Angle-resolved photoemission study of insulating and metallic Cu-O   chains in PrBa$_2$Cu$_3$O$_7$ and PrBa$_2$Cu$_4$O$_8$
  We compare the angle-resolved photoemission spectra of the hole-doped Cu-O chains in PrBa$_2$Cu$_3$O$_7$ (Pr123) and in PrBa$_2$Cu$_4$O$_8$ (Pr124). While, in Pr123, a dispersive feature from the chain takes a band maximum at $k_b$ (momentum along the chain) $\sim$ $\pi/4$ and loses its spectral weight around the Fermi level, it reaches the Fermi level at $k_b$ $\sim$ $\pi/4$ in Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4-filled, they show contrasting behaviors: While the chains in Pr123 have an instability to charge ordering, those in Pr124 avoid it and show an interesting spectral feature of a metallic coupled-chain system. 
T. Mizokawa, C. Kim, Z.-X. Shen, A. Ino, T. Yoshida, A. Fujimori, M.   Goto, H. Eisaki, S. Uchida, M. Tagami, K. Yoshida, A....