We construct an effective Hamiltonian for bilayer quantum Hall ferromagnets, where the SU(4) nonlinear sigma model supports the SU(4) quantum coherence, At v = 1 there are three independent soft waves and three basic skyrmions. At v = 2 there exist two phases in each of which there are one soft wave and one biskyrmion. Interlayer tunneling occurs via the pseudospin soft mode not only at v = 1 but also at v = 2. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 18]
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