Schematic of quantum Hall edge states in graphene probed by an atomic force microscope. In the interior of the graphene sheet, electrons under the influence of a perpendicular applied magnetic field are constrained to undergo circular motion. This turns the interior of the graphene sheet into a poor electrical conductor and even into an insulator under certain conditions. In contrast, along the boundary of the graphene, so-called topologically protected edge states provide runways for charge carriers to flow easily, creating conducting edge channels.
Credit: Sungmin Kim/NIST
They may not be impervious to bullets like Superman, but groups of electrons that gather along the edges of some ultrathin materials have their own superpowers. Defying such disturbances as bending, stretching, the introduction of an external magnetic field, and distortions that wreck the orderly arrangement of atoms in a crystal, these groups of electrons, known as quantum Hall (QH) edge states, still maintain their properties and positions.
The persistence of QH edge states makes for some weird and wonderful physics. For instance, the states help explain why the interior of atomically thin layers of some materials act as insulators, preventing the flow of electric current, while their edges are excellent conductors — enabling current to flow freely along the perimeter. The discovery of these materials, known as topological insulators, and the explanation for how they work, have garnered two Nobel Prizes in Physics.
In addition, physicists are exploring whether the durability of the QH edge states may qualify them as good candidates for quantum bits, or qubits, the fundamental unit of quantum information. Although qubits can carry much more information than classical computer bits, their quantum properties can easily be disturbed, destroying the data they ..
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