ARPES Study of Nodal Quasiparticles Using Low-Energy Tunable Photons

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, nodal quasiparticles are crucial excitations starting from zero energy. So far, however, the nodal quasiparticle dynamics of high-Tc cuprates has been controversial. For example, it has been reported by an angle-resolved-photoemission (ARPES) experiment that the marginal-Fermi-liquid behavior persists into the superconducting state without appreciable change in the scattering rate, while microwave conductivity increases upon the superconducting transition. Here, we show a new ARPES result that solves the controversies with unprecedented momentum-resolution. Low-energy tunable photons have enabled us to resolve a small nodal bilayer splitting clearly, and to reveal the detailed temperature- and energy-dependence of the scattering rate, indicating the behaviors unique to 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 impurities involved. This work was done in collaboration with T. Yamasaki, T. Kamo, K. Yamazaki, H. Anzai, M. Arita, H. Namatame, M. Taniguchi, \textit{Grad.~Sch.~of Science and Hiroshima Synchrotron Radiation Center, Hiroshima Univ.}, A. Fujimori, \textit{Dept.~of Complexity Science and Engineering, Univ.~of Tokyo}, Z.-X. Shen, \textit{Dept.~of Physics, Applied Physics and SSRL, Stanford Univ.}, M. Ishikado, K. Fujita, and S. Uchida, \textit{Dept.~of Physics, Univ.~of Tokyo}.