A detailed study of the normal state photoemission lineshapes and quasiparticle dispersion for the single layer Bi2Sr 2-xLaxCuO6+δ (Bi2201) superconductor is presented. We report the first experimental evidence of a double peak structure and a dip of spectral intensity in the energy distribution curves (EDCs) along the nodal direction. The double peak structure is well identified in the normal state, up to 10 times the critical temperature. As a result of the same self-energy effect, a strong mass renormalization of the quasiparticle dispersion, i.e. kink, and an increase of the quasiparticle lifetime in the normal state are also observed. Our results provide unambiguous evidence on the existence of bosonic excitation in the normal state, and support a picture where nodal quasiparticles are strongly coupled to the lattice.
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This work was supported by the Department of Energy's Office and Basic Energy Science, Division of Materials Sciences and Engineering of the US Department of Energy, under Contract No. DE-AC03-76SF00098 and by the National Science Foundation through Grant DMR-0349361. SSRL work is supported by DOE contract DE-ACO3-765F00515. Stanford work is also supported by NSF DMR-0304981.