Itinerant versus localized transport in La1-xSrxMnO3 for x <= 0.175

The complex dielectric function epsilon(1) + i epsilon(2) of La1-xSrxMnO3 h as been investigated for concentrations 0.1 less than or equal to x less th an or equal to 0.175 near the metal-to-insulator transition. Quasioptical s pectroscopy technique were employed for frequencies 100 GHz less than or eq ual to nu less than or equal to 1100 GHz and temperatures 10 K less than or equal to T less than or equal to 300 K. Both, the dielectric constant Ei a nd the conductivity sigma(1) = epsilon(0)epsilon(2)omega Of all samples wer e found to increase on cooling through the magnetic-ordering transition. Fr om this observation and from the analysis of the frequency dependencies we conclude that the charge carriers are localized in a broader range of the p hase diagram than generally accepted. We show that hopping or tunneling bet ween localized states dominates the conductivity for Sr concentrations x le ss than or equal to 0.15 and temperatures T<300 K. Even for La0.725Sr0.175M nO3 the localization effects are observed at least 10 K below the magnetic- phase transition. Finally, the dielectric constant is observed to diverge o n approaching the metal-insulator transition. [S0163-1829(99)02519-9].