Dual character of the electronic structure in YBa$_2$Cu$_4$O$_{8}$: conduction bands of CuO$_2$ planes and CuO chains

We use microprobe Angle-Resolved Photoemission Spectroscopy ($\mu$ARPES) to separately investigate the electronic properties of CuO$_{2}$ planes and CuO chains in the high temperature superconductor, YBa$_2$Cu$_4$O$_{8}$. In the CuO$_2$ planes, a two dimensional (2D) electronic structure with nearly momentum independent bilayer splitting is observed. The splitting energy is 150 meV at ($\pi$,0), almost 50\% larger than in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and the electron scattering at the Fermi level in the bonding band is about 1.5 times stronger than in the antibonding band. The CuO chains have a quasi one dimensional (1D) electronic structure. We observe two 1D bands separated by $\sim$ 550meV: a conducting band and an insulating band with an energy gap of $\sim$240meV. We find that the conduction electrons are well confined within the planes and chains with a non-trivial hybridization.