  In strained graphene, lattice deformation can create pseudo-magnetic fields
and result in zero-field Landau level-like quantization. In the presence of an
external magnetic field, valley-polarized Landau levels are predicted to be
observed because the pseudo-magnetic fields are of opposite directions in the K
and K' valleys of graphene. Here, we present experimental spectroscopic
measurements by scanning tunneling microscopy of strained graphene on Rh foil.
We direct observed valley splitting of the Landau level induced by the
coexistence of the pseudo-magnetic fields and external magnetic fields. The
observed result paves the way to exploit novel electronic properties in
graphene through the combination of the pseudo-magnetic fields and the external
magnetic fields.
