High-resolution angle-resolved photoemission study of Sr_14-xCa_xCu₂₄O₄₁ (x=0 and 6)

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of ladder compounds Sr_14-xCa_xCu₂₄O41 (x = 0 and 6) to study the change of the electronic structure near the Fermi level (Ep) by substitution of Sr with Ca. The experimental band structures of both compounds determined by ARPES show a dispersive band near E_F, which has a minimum binding energy at ka = (2n+1)π/2 (n=0, ±1, ±2...) with the periodicity of a ladder sublattice. This result indicates that the electronic states near E_F originate in the ladder and form aMott-Hubbard band with an energy gap at ka= (2n+1)π/2. This suggests that Cu atoms in the ladder take essentially a divalent state (Cu²⁺) and "self-doped" holes reside on the chain. It was also found that the dispersive band of Sr₈Ca₆Cu₂₄O₄₁ is closer to E_F than that of Sr₁₄Cu₂₄O₄₁, indicating that the ladder band is "doped" with holes by Ca-substitution. This suggests that the superconductivity observed in a heavily Ca-substituted sample occurs on the ladder.