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Scientific Method / Science & Exploration

A magnetic quantum ratchet made of graphene turns AC to DC

Graphene's orbitals shift to allow current to flow in only one direction.

A ratcheting socket wrench happily turns one way, but resists rotation in the opposite direction. A magnetic quantum ratchet allows flow of electrons one way, but not the other.

In a common type of mechanical ratchet, back and forth motion provided by a human arm gets converted to rotating motion that acts to tighten a screw or bolt. The role of the ratchet is to convert a force that changes direction into a torque acting in one direction only. That principle is generalized in many other systems that convert fluctuations (some of which may be random) into usable work. Many types of ratchets exist, in mechanical, quantum, and biological systems.

Researchers have now fabricated a magnetic quantum ratchet out of graphene, a two-dimensional hexagonal lattice of carbon atoms. C. Drexler and colleagues introduced asymmetries in the electronic structure by disrupting graphene's structure with hydrogen and modifying the substrate on which the carbon sat. When they exposed the modified graphene to an alternating electric current and a strong magnetic field, its electrons preferentially moved in one direction, setting up a directed current. So the modified graphene acted as an AC/DC converter. Although it's not practically useful, the behavior may tell us more about the rules that govern graphene-like materials.

Under ordinary circumstances, graphene is a symmetrical hexagonal lattice of carbon atoms. When exposed to an alternating electric current, the electrons oscillate, producing no direct current on average. Similarly, imposing a steady magnetic field in the presence of the alternating current alters the electronic properties of the graphene slightly, but doesn't tend to make the electrons move preferentially in one direction.

However, the researchers found that introducing even a relatively small number of hydrogen atoms on top of the graphene changed the situation. The same oscillating electric current and steady magnetic field produced a regular flow of electrons. They found a similar effect by modifying the substrate—the material underlying the graphene lattice.

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.

In the analogy used in the research paper, the alternating current acts like the wheel of the ratchet, while the distorted electron orbits act like the pawl, which stops the ratchet from rotating in one direction but not the other. Due to its source and character, in graphene this is known as a magnetic quantum ratchet.

Given how simple it is as a substance, the existence of a structure inversion asymmetry is potentially interesting for understanding graphene's novel electronic and magnetic properties. Other systems exhibiting such behavior are three-dimensional lattices, while graphene is two-dimensional. The authors proposed examining other graphene-like two-dimensional structures, such as boron nitride.

As a practical means of making direct current out of oscillating electric currents, this experiment leaves much to be desired. The electric current was from a 3.34 terahertz (3.34 THz, or 3.34 × 1012 Hz) electromagnetic wave, which falls at the border between microwave and infrared light; household alternating current in the US is 60 Hz. Similarly, the magnetic field the researchers used was 7 teslas, more than twice the strength of a powerful MRI magnet.

However, the purpose of this experiment was the exploration of the electronic properties of graphene, not direct application. Nevertheless, the hidden structure inversion symmetry could help us understand some of the material's behavior, and pave the way to improved magnetic and electron-spin technology that uses graphene and graphene-like substances.

Nature Nanotechnology, 2013. DOI: 10.1038/nnano.2012.231  (About DOIs).Imp

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