arXiv2009
We report Cu-NMR/NQR and F-NMR studies on the multilayered high-T_c copper oxides Ba_2Ca_{n-1}Cu_nO_{2n}F_2 with n=2,3,4, where n is the number of CuO_2 planes. It is revealed that bi-layered Ba_2CaCu_2O_4F_2 is an underdoped superconductor with hole carriers, which are introduced into CuO_2 planes by an unexpected deviation from the nominal content of apical fluorines. In a previous paper, we proposed a self-doping mechanism as the origin of carrier doping in n=3 and n=4; in the mechanism, electrons are transferred from the inner CuO_2 plane (IP) to the outer one (OP). However, since it has been found that the bi-layered compound is hole doped, we have reexamined the superconducting and magnetic properties in n=3 and n=4 by Cu-NMR/NQR and F-NMR. The extensive NMR studies have confirmed that the apical-fluorine compounds are not self-doped but hole-doped, and that antiferromagnetism (AFM) and superconductivity (SC) coexist in a single CuO_2 plane. In n=4, the AFM ordering occurs at T_N = 80 K, well above T_c=55 K, where the respective AFM moments are M_AFM=0.11 mu_B and 0.18 mu_B at the OP and the IP. In n=3, on the other hand, the underdoped single IP exhibits a spontaneous moment M_AFM=0.12 mu_B at low temperatures and a peak in the nuclear-spin-lattice relaxation rate 1/T_1 of F at T_N=23 K, much lower than T_c = 76 K. We note that the increase in the number of IPs from one to two leads to an increase in T_N due to strengthening the interlayer coupling, although the doping levels for both compounds are almost comparable. The present results strongly suggest that the uniform mixing of AFM and SC is a general property inherent to a single CuO_2 plane in the underdoped regime for hole-doping.