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…]

Turning graphene into an AC/DC quantum ratchet


Scanning electron micrograph of graphene. [Credit: Lawrence Berkeley National Laboratory]

In his science fiction novel The Diamond Age, Neal Stephenson describes a world filled with electronic paper, tiny flexible computers, and transparent displays. Graphene—a crystal comprised of a single layer of carbon atoms—is perhaps the most promising material to make that world real (though hopefully without the universal surveillance state and environmental collapse that are also part of the book). However, promises aren’t the same thing as practical technology, so it behooves us to take a critical look at what graphene can and can’t currently do.

…Nobel Laureate Konstantin Novoselov and colleagues have written a critical, yet optimistic, assessment of the state of graphene research and production.

As they point out, there is a big question that must be answered before widespread adoption of graphene technology is possible: are graphene’s advantages sufficient to use it in place of the materials we use in existing devices? The authors conclude that, to some extent, that’s the wrong question. Graphene’s biggest promise lies in novel applications, designed especially for the advantages that graphene offers. [Read more…]

Not quite ready for the Diamond Age