Identification of a new type of electronic state in the magnetoresistive orthomanganites

PEROVSKITES of composition Ln(1-x)q(x)MnO(3) (where Ln and A are rare-earth and alkaline-earth elements respectively) have become the focus of scientific and technological interest because of their extraordinary electronic and magnetic properties, For example, compositions with x = 0.3 exhibit an unusual combination of ferromagnetism and good electrical conductivity(1); moreover. the observation(2) of a 'colossal' magnetoresistive response near the Curie temperature (T-C), which can be changed dramatically by altering the strength of the Mn-O-Mn interactions(3,15), points to potential practical applications for these materials. Recent resistivity studies? of these materials have suggested that the accepted 'tight-binding' band description(5) of the conduction electrons below T-C may be inadequate and that strong interactions between the electrons and dynamic local lattice distortions associated with the Mn sites may be important. Here we report resistivity and thermopower measurements at high pressure, which indicate a more exotic origin for the electronic properties of these materials. We find that below T-C the conduction electrons lose their localized character and condense into extended electronic states that exhibit no energy dispersion, We attribute the formation of this new state to the strong coupling of the conduction electrons to cooperative oxygen vibrations along the Mn-O bond axes.