Low-energy properties of the ferromagnetic metallic phase in manganites: Slave-fermion approach to the quantum double-exchange model

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.