Topological excitations in quantum Hall ferromagnets

Single-layer quantum Hall systems at filling factor nu = 1 spontaneously develop ferromagnetic spin order. In addition to collective spin wave excitations, such itinerant ferromagnets contain charged 'skyrmion' like topological excitations. As the filling factor deviates from unity, the spin polarization rapidly collapses due to the creation of an increasing density of skyrmions. This has recently been observed experimentally by Barrett et al.. Double-layer quantum Hall systems exhibit analogous effects with layer index playing the role of spin quantum number. Due to the capacitive charging energy of the layers, such 'pseudo-spin' systems have an easy-plane anisotropy and thus are XY ferromagnets. Spontaneous interlayer phase coherence has recently been observed experimentally in double-layer systems by Murphy et al. and related wide single well systems by Santos et al..