Al-substitution effect on charge transport in La1-xSrxMnO3: Incoherent metallic state in a double-exchange ferromagnet

Resistivity rho(T) was measured on (La1-xSrx)(Mn1-zAlz)O-3 single crystals over a temperature T range 4-1000 K The doped Al decreases Curie temperatur e T-C and increases rho(T). At low temperatures, the resistivity shows the upturn when the residual resistivity reaches the critical value estimated a bout 300 mu Omega cm. This upturn is most likely to originate from the carr ier localization and therefore mean free path I reaches Fermi wavelength la mbda(F) at this critical resistivity. This suggests that the basic assumpti on of an ordinary metal, l>lambda(F), is violated when rho(T) exceeds this critical value even though rho(T) is metallic (d rho/dT>0). This "coherent- incoherent" crossover is also suggested by the recent optical studies. The metallic rho(T) above T-C, on the other hand, becomes insulating by only a few percent Al. The conduction above T-C is dominated by a short-range corr elation between the t(2g) spins.