Anomalous Doping Dependence of Many-Body Effects in (La_2-xSr_x)CuO_4

High resolution angle-resolved photoemission measurements have been carried out on (La_2-xSr_x)CuO₄ over the whole doping range (0<=x<=0.3) to investigate the doping dependence of quasiparticle dynamics. We have invariably observed an energy scale at 70 ~100 meV, showing up as an kink in dispersion, a drop in the scattering rate, and a peak-dip-hump structure in photoemission spectra. All these observations point to a strong coupling of quasiparticles with phonons in LSCO system. On the other hand, the electron self-energy also exhibits evidence of strong electron-electron correlation. This has demonstrated the necessity and importance of combining electron-electron correlation and electron-phonon coupling in understanding many unusual properties of high temperature cuprate superconductors. These many-body effects also exhibit anomalous doping dependence, as manifested in quasiparticle velocities and electron-phonon coupling strength.