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Re-entrant charge-ordering behaviour in the layered manganites La2-2xSr1+2xMn2O7

We have investigated the charge-ordering (CO) behaviour of the layered manganites La2-2xSr1+2xMn2O7 (0.3⩽x⩽0.8) by means of transport and magnetization measurements. It is found that the CO transitions occur in the very high-temperature region (200-340 K) and moreover the re-entrant CO behaviours are exhibited over a broad doping range (0.47⩽x⩽0.62). The electronic phase diagram based on the transport data shows features similar to those suggested by a two-dimensional Hubbard model with electron-phonon interaction (Yuan Q and Thalmeier P 1999 Phys. Rev. Lett. 83 3502). Our present results reveal that the temperature-dependent polaron bandwidth plays a crucial role as regards the occurrence of re-entrant CO transitions in the layered manganites.