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 graphene’s 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.