Caltech Young Investigator Lecture
If we stack two sheets of graphene, atomically thin carbon, on top of each other, we might expect the
composite system would behave similarly to two copies of monolayer graphene. Remarkably, this
intuition fails completely for electronic properties. If the two graphene lattices are stacked with a slight
twist, they drift in and out of registry, forming a periodic pattern called a moiré superlattice with a period
much larger than the lattice constant. When twisted by one degree, so called magic-angle twisted bilayer
graphene (TBG), the two layers of graphene hybridize to form nearly flat electronic bands with a total
bandwidth of approximately 10 millielectronvolts.
In many conventional materials, such as aluminum and silicon, electrons ignore each other and move
about the lattice independently. However, because of the narrow bandwidth of TBG, electrons become
well localized on the moiré superlattice and will arrange themselves collectively to minimize the system's
total energy. As the electrons aim to avoid each other at different fillings of the lattice, TBG exhibits
superconductivity and other interesting states. Here we present evidence that these strongly enhanced
interactions can drive TBG into a ferromagnetic state that can be electronically measured via an extremely
large anomalous Hall effect: a measurable Hall resistance that persists to zero applied magnetic field.
Graphene has no magnetic elements, suggesting that this is a new kind of magnetism that may be
potentially useful for metrology or ultra-low power electronics.
More about the Speaker:
Aaron Sharpe is a PhD candidate in Applied Physics at Stanford University, working in David
Goldhaber-Gordon's group. He received his Bachelor's in Physics from Rice University in 2014. His
research interests focus on exploring complex emergent behavior in quantum materials, specifically van
der Waals heterostructures. His work has helped provide experimental insights about systems where, with
the appropriate choices, the electrons become strongly interacting, a regime where it is theoretically
challenging to make predictions. Understanding how electrons interact is crucial for understanding
phenomena such as superconductivity and magnetism, and may have far-reaching technological
implications for areas such as quantum computation.
This lecture is part of the Young Investigators Lecture Series sponsored by the Caltech Division of Engineering & Applied Science.