| La2-2xSr1+2xMn2O7 is a typical colossal magnetoresistive oxide, showing a drastic transition from a low-temperature metal to a high-temperature insulator near 130K. The famous CMR (colossal magnetoresistive) effect accompanies the metal-insulator transition, though the mechanism for the metal-insulator transition and the CMR effect remain unknown. Using angle-resolved photoemission spectroscopy (ARPES) we studied the electronic structure of the bi-layer compound La2-2xSr1+2xMn2O7with a variety of doping levels, uncovering a number of critical new features. We observe the first evidence of quasiparticle peaks in the antinodal region of the Brillouin zone, and we study their detailed dispersion including "kink" behavior. With our collaborators we performed inelastic neutron scattering experiments to map out the phonon dispersion relations, allowing us to pinpoint which phonons in energy and momentum space couple most strongly to the electrons, also obtaining the electron-phonon coupling strength. Furthermore, we found that in the insulating state there remain local metallic regions up to a very high temperature. In these metallic regions, the electronic behavior has minimal change with temperature. Our data indicate that the metal-insulator transition is a new type and an "emergent" phenomenon driven by the phase separation and percolation effect. The CMR effect can also be understood in the framework of the phase separation and percolation effect. |
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