Dawei Zhai

Tuning the Pseudospin Polarization of Graphene by a Pseudomagnetic Field

One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudomagnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudomagnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudomagnetic field is a local sublattice symmetry breaking observable as a redistribution of the local density of states. This can be interpreted as a polarization of graphenes pseudospin due to a strain induced pseudomagnetic field, in analogy to the alignment of a real spin in a magnetic field. We reveal this sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunneling microscope. The tip locally lifts the graphene membrane from a SiO2 support, as visible by an increased slope of the I(z) curves. The amount of lifting is consistent with molecular dynamics calculations, which reveal a deformed graphene area under the tip in the shape of a Gaussian. The pseudomagnetic field induced by the deformation becomes visible as a sublattice symmetry breaking which scales with the lifting height of the strained deformation and therefore with the pseudomagnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models, revealing fields of 1000 T. These results might be the starting point for an effective THz valley filter, as a basic element of valleytronics.

Quantum wires and waveguides formed in graphene by strain

Confinement of electrons in graphene to make devices has proven to be a challenging task. Electrostatic methods fail because of Klein tunneling, while etching into nanoribbons requires extreme control of edge terminations, and bottom-up approaches are limited in size to a few nanometers. Fortunately, its mechanical flexibility raises the possibility of using strain to alter graphenes properties and create novel straintronic devices. Here, we report transport studies of nanowires created by linearly-shaped strained regions resulting from individual folds formed by layer transfer onto hexagonal boron nitride. Conductance measurements across the folds reveal Coulomb blockade signatures, indicating confined charges within these structures, which act as quantum dots. Along folds, we observe sharp features in traverse resistivity measurements, attributed to an amplification of the dot conductance modulations by a resistance bridge incorporating the device. Our data indicates ballistic transport up to ∼1 μm along the folds. Calculations using the Dirac model including strain are consistent with measured bound state energies and predict the existence of valley-polarized currents. Our results show that graphene folds can act as straintronic quantum wires.

Correction to Tuning the Pseudospin Polarization of Graphene by a Pseudomagnetic Field

T following NSF grant number should be added to the Acknowledgment: DMR-1508325 (D.F., D.Z., and N.S.). With this, the correct Acknowledgment section should read: We acknowledge discussions with M. I. Katsnelson, A. Bernevig, M. Kra ̈mer, W. Bernreuther, F. Libisch, C. Stampfer,and C. Wiebusch, assistance at the STM measurements and sample preparation by C. Pauly, C. Saunus, S. Hattendorf, V. Geringer. We acknowledge financial support by the Graphene Flagship (Contract No. NECT-ICT-604391) and the German Research Foundation via Li 1050/2-2 (A.G., P.N.I., M.P., M.L. and M.M.); DFG SPP 1459 and the A. v H. Foundation (M.S., S.V.K.); CNPq No.150222/2014-9 (D.F.); NSF No. DMR-1108285 (D.F., R.C-B., D.Z., and N.S.) and DMR-1508325 (D.F., D.Z., and N.S.); PRODEP (R.C.B). FNR Luxembourg INTER/ANR/13/20/NANOTMD (L.W).