Antiferromagnetism, superconductivity, and pseudogap in three-layered high-Tc cuprates Ba₂Ca₂Cu₃O₆(F,O)₂ probed by Cu-NMR

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We report on the phase diagram of antiferromagnetism (AFM) and superconductivity (SC) in three-layered Ba₂Ca₂Cu₃O₆(F,O)₂ by means of Cu-NMR measurements. It is demonstrated that AFM and SC uniformly coexist in three-layered compounds as well as in four- and five-layered ones. The critical hole density pc for the long range AFM order is determined as pc 0.075, which is larger than pc 0.02 and 0.055 in single- and bi-layered compounds, and smaller than pc 0.08-0.09 and 0.10-0.11 in four- and five-layered compounds, respectively. This variation of pc is attributed to the magnetic interlayer coupling which becomes stronger as the stacking number of CuO₂ layers increases; that is, the uniform coexistence of AFM and SC is a universal phenomenon in underdoped regions when a magnetic interlayer coupling is strong enough to stabilize an AFM ordering. In addition, we highlight an unusual pseudogap behavior in three-layered compounds -- the gap behavior in low-energy magnetic excitations collapses in an underdoped region where the ground state is the AFM-SC mixed phase.