Anomalous Doping Variation of the Nodal Low-Energy Feature of Superconducting $(\mathrm{Bi},\mathrm{Pb}{)}_2(\mathrm{Sr},\mathrm{La}{)}_2{\mathrm{CuO}}_{6+\delta }$ Crystals Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

The nodal band dispersion in $(\mathrm{Bi},\mathrm{Pb}{)}_2(\mathrm{Sr},\mathrm{La}{)}_2{\mathrm{CuO}}_{6+\delta }$ (Bi2201) is investigated over a wide range of doping by using 7-eV laser-based angle-resolved photoemission spectroscopy. We find that the low-energy band renormalization (“kink”), recently discovered in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212), also occurs in Bi2201, but at a binding energy around half that in Bi2212. Surprisingly, the coupling energy dramatically increases with a decrease of carrier concentration, showing a sharp enhancement across the optimal doping. These properties (material and doping dependence of the coupling energy) demonstrate the significant correlation among the mode coupling, the energy gap close to the node, and the strong electron correlation. Our results suggest forward scattering arising from the interplay between the electrons and in-plane polarized acoustic phonon branch as the origin of the low-energy renormalization.