Caltech Young Investigator Lecture

Abstract:

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.