| Low-energy quasiparticle excitations govern the thermodynamic properties of a superconductor both in the zero-field and vortex-mixed states. For a d-wave superconductor, the quasiparticle excitations starting from the nodal point at zero energy are unique and crucial. In transport experiments, however, the quasiparticle properties are intricately-integrated. Here, we have performed the angle-resolved photoemission study of single-layer and bilayer high-Tc cuprates using low-energy synchrotron radiation, directly resolving the quasiparticle properties in energy- and momentum-spaces. The tunability of photon energy has provided us both the specific and perspective views of the quasiparticle structure. With high momentum- and energy-resolution, we have identified the fine structures of quasiparticle dispersion, indicating that multiple bosonic modes are coupled with low-energy electrons. Detailed temperature- and energy-dependence of the scattering rate indicates unique behaviors for the nodal quasiparticles. Due to the opening of the d-wave gap, the nodal scattering rate is remarkably suppressed, and shows a linear energy dependence. The difference in the energy-linear term between the bilayer-resolved scattering rates hints the nature of scatterers involved. |
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