Scanning tunneling spectroscopy of quantum Hall ferromagnetic states in graphene

Under strong magnetic fields, cyclotron motion of two-dimensional electrons causes Landau quantization. In graphene, the quantized Landau levels are four-fold degenerate, due to spin and valley degrees of freedom. Electron-electron interactions, however, breaks the Landau level degeneracy, and establish quantum Hall ferromagnetic (QHFM) states. The unique SU(4) symmetry breaking of QHFM states in graphene have attracted extensive studies, mostly using transport and capacitance techniques. However, these studies only probe the system near the Fermi energy. Exotic excitations of QHFM in graphene, such as skyrmions, still wait to be explored. In this work, employing scanning tunneling microscope, we measure tunneling spectrum as a function of electron filling. We demonstrate well-developed symmetry breaking QHFM states at all integer fillings and directly measure the exchange energy. Besides Laudau levels, we discover rich spectral features stemming from the symmetry broken gaps. These excitation features near /nu=-1 and /nu=1 are very similar and are distinctly different from excitation features near \nu=0, suggesting they are related to the nature of the symmetry breaking.