Collapse of Superconductivity in the Y123/Y124 Superlattice System Y₂Ba₄Cu₇O_15-δ

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Magnetic susceptibility and electric resistivity were measured on Y₂Ba₄Cu₇O_15-δ over the whole oxygen deficiency range, 0≤δ≤1. Complete collapse of superconductivity was observed for Y₂Ba₄Cu₇O₁₄ in both resistivity and susceptibility measurements, although there still reside the metallic conduction. This is the first observation of under-doped normal metallic state in high-T_c cuprates without applying any external factors such as magnetic field, hydrostatic pressure or substitution of impurity for plane-Cu site for breaking the superconducting state. The behavior of electric conduction due to the CuO₂ plane was observed in resistivity measurement as changing from the T-linear dependence to the under-doped S-shaped temperature dependence, giving the spin-gap temperature which agree well with the results from the spin-lattice relaxation rate 1/T₁ for Cu NQR at plane-site in various YBCO compounds. The temperature dependence of 1/T₁ on plane-site Cu in Y₂Ba₄Cu₇O₁₄ is discussed to be of itinerant-electron type with finite magnetic correlation between them as in the case of nearly and/or weak magnetic systems. The temperature dependence of 1/T₁ on chain-site is also discussed among various YBCO compounds.