Uniform mixing of antiferromagnetism and high-Tc superconductivity in multilayer copper oxides Ba2Can−1CunO2nF2 (n=2,3,4) with apical fluorines: C63u-NMR/NQR and F19-NMR studies

Abstract

We report C63u-NMR/NQR and F19-NMR studies on the multilayered high-Tc copper oxides Ba2Can−1CunO2nF2 with n=2,3,4, where n is the number of CuO2 planes. It is revealed that bilayered Ba2CaCu2O4F2 is an underdoped superconductor with hole carriers, which are introduced into CuO2 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 CuO2 plane (IP) to the outer one (OP). However, since it has been found that the bilayered compound is hole doped, we have re-examined the superconducting and magnetic properties in n=3 and n=4 by C63u-NMR/NQR and F19-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 CuO2 plane. In n=4, the AFM ordering occurs at TN=80 K well above Tc=55 K, where the respective AFM moments are MAFM=0.11μB and 0.18μB at the OP and the IP. In n=3, on the other hand, the underdoped single IP exhibits a spontaneous moment MAFM=0.12μB at low temperatures and a peak in the nuclear-spin-lattice relaxation rate 1/T1 of F19 at TN=23 K much lower than Tc=76 K. We note that the increase in the number of IPs from one to two results in the strengthening of the interlayer coupling; TN increases as the interlayer coupling becomes stronger, although the doping levels for both compounds are comparable. Consequently, we conclude that the uniform mixing of AFM and SC is a general property inherent to a single CuO2 plane in an underdoped regime for hole doping. This conclusion incorporates the angle-resolved photoemission spectroscopy results on the n=4 compound [Chen et al., Phys. Rev. Lett. 97, 236401 (2006)]; it was found that the two Fermi sheets of the IP and OP are observed and that the SC gap opens at the IP and OP below Tc=55 K.

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