Observation of non-conventional spin waves in composite fermion   ferromagnets
  We find unexpected low energy excitations of fully spin-polarized composite-fermion ferromagnets in the fractional quantum Hall liquid, resulting from a complex interplay between a topological order manifesting through new energy levels and a magnetic order due to spin polarization. The lowest energy modes, which involve spin reversal, are remarkable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin-flip excitonic state of composite-fermion particle-hole pairs. The striking properties of spin-flip excitons imply highly tunable mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets. 
U. Wurstbauer, S. S. Mandal, D. Majumder, I. Dujovne, T. D. Rhone, B.   S. Dennis, A. F. Rigosi, J. K. Jain, A. Pinczuk, K. ...
cond-mat.str-el