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In most conventional materials, such as copper and silicon, electrons move about the
lattice independently, effectively ignoring each other. Despite some idiosyncrasies from
being an atomically thin layer of carbon, graphene is no exception to this behavior. If
we stack two sheets of graphene on top of each other, we might expect the composite
system would behave similarly to two copies of monolayer graphene. Remarkably, this
intuition is completely wrong.
If the two layers are stacked with a relative twist near one degree, they hybridize to
form new electronic bands with the remarkable property that all the electrons have
nearly the same kinetic energy. Freed to fill states in those bands without regard to
kinetic energy, electrons can collectively arrange themselves to minimize their mutual
Coulombic repulsion. This may explain the superconductivity surprisingly seen in such
stacks. Here, we will discuss the discovery that, despite containing none of the
traditional magnetic elements, twisted bilayer graphene can become magnetic. Unlike
conventional magnets, where magnetism arises from the ordering of electron spins,
twisted bilayer graphene’s magnetic state originates from perpetually swirling current
loops. Such "orbital magnetism" has now been seen in multiple different stacks of
atomically-thin materials.