Atomic Valley Filter Effect Induced by an Individual Flower Defect in Graphene

Owing to the bipartite nature of honeycomb lattice, the electrons in graphene host valley degree of freedom, which gives rise to a rich set of unique physical phenomena including chiral tunneling, Klein paradox, and quantum Hall ferromagnetism. Atomic defects in graphene can efficiently break the local sublattice symmetry, and hence, have significant effects on the valley-based electronic behaviors. Here we demonstrate that an individual flower defect in graphene has the ability of valley filter at the atomic scale. With the combination of scanning tunneling microscopy and Landau level measurements, we observe two valley-polarized density-of-states peaks near the outside of the flower defects, implying the symmetry breaking of the K and K ' valleys in graphene. Moreover, the electrons in the K valley can highly penetrate inside the flower defects. In contrast, the electrons in the K ' valley cannot directly penetrate, instead, they should be assisted by the valley switch from the K ' to K. Our results demonstrate that an individual flower defect in graphene can be regarded as a nanoscale valley filter, providing insight into the practical valleytronics.