Underlying Fermi surface of Sr${}_{14-x}$Ca${}_x$Cu${}_{24}$O${}_{41}$ in two-dimensional momentum space observed by angle-resolved photoemission spectroscopy

We have performed an angle-resolved photoemission study of the two-leg ladder system Sr${}_{14-x}$Ca${}_x$Cu${}_{24}$O${}_{41}$ with $x$= 0 and 11. “Underlying Fermi surfaces” determined from low energy spectral weight mapping indicates the quasi-one dimensional nature of the electronic structure. Energy gap caused by the charge density wave has been observed for $x$=0 and the gap tends to close with Ca substitution. The absence of a quasi-particle peak even in $x$=11 is in contrast to the two-dimensional high-$T_c$ cuprates, implying strong carrier localization related to the hole crystalization.