Sommaire:

In the presence of a strong magnetic field, and for an integer filling of the Landau levels, Coulomb interactions favor a ferromagnetic ground-state. It has been shown already twenty years ago that when extra charges are added or removed to such system, the ferromagnetic state becomes unstable and is replaced by spin textures called Skyrmions. We (*) have generalized this notion to an arbitrary number d of internal states for the electrons, which may correspond to the combination of spin, valley, or layer indices. We first consider a fully SU(d) symmetric model, that differs from the usual CP(d-1) model by the long ranged Coulomb interaction. This additional term lifts the massive ground-state degeneracy present in the short range CP(d-1) model, and selects a hexagonal Skyrmion lattice which spontaneously breaks the underlying SU(d) symmetry. The ground state charge density modulation, which inevitably exists in these lattices, vanishes exponentially in d. We compute analytically the complete low-lying excitation spectrum, which separates into d2-1 gapless acoustic magnetic modes and a magnetophonon. We discuss the role of effective mass anisotropy for SU(3) valley Skyrmions relevant for experiments with AlAs quantum wells. Here, we find a transition, which breaks a six-fold rotational symmetry of a triangular lattice, appearing at weak anisotropy, followed by a formation of a square lattice at large values of anisotropy strength. (*) This work is the result of a collaboration with D. Kovrizhin and R. Moessner

Pour plus d'informations, merci de contacter Moultaka G.