Different electronic states at crystallographically inequivalent CuO₂ planes on four-layered cuprates HgBa₂Ca₃Cu₄O_10+δ

We report tunneling conductances due to two kinds of crystallographically inequivalent CuO₂ planes on multi-layered cuprates Hg_0.95Ba₂Ca₃Cu₄O_10.05 (Hg1234) measured by a point-contact tunneling technique. One is an outer CuO₂ plane (OP) which has a pyramidal five-oxygen coordination, another is an inner CuO₂ plane (IP) which has a square four oxygen coordination. These tunneling conductances exhibit two kinds of superconducting gaps Δ with clearly different sizes. That is, Δ on Hg1234 was 36 ± 2 meV and 55 ± 2 meV for OP and IP, respectively. Moreover, we report the correlation between the mode energy Ω and Δ. The Ω/Δ exhibits the common feature with other cuprates and does not exceed 2. This behaviour implies that the collective spin excitation is a candidate for the mediator on pair formation.