DIRECT MEASUREMENT OF THE CHARGE ORDERING GAP IN ND0.5SR0.5MNO3 USINGVACUUM TUNNELING

We have used tunnelling spectroscopy (TS) to probe the density of stat es (DOS) of the rare earth manganate Nd0.5Sr0.5MnO3, which shows a tra nsition to a charge ordered (CO) antiferromagnetic insulating state fr om a ferromagnetic metallic state. We find that a gap opens up in the DOS around the Fermi level, E-F, as the solid is cooled below the char ge ordering temperature T-CO. The charge ordering gap (Delta(CO)) was found to be strongly temperature dependent for T approximate to T-CO a nd for T/T-CO < 0.6 it reaches a limiting value of nearly 0.27 eV. Abo ve T-CO the temperature dependence of the DOS at E-F was found to be s imilar to that of the solids such as La0.8Ca0.2MnO3 which show colossa l magnetoresistance but no charge ordering.