  We study the low energy properties of the one-orbital quantum double-exchange
model by using the slave fermion formulation. We construct a mean-field theory
which gives a simple explanation for the magnetic and thermodynamic properties
of the ferromagnetic metallic phase in manganites at low energy. The resulting
electron spectral function and tunneling density of states show an incoherent
asymmetric peak with weak temperature dependence, in addition to a
quasiparticle peak. We also show that the gauge fluctuations in the
ferromagnetic metallic phase are completely screened due to the Anderson-Higgs
mechanism. Therefore, the mean-field state is robust against gauge fluctuations
and exhibits spin-charge separation at low energy.
