Selective Observation of Bilayer-Split Nodal Quasiparticles in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ by ARPES Using Low-Energy Tunable Photons

Doubling the CuO$_{2}$ layer is considered to enhance the transition temperature of cuprate superconductors. However, since a recent breakthrough in angle-resolved-photoemission spectroscopy (ARPES) using low-energy excitation photons, it has been controversial whether the nodal bilayer splitting is resolvable or not. We have performed the photon-energy-dependent ARPES study of bilayer cuprate Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ using low-energy synchrotron radiation (h$\nu $ = 7-8 eV). While the bonding and antibonding band are consistently resolved by a constant splitting width of $\Delta k\cong 0.008\mbox{A}^{\mbox{-1}}$, the spectral weight drastically transfers between them with changing photon energy. The tunable photons enabled us to control the bonding-to-antibonding ratio from 0{\%} to 162{\%}. We will discuss the nodal quasiparticle properties using this bilayer-selective probe.