Since its discovery in 2018, magic angle twisted bilayer graphene (MATBG) has amazed physicists. Though still a scientific curiosity, researchers have made remarkable progress in the last several years with respect to understanding its exotic characteristics. Such moiré material supports an array of exotic quantum phases, such as unconventional superconductivity and new types of ferromagnetism. In ultra-pure MATBG devices, recent experiments have revealed cascades of topological states known as symmetry-broken Chern insulators (SBCIs). This finding underlines the material’s promise for leading new cutting-edge developments in quantum mechanics.
Research of MATBG is focused on its strongly correlated quantum states and emergence of topological phases of matter. Experimentalists had previously detected a ladder of fractional quantum Hall (FQH) states in MATBG when they subjected the material to a large magnetic field. Specifically, they discovered the Hofstadter butterfly, a fractal electronic structure first predicted more than 50 years ago. These discoveries contribute to our knowledge base of MATBG. They further underscore promising uses in quantum computing and materials science.
Exotic Quantum Phases
Since its breathtaking discovery, MATBG has taken the world by storm with its novel, exotic quantum phases. Such as unconventional superconductivity, and rare forms of ferromagnetism. These properties are a direct result of the material’s nontrivial electronic interactions and structure. Researchers are now exploring further with entangled quantum states. These states could provide the key to understanding the nature of exotic states of matter seen in condensed matter physics.
“Since its discovery in 2018, there has been great interest in MATBG, owing to its exotic quantum phases of matter, including unconventional superconductivity, new forms of ferromagnetism, and topology. Our original intention was to study orbital ferromagnetic states underlying the anomalous Hall effect in MATBG.” – Matthew Yankowitz
These exotic quantum phases have been intricately connected to the non-trivial topology of the material. The ability to control these states opens avenues for research that could revolutionize technology. These complex interactions at play in this material could, one day, give rise to entirely new kinds of electronic devices that harness these exotic properties.
An enormous amount of research has been directed towards understanding how to control the anomalous Hall effect. They do this by optically pumping WSe2 with circularly polarized light. Such a control mechanism would allow for fine-tuning of device performance according to the distinctly MATBG’s properties.
Cascades of Topological States
The recent observations of cascades of SBCI states in MATBG represent a major breakthrough in the field. Unlike the rest of these states which show a sequence of Chern numbers that perfectly follow from their parent correlated Chern insulators. This signals a newly discovered trend.
“Although SBCI states have previously been observed in MATBG, they have so far only appeared at apparently random moiré filling factors. What we have uncovered is a remarkable cascade sequence of SBCI states, with a sequence of Chern numbers mimicking their parent correlated Chern insulators.” – Minhao He et al.
This finding highlights the complex, rich nature of MATBG’s phase diagram. It shows the onset of several strongly-coupled Hofstadter states, which are dominantly sculpted under electron interacting. To explore these effects at milli-Kelvin temperatures, the researchers employed Landau fan diagrams. Their findings showed dramatic observations of MATBG’s response to different magnetic fields.
Fractional states manifest themselves in MATBG as a consequence of strained magnetic sub-bands with finite bandwidth. These sub-bands further display highly non-uniform and non-ideal quantum geometric characteristics. This arcane-looking geometric structure raises the prospect of deep connections to magnetic field induced FCI s. This find deepens understanding of an already remarkably rich and varied quantum materials landscape.
Understanding the Hofstadter Butterfly
The Hofstadter butterfly phenomenon, first realized in MATBG, serves as a powerful lens through which to view these mysterious interactions. Under appropriate conditions, the FQH states in typical conventional two-dimensional electron gas systems are found to grow stronger with increasing magnetic fields. The FQH states seen so far in MATBG disappear suddenly when the magnetic field goes beyond ~10 Tesla.
“In stark contrast to FQH states in conventional 2D electron gas systems that strengthen upon increasing magnetic field, the observed FQH states in our sample abruptly disappear above a magnetic field of ≈10 T,” – Xu
These results highlight the complex nature of MATBG’s electronic structure and its response to externally applied magnetic fields. The high-energy team has taken their experiments to successfully map the Hofstadter energy spectrum, showing how electron interactions ultimately take center stage.
“Our Hartree–Fock band analysis shows that these fractional states arise out of strained magnetic sub-bands with finite bandwidth and non-uniform, non-ideal quantum geometric properties. This unusual quantum geometric structure points to a potential description of these states within the framework of magnetic field-induced FCIs,” – Oskar Vafek
These are more of the correlational or experimental kind—the studies that try to uncover the mechanics behind what already exists. They hope to motivate ongoing research into the connections between fractional Chern insulators and fractional quantum Hall states.
“An exciting opportunity for our next studies will be to investigate the connection and interplay between fractional Chern insulator and fractional quantum Hall effect,” – Xu