Unusual magnetic and superconducting characteristics in multilayered high-T_c cuprates: ⁶³Cu NMR study

We report unusual magnetic and superconducting (SC) characteristics in multilayered CuO₂ planes in Hg- and Cu-based high-T_c cuprates through the ⁶³Cu-NMR measurements. These compounds, in which the number of CuO₂ planes (n) ranges from 3 to 5 in a unit cell, include crystallographically inequivalent outer (OP) and inner (IP) CuO₂ plane that are surrounded by pyramidal and square oxygen, respectively. The Knight shift (⁶³K) at the OP and IP exhibits respective characteristic temperature dependence, consistent with its own doping level. Using an experimental relation between the spin part in ⁶³K at room temperature and the doping level in a CuO₂ layer N_h, we show that N_h(OP) at the OP is larger than N_h(IP) at the IP for all the systems and its difference ΔN_h=N_h(OP)-N_h(IP) increases as either a total carrier content σ or n increases. At ΔN_h's exceeding a critical value, the pseudogap behavior in the normal state is seen alone at the IP, and a bulk SC transition does not set in simultaneously at the IP and OP. A SC nature at the OP becomes consistent with a mean-field behavior only below T_c2 that is significantly lower than T_c. Reduction in T_c with increasing n is associated with an increase in ΔN_h. It is a rather remarkable aspect that a T_c is not always reduced even though these multilayered high-T_c compounds are heavily overdoped. This arises, we show, because the IP remains underdoped and keeps a high value of T_c, while the OP is predominantly overdoped. This may be a microscopic origin for the lowest anisotropic SC characteristics reported to date in Cu-based multilayered high-T_c compounds.