Doping-dependence of nodal quasiparticle properties in high- Tc cuprates studied by laser-excited angle-resolved photoemission spectroscopy

We investigate the doping dependent low-energy, low temperature (T=5 K) properties of nodal quasiparticles in the d -wave superconductor Bi2.1 Sr1.9 Ca Cu2 O8+δ (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near EF, mean free paths, and scattering rates (Γ) of quasiparticles. For optimally and overdoped samples, we obtain very sharp quasiparticle peaks of 8 and 6 meV full width at half maximum, respectively. The value of Γ for optimally doped sample is in good accordance with terahertz conductivity. For all doping levels, we find the energy dependence of Γ∼ ω, while Γ (ω=0) shows a monotonic increase from overdoping to underdoping. The doping-dependence suggests the role of electronic inhomogeneity on the nodal quasiparticle scattering at low temperature (5 K0.07 Tc) pronounced in the underdoped region. The quasiparticle peak spectra match well with a single Lorentzian function, thus indicating that the nodal carriers can be described as well-defined quasiparticles, even in the underdoped region.