Tuning the pseudospin polarization of graphene by a pseudo-magnetic field

TL;DR

This paper demonstrates how mechanical strain in graphene induces a pseudo-magnetic field that causes sublattice symmetry breaking, revealing pseudospin polarization, with potential applications in valleytronics.

Contribution

It provides experimental evidence of pseudospin polarization in graphene caused by strain-induced pseudo-magnetic fields, measured via sublattice symmetry breaking using STM.

Findings

Pseudo-magnetic fields up to 1000 T were estimated.

Sublattice symmetry breaking scales with deformation height.

Strain-induced pseudo-magnetic fields can be tuned locally.

Abstract

One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudo-magnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudo-magnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudo-magnetic 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 pseudo-magnetic 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 SiO$_2$ 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 pseudo-magnetic 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 pseudo-magnetic 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.