The effect of high polaron concentration on the polaron transport in NbO2.5-x: optical and electrical properties

The electrical conductivity and optical absorption of NbO 2.5-x (0.005<x<0.08) are described by polaron ground states. Correlated hopping and polaronic absorption of weakly disordered systems are observed in the regime of low carrier concentration (x<0.05). Bipolarons and single polarons are both stable in this regime. At higher carrier concentrations, quasi-metallic conductivity and saturated polaron concentrations are described as being due to the superposition of polaron groundstates and quasi-free electrons. The saturation concentration yields a critical distance of about 10 AA between polarons. The threshold value of x=0.05 is related to the geometrical properties of the NbO 2.5-x structures.