A recent study led by Weizmann Institute's Prof. Shahal Ilani and Prof. Pablo Jarillo-Herrero from MIT, and involving AQC's Yuval Oreg, has made significant strides in understanding graphene's quantum behaviors, particularly focusing on Dirac electrons in magic-angle graphene. Published on EurekAlert on 21 June 2020, this research delves into a new symmetry-broken parent state discovered in twisted bilayer graphene, attracting considerable attention in the quantum physics community.
Graphene, a single-atom-thick layer of carbon, displays a range of fascinating quantum phases when two layers are twisted at a specific "magic" angle. These phases include superconductivity, magnetism, and insulating behaviors. The study revealed that these quantum phases originate from a high-energy parent state with a unique symmetry-breaking characteristic.
In twisted bilayer graphene, electrons come in four 'flavors' - spins 'up' or 'down', combined with two 'valleys' from the hexagonal lattice. This allows each moiré site in the lattice to hold up to four electrons. The research team utilized a carbon nanotube single-electron transistor microscope to image the 'compressibility' of the electrons. This method revealed surprising patterns of electronic compressibility, indicating a sharp transition where heavy carriers abruptly give way to light Dirac-like electrons.
The study suggests that electrons fill the energy bands in magic-angle twisted bilayer graphene (MATBG) in a highly unusual 'Sisyphean' manner. As electrons approach an integer number per moiré superlattice site, a dramatic phase transition occurs, with one flavor dominating and resetting the others to the charge-neutral Dirac point.
This discovery is significant as it suggests that the broken symmetry state is the parent state for the superconducting and correlated insulating states observed in magic-angle graphene. The findings challenge current understanding of superconductivity, particularly how light Dirac electrons can give rise to superconductivity in this system.
Yuval Oreg's involvement in this groundbreaking research highlights the ongoing efforts to unravel the mysteries of quantum materials like graphene. These discoveries not only deepen our understanding of quantum physics but also open new possibilities for developing advanced quantum technologies.
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