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Observation of the Chern tiling and Berry curvature magnetism in graphene at a magic angle

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

Researchers from the Weizmann Institute of Science, the Barcelona Institute of Science and Technology and the National Institute of Materials Science in Tsukuba (Japan) have recently investigated the Chern tiling topology and Berry curvature magnetism in magic angle graphene. Their article, published in Physics of natureoffers new insights into the topological disorder that can occur in condensed state physical systems.

“Magic Angle Twisted Bilayer Graphene (MATBG) has attracted huge interest over the past few years due to its experimentally accessible flat bands creating a playground for highly correlated physics,” Matan Bokarsli, one of the researchers who led the study. , said 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 charge-transfer phenomenon in which the Hall resistance of a material is quantized to what is known as the von Klitzing constant. It is reminiscent of the so-called integer quantum Hall effect, which Bocarsley and colleagues have extensively studied in their previous work, especially in graphene and MATBG.

Based on their past findings, the researchers set out to further study 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 tip of a sharp pipette. This device is an extremely sensitive local magnetometer (that is, a sensor that measures magnetic fields) that can collect images at a scale of 100 nm.

“By varying the carrier density in our sample, we measured the response of the local magnetic field,” Bocarsley explained. “At low applied fields, this magnetic response correlates exactly with the intrinsic orbital magnetization of the Bloch wavefunctions, 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 caused by Berry’s local curvature in MATBG is a very difficult task that has never been solved before. This is because the signal is extremely weak, so it eludes most existing magnetic measurement tools.

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

“The Chern number, or the topology of an electronic system, is usually considered a global topological invariant,” Bocarsley said. “We noticed that at the scale of the device (on the order of microns), the C number is not constant, but fluctuates between +1 and -1. This introduces a new type of disorder, topological disorder, into condensed matter systems that must be taken into account. for device fabrication and theoretical analysis”.

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

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


Direct detection of a topological phase transition by sign reversal of a Berry curvature dipole


Additional Information:
Samir Grover et al., Chern Mosaic and Berry Curvature Magnetism in Magic Angle Graphene, Physics of nature (2022). DOI: 10.1038/s41567-022-01635-7

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Quote: Observation of Chern Mosaic and Berry Curvature Magnetism in Graphene at Magic Angle (July 22, 2022), retrieved July 23, 2022 from https://phys.org/news/2022-07-chern-mosaic-berry-curvature- magnetism-. magic.html

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