THERMOPOWER AND NATURE OF THE HOLE-DOPED STATES IN LAMNO3 AND RELATEDSYSTEMS SHOWING GIANT MAGNETORESISTANCE

We have measured the thermopower (S) of hole-doped LaMnO3 systems in o rder to see its dependence on the Mn4+ content as well as to investiga te other crucial factors that determine S. We have carried out hole do ping (creation of Mn4+ by two distinct means, namely, by the substitut ion of La by divalent cations such as Ca and Sr and by self-doping wit hout aliovalent substitution). The thermopower is sensitive not only t o the hole concentration but also to the process employed for hole dop ing, which we explain as arising from the differences in the nature of the hole-doped states. We also point out a general trend in the depen dence of S on hole concentration at high temperatures (T> T-c), simila r to that found in the normal-state thermopower of the cuprates.