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Valleytronics in graphene

Seminar by Carlo Beenakker, Instituut-Lorentz for Theoretical Physics, Leiden University.

Abstract
Conduction and valence bands in graphene form conically shaped alleys, touching at a point called the Dirac point. There are two inequivalent Dirac points in the Brillouin zone, related by time-reversal symmetry. Intervalley scattering is suppressed in pure samples. The independence and degeneracy of the valley degree of freedom suggests that it might be used to control an electronic device, in much the same way as the electron spin is used in spintronics or quantum computing. We discuss three building blocks of this emerging field of "valleytronics".

The first building block provides a controllable way of occupying a single valley in graphene, thereby producing a valley polarization. Such a valley filter can be formed by a ballistic point contact with zigzag edges. The polarity can be inverted by local application of a gate voltage to the point contact region. Two valley filters in series may function as an electrostatically controlled valley valve. This second building block represents a zero-magnetic-field counterpart to the familiar spin valve. Thirdly, a detector of valley polarization can be created by reflecting electrons from one valley as holes from the other valley at the interface with a superconductor.

Based on research reported in:

[1] A. Rycerz, J. J. Tworzydlo, and C. W. J. Beenakker, Nature Physics 3, 172 (2007).

[2] A. R. Akhmerov and C. W. J. Beenakker, Phys. Rev. Lett. 98, 157003 (2007).