Researchers at the Weizmann Institute of Science, the Barcelona Institute of Science and Technology and the National Institute for Materials Science in Tsukuba (Japan) recently investigated the topology of the Chern mosaic and Berry curvature magnetism in magic angle graphene. Their paper was published in Nature Physicsprovides new insight into the topological perturbation that can occur in condensed matter physical systems.
One of these interrelated phases observed in transport measurements is the quantum anomalous Hall effect, where topological edge currents exist even in the absence of an applied magnetic field, Matan Bokarsli, one of the researchers who conducted the study, told Phys.org.
The anomalous quantum Hall effect is a phenomenon associated with charge transfer, in which the Hall resistance of matter is measured by what is called the von Klitzing constant. It is similar to the so-called correct quantum Hall effect, which Bucarsley and colleagues have extensively studied in their previous work, particularly in graphene and MATBG.
Building on their previous results, the researchers set out to further investigate the anomalous quantum Hall effect using the measurement tools they found to be most effective. To do this, they used a scanning quantum interference superconducting device (SQUID), which was made on top of a sharp pipette. This device is a highly sensitive local magnetometer (that is, a sensor that measures magnetic fields), which can collect images at a scale of 100 nanometers.
“By varying the carrier density of our sample, we measured the response of the local magnetic field,” Bucarsley explained. “At low applied fields, this magnetic response is completely related to the internal orbital magnetization of the Bloch wave functions, which is caused by Berry’s curvature. So, in essence, we have a local probe that measures the local Berry curvature.”
Direct measurement of the orbital magnetism resulting from the local Berry curvature in MATBG is a very challenging task, and has not been achieved before. This is because the signal is very weak, and therefore elusive to most current magnetometers.
Bucarsley and colleagues were the first to directly measure this elusive signal. During their experiments, they also observed a chirn mosaic topology in their sample and thus identified a new topological disorder in MATBG.
“It is generally believed that the Chirn number, or the topology of an electronic system, is a universal topological constant,” Bucarsley said. “We have observed that on the device scale (the order of microns), the number C is not a constant, but rather alternates between +1 and -1. This introduces a new type of disorder, the topological disorder, into condensed matter systems that needs to be accounted for in device fabrication and theoretical analysis. “.
The latest study by this team of researchers contributes significantly to understanding MATBG, both in terms of magnetism and structure. In the future, it could help develop more accurate theoretical models of this substance, with the potential to facilitate its implementation in many quantum computing devices.
“The Low Field Local Orbital Magnetization Probe can also be used to investigate other fundamental properties such as local time inversion symmetry breaking,” Bucarsley added. “There are still many open questions about the integer-population states of MATBG and the symmetries they are subject to, which could be an interesting direction to explore in the future.”
Direct detection of a topological phase transition by a sign change in Berry dipole curvature
Samir Grover et al., Chern mosaic and Berry bend magnetism in magic-angle graphene, Nature Physics (2022). DOI: 10.1038 / s41567-022-01635-7
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