Quantum Hall skyrmions at ν=0,±1 in monolayer graphene

Monolayer graphene under a strong perpendicular magnetic field exhibits quantum Hall ferromagnetism with spontaneously broken spin and valley symmetry [K. Nomura and A.H. MacDonald, Phys. Rev. Lett. 96, 256602 (2006)]. The approximate SU(4) spin/valley symmetry is broken by small lattice-scale effects in the central Landau level corresponding to filling factors. = 0,+/- 1. Notably, the ground state at. = 0 is believed to be a canted antiferromagnetic (AF) or a ferromagnetic (F) state depending on the component of the magnetic field parallel to the layer and the strength of small anisotropies. We study the skyrmions for the filling factors. = +/- 1, 0 by using exact diagonalizations on the spherical geometry. If we neglect anisotropies, we confirm the validity of the standard skyrmion picture generalized to four degrees of freedom. For filling factor. = -1, the hole skyrmion is an infinite-size valley skyrmion with full spin polarization because it does not feel the anisotropies. The electron skyrmion is also always of infinite size. In the F phase it is always fully polarized, while in the AF phase it undergoes continuous magnetization under increasing Zeeman energy. In the case of nu = 0, the skyrmion is always maximally localized in space both in F and AF phases. In the F phase it is fully polarized, while in the AF phase it has also progressive magnetization with Zeeman energy. The magnetization process is unrelated to the spatial profile of the skyrmions, contrary to the SU(2) case. In all cases, the skyrmion physics is dominated by the competition between anisotropies and Zeeman effect but not directly by the Coulomb interactions, breaking universal scaling with the ratio Zeeman to Coulomb energy.