Just as a ratchet allows rotation in one direction but not the other, quantum ratchets break the symmetry of a microscopic system to facilitate preferential motion in one direction or another. Graphene is a two-dimensional hexagonal lattice of carbon atoms. As such, it’s highly symmetrical, but beneath that lurks a potentially exploitable hidden asymmetry. If you add hydrogen atoms (for example) to the top of graphene, an applied alternating current (in the form of a microwave-frequency light wave) induces electrons to flow preferentially one direction: a quantum ratchet.
The reason for this striking change in behavior is due to what’s called a structure inversion asymmetry in graphene. In the presence of an external influence—in this case, the introduction of hydrogen atoms and a strong magnetic field—the shape of the electron orbits in the carbon atoms gets distorted in one direction. When exposed to the oscillating electric field, the electrons felt a strong resistive force in one direction (which the authors liken to friction), but increased mobility in the opposite direction. [Read more…]