63Cu-NMR/NQR studies on apical-F bi-layered cuprates Ba2CaCu2O4F2 and Ba2CaCu2O4(F1.6O0.4)

Abstract We report 63 Cu-NMR/NQR on the bi-layered high- T c copper oxide Ba 2 CaCu 2 O 4 F 2 , which is expected to be a Mott insulator in an ideal case of a nominal content of fluorine, and less fluorinated Ba 2 CaCu 2 O 4 ( F 1.6 O 0.4 ) to clear the doping mechanism in apical-fluorine multilayered system Ba 2 Ca n - 1 Cu n O 2 n F 2 , where n is the number of the layers in a unit cell. In the case of n = 3 and, especially, n = 4 , the origin of carrier doping has been attributed to a self-doping mechanism based on the nominal stoichiometry; electrons are transferred between the inner CuO 2 plane (IP) and the outer one (OP), keeping Cu + 2 on average. It is revealed that Ba 2 CaCu 2 O 4 F 2 is an underdoped superconductor with hole carriers , doped by an inevitable deviation from the nominal content of apical fluorines. This result suggests that hole carriers are doped in the same manner for the case of n = 3 and 4, and that we should reconsider the self-doping mechanism as the carrier source in the apical-fluorine system.