Li-Qiao Xia—Engineering moiré ferromagnetism with helical trilayer graphene

Li-Qiao Xia, a PhD Candidate in the Department of Physics at MIT, delivered the Nano Explorations talk on December 5, 2023.

Two-dimension van der Waals moiré materials recently emerged as highly-tunable platforms for studying various exotic electronic states. Among them, orbital magnetism and anomalous Hall effects (AHE) have been observed in several systems with broken C2z symmetry. Existing C2z-symmetry-broken moiré systems include at least one constituent layer that lacks this symmetry on its own.

This talk introduces magic-angle helical-twisted trilayer graphene, which acts as the first example of breaking C2z symmetry locally on the moiré scale using C2z-symmetric constituent layers. The electrical transport measurements reveal correlated states and AHE with the highest reported Curie temperature among graphene-moiré systems, which shows the non-trivial band topology, and correlation-driven spontaneous time-reversal symmetry breaking. The hysteretic switching of magnetic states upon density sweeping points to first-order phase transitions across a spatial mosaic of Chern domains reflects the supermoiré-scale C2z symmetry.

This talk looks to the future of how engineering multiple moiré lattices could lead to emergent broken symmetries and flat topological bands, and stimulate the engineering of new moiré structures.

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Nano Explorations is a virtual seminar series powered by MIT.nano that features presentations by MIT students and postdocs on their work in nanoscience, nanotechnology, and other advanced research fields. The series was launched as a way to keep our nano community connected during the COVID-19 pandemic. Presentations take place once a month and are open to any interested viewers.

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