traditionally, scientists have addressed this issue by separating electrons and holes into different layers or hosts. While this approach has demonstrated electron-hole states in multi-layered structures, achieving these states in a single natural material has been elusive. The challenge lies in finding exotic quantum materials that can sustain electron-hole crystals without them neutralising each other.
The NUS research team, led by Associate Professor Lu Jiong from the Department of Chemistry and the Institute for Functional Intelligent Materials (I-FIM), and Professor Kostya S. Novoselov, Director of NUS I-FIM, has overcome these challenges by creating and visualising electron-hole crystals using a Mott insulator called Alpha-ruthenium(III) chloride (α-RuCl3). This material, known for its unique quantum properties, provides a stable environment for electron-hole pairs to coexist and form distinct quantum states.
The breakthrough was achieved using scanning tunnelling microscopy (STM), a technique that utilises quantum tunnelling to produce images at the atomic level. Traditionally, STM has been limited to studying conductive materials, excluding insulators. The NUS team addressed this limitation by integrating graphene with α-RuCl3. Graphene, a single-atom-thick layer of carbon atoms, is highly conductive and allows electrons to pass through, revealing the electronic structure of the Mott insulator beneath. Graphene also serves as an adjustable electron source, enabling non-invasive and tunable doping of α-RuCl3.
Through this innovative setup, the researchers were able to visualise two distinct ordered patterns at different energy levels – the lower Hubbard band and the upper Hubbard band energies – each exhibiting unique periodicities and symmetries. By adjusting carrier densities in the system through electrostatic gating, they observed transitions between these orderings, providing strong evidence of electron-hole crystals. The direct visualisation at the atomic level revealed the shapes and structures of these crystals with unprecedented clarity, offering insights previously inferred from mesoscopic studies.
Assoc Prof Lu Jiong commented on the significance of the discovery, stating, “Typically, when a Mott insulator is doped, the strong interactions between electrons cause excess carriers to arrange into orderly patterns. While new charge orderings in a doped Mott insulator are expected, it was surprising to see two distinct orderings emerge simultaneously. This discovery is attributed to the formation of electron-hole crystals with coexisting electrons and holes.”
The implications of this research are profound. Visualising and controlling electron-hole crystals could revolutionise computing technologies by enabling materials with rapid state-switching capabilities. This advancement may lead to powerful new computers and materials for simulating complex quantum systems with high precision.
Moreover, the discovery holds promise for developing innovative materials for applications such as satellite communication and data networks, leveraging the precision of interferometry technology. The NUS team’s work not only enhances the understanding of quantum materials but also paves the way for future innovations in material science and quantum computing.
The successful visualisation of electron-hole crystals by the NUS research team represents a landmark achievement in quantum materials science. Their innovative approach provides new insights into electron-hole interactions and sets the stage for groundbreaking advancements in technology and material science.
