Observing the Chern mosaic and Berry curvature magnetism in magic angle graphene

Observing the Chern mosaic and Berry curvature magnetism in magic angle graphene

Observing the Chern mosaic and Berry curvature magnetism in magic angle graphene

a, Schematic layout showing the backgate voltage V_bg^dc + V_bg^ac applied to the MATBG sample and the corresponding change in the local magnetic field B_z^ac(x,y) is imaged using SOT scanning. The Chern mosaic is represented schematically in the MATBG. b, m_z (x,y,ν_↑) measured at B_a=50 mT and ν= 0.966. Red (blue) colors indicate local differential magnetization of the paramagnetic type (diamagnetic type). c, Chern mosaic map derived from the evolution of m_z (x,y,ν_↑) showing the intermediate regions C=1 (KB polarization, blue), C=-1 (KA, red) and C=0 or semi-metallic (green). Credit: Grover et al.

Researchers from the Weizmann Institute of Science, the Institute of Science and Technology in Barcelona, ​​and the National Institute of Materials Science in Tsukuba (Japan) recently probed a Chern mosaic topology and a curvature magnetism of Berry in magic angle graphene. Their article, published in Natural Physicsoffers new insights into the topological disorder that can occur in condensed matter physical systems.

“Magic-angle twisted bilayer graphene (MATBG) has attracted enormous interest in recent years due to its experimentally accessible flat bands, creating a playground of highly correlated physics,” said Matan Bocarsly, one of the researchers who conducted the study. , told Phys.org, “One such correlated phase observed in transport measurements is the quantum anomalous Hall effect, where topological edge currents are present even in the absence of an applied magnetic field. “

The quantum anomalous Hall effect is a phenomenon related to charge transport, in which the Hall resistance of a material is quantized to the so-called von Klitzing constant. It resembles the so-called integer quantum Hall effect, which Bocarsly and his colleague had studied extensively in their previous work, particularly on graphene and MATBG.

Building on their past findings, the researchers set out to further investigate the quantum anomalous Hall effect using the measurement tools they found most effective. To do this, they used a scanning superconducting quantum interference device (SQUID), which was fabricated on the top of a pointed pipette. This device is an extremely sensitive local magnetometer (i.e. a sensor that measures magnetic fields), which can collect images at the 100 nm scale.

“By varying the carrier density of our sample, we measured the local magnetic field response,” Bocarsly explained. “At low applied fields, this magnetic response is exactly correlated with the inner orbital magnetization of the Bloch wave functions, which is induced by the Berry curvature. So essentially we have a local probe that measures the local Berry curvature. “

Directly measuring the orbital magnetism induced by local Berry curvature in MATBG is a very challenging task, which had never been achieved before. Indeed, the signal is extremely weak, so it escapes most existing magnetic measurement tools.

Bocarsly and his colleagues were the first to directly measure this elusive signal. During their experiments, they also observed a Chern mosaic topology in their sample, thus identifying a new topological disorder in MATBG.

“The Chern number, or the topology of an electronic system, is generally considered to be a global topological invariant,” Bocarsly said. “We observed that at the device scale (micron order), the number C is not invariant, but alternates between +1 and -1. This introduces a new type of disorder, topological disorder, in condensed matter systems that must be considered for device fabrication and theoretical analysis.”

The recent study by this team of researchers greatly contributes to the understanding of MATBG, both in terms of its magnetism and its topology. In the future, this could inform the development of more accurate theoretical models of this material, while potentially facilitating its implementation in various quantum computing devices.

“Our low-field local orbital magnetization probe can also be used to study other fundamental properties such as local time reversal symmetry breaking,” Bocarsly added. “There are still many open questions about the integer filling states of MATBG and the symmetries they obey, which could be an interesting direction for future exploration.”


The direct detection of a topological phase transition by a sign change in the Berry curvature dipole


More information:
Sameer Grover et al, Chern mosaic and Berry curvature magnetism in magic angle graphene, Natural Physics (2022). DOI: 10.1038/s41567-022-01635-7

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Quote: The observation of Chern mosaic and Berry-curvature magnetism in magic angle graphene (2022, July 22) retrieved July 23, 2022 from https://phys.org/news/2022-07-chern-mosaic-berry-curvature-magnetism – magic.html

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