Harnessing chirality for valleytronics

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Science  24 Oct 2014:
Vol. 346, Issue 6208, pp. 422-423
DOI: 10.1126/science.1260989

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One of the unusual electronic characteristics of graphene is that the direction of motion of its charge carriers is locked to an extra quantum mechanical degree of freedom, known as pseudospin. Graphene is in this way similar to the conducting surface layer of a topological insulator, where the direction of motion of the carriers is locked to their true spin—that is, to their magnetic moment. Whereas the true spin state of an electron can be described as a superposition of spin-up and spin-down components, pseudospin in graphene is a superposition of electron orbitals of the two carbon atoms in a hexagonal lattice unit cell. To date, graphene's pseudospin has played only a subtle role in carrier scattering, and in phenomena that are directly sensitive to the phase of a quantum mechanical wave function. On page 448 of this issue, Gorbachev et al. (1) show that pseudospin, modified in the right way, can be used to drive a so-called valley current in a graphene device. That is, a voltage applied across the device gives rise to counter-propagating streams of carriers in graphene's two band structure valleys. By harnessing its built-in chirality, an all-electrical valleytronic circuit using graphene is demonstrated.