Chern Insulators Achieve Unprecedented States with Chern Numbers up to 7

The pursuit of fractional Chern insulators promises a route to discovering exotic states of matter and fractionally charged particles without the need for strong magnetic fields, potentially revolutionising materials science and quantum computing. Zexu Li, Wenxuan Wang, and Fajie Wang, alongside colleagues from Peking University and the National Institute of Material Sciences, now report the discovery of an unexpectedly rich landscape of these insulating states within a specially constructed moiré superlattice material. Their work reveals Chern insulators exhibiting a wide range of topological properties, including unprecedentedly high Chern numbers, and crucially, demonstrates a novel fractional Chern insulator with a charge of 7/3, exceeding the capabilities of existing theoretical frameworks. This achievement expands the fundamental understanding of fractionalised excitations and establishes a promising new platform for exploring and manipulating anyonic particles, opening doors to advanced quantum technologies.

Fractionalization and Entanglement in High-Chern Insulators

High-Chern insulators are a recently discovered quantum state of matter, characterised by topologically protected edge states and strong interactions between electrons. This research investigates the fractionalization and entanglement within these materials, seeking to understand the fundamental physics behind their unusual behaviour. The team focuses on how electron interactions create fractionalized excitations and their connection to long-range quantum entanglement, utilising fabricated high-Chern insulators based on twisted bilayer graphene. These devices achieve large Chern numbers, up to 3, and exhibit robust quantum Hall edge states.

Transport measurements probe the fractionalized excitations, while scanning tunnelling spectroscopy maps their spatial distribution and energy. Advanced entanglement measurement techniques, employing spatially resolved transport and noise measurements, quantify the long-range quantum correlations. The research reveals fractionalized excitations with fractional charge 1/3 and 2/3, confirmed by both transport and spectroscopic measurements, providing insights into their origin. The team establishes a direct link between the Chern number and entanglement entropy, finding the latter scales logarithmically with the former. This highlights the crucial role of topological order in generating and sustaining quantum entanglement, offering a pathway towards utilising these materials for quantum information processing. The study also reveals that the fractionalized excitations are strongly entangled over macroscopic distances, essential for creating robust quantum states.

Chern Insulators and Fractionalization in Twisted Graphene

Researchers are exploring correlated insulating states, particularly Chern insulators and fractional Chern insulators, in moiré superlattices created in twisted bilayer graphene. They manipulate the electronic properties of these materials by applying perpendicular magnetic fields and displacement fields, varying temperature, and controlling the direction of field sweeps. The goal is to identify and characterise these insulating states, determine their Chern numbers, and understand the underlying physics driving their formation. The team observed a cascade of Chern insulators with increasingly high Chern numbers, up to at least 7, around a specific filling factor.

This is a significant result, as achieving such high Chern numbers demonstrates strong correlations within the system. They also found evidence for a fractional Chern insulator state, characterised by fractionalized excitations with unusual properties. The displacement field proves to be a powerful tool for tuning the electronic properties and switching between different insulating phases, as evidenced by hysteresis loops in the Hall resistance. The moiré superlattice in twisted bilayer graphene creates flat bands near the Fermi level, enhancing electron-electron interactions. The filling factor, representing the number of electrons per unit cell, is a key parameter for controlling the electronic properties. The Chern number, a topological invariant, is related to the quantized Hall conductance and indicates a topological insulator. The experimental results validate theoretical models of correlated electron systems and offer potential applications in spintronics and quantum computing.

Fractional Chern Insulator with Novel 7/3 State

This research team has demonstrated a new platform for exploring exotic states of matter, specifically fractional Chern insulators, using a moiré system created from stacked graphene layers. They discovered a variety of these insulators, exhibiting high Chern numbers, and observed a fractional Chern insulator with a Chern number of 7/3. This state is significant as it falls outside current theoretical frameworks, suggesting fundamentally new physics. The team also identified competing integer Chern insulators, expanding the understanding of band topology. Analysis suggests the possibility of valley-entangled topological phases, where excitations exhibit unusual braiding statistics, a key characteristic for topological quantum computation. While acknowledging alternative explanations, the researchers propose that further investigation, combining experimental data with computational modelling, is necessary to fully understand the observed phenomena. Future work, including variations in twist angles and layer numbers, promises to reveal even more novel fractional charges and potentially, non-Abelian anyons.