Charge Excitations in Easy-Axis and Easy-Plane Quantum Hall Ferromagnets

We study charge excitations in quantum Hall ferromagnets realized in a symmetric quantum well. Landau levels (LLs) with different subband and orbital indices crossing at the Fermi level act as up and down pseudospin levels. The activation energy measured as a function of the pseudospin Zeeman energy, Δ_Z, reveals easy-plane and easy-axis ferromagnetism for LL filling of ν = 3 and 4, respectively, for which the crossing levels have parallel and antiparallel spin. For ν = 4, we observe a sharp reduction in the gap for Δ_Z→0, which we discuss in terms of a topological excitation in domain walls akin to Skyrmions.