Spin-Pseudospin Coherence and CP$^{3}$ Skyrmions in Bilayer Quantum Hall Ferromagnets

We analyze bilayer quantum Hall ferromagnets, whose underlying symmetry group is SU(4). Spin-pseudospin coherence develops spontaneously provided both the Zeeman and tunneling interactions are small. Quasiparticles are CP$^{3}$ skyrmions. It is shown that it possesses the electric charge $\nu e$ at the filling factor $\nu \leq 2$. Hence, the elementary charge of charged excitations is $e$ at $\nu =1$ but $2e$ at $\nu =2$. Recent experimental data due to Sawada et al. [PRL80(1998)4534] support this conclusion.