Re-entrant charge-ordering behaviour in the layered manganites La_2-2xSr_1+2xMn₂O₇

We have investigated the charge-ordering (CO) behaviour of the layered manganites La_2-2xSr_1+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.