Underlying Fermi surface of Sr₁₄₋ₓCaₓCu₂₄O₄₁ in two-dimensional momentum space observed by angle-resolved photoemission spectroscopy

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We have performed an angle-resolved photoemission study of the two-leg ladder system Sr₁₄₋ₓCaₓCu₂₄O₄₁ 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-Tc cuprates, implying strong carrier localization related to the hole crystalization.