The Electronic Structure, Fermi Surface and Pseudogap in Manganites

Abstract

This chapter presents an overview of the electronic structure and the Fermi Surface of the colossal magnetoresistive (CMR) oxides, with an emphasis on recent angle resolved photoemission results of the bilayer manganite La2–2x Sr1+2x Mn2O7. These experiments have now advanced to the stage that the low energy excitations very near the Fermi level can be directly measured. The key transport parameters and energy scales can be directly determined from the data. One of the critical outcomes of these and other experiments is the observation and study of a pseudogap: a suppression of spectral weight near the Fermi energy. This effect is shown to be crucial in understanding the very high resistivities as well as the CMR effect. We argue that this pseudogap is likely to be an unusual type of fluctuating charge density wave (CDW) gap, in which the variations in charge density cooperate with the lattice distortions (e.g. Jahn-Teller distortions) as well as with the spin degrees of freedom (double exchange). An other important factor consistent with this data appears to be nanoscale electronic phase separation.