CHARGE-CARRIER DENSITY AND INTERPLANE COUPLING IN Y2BA4CU7O15 - A CU NMR-NQR STUDY

We report an observation of the Cu-63, Cu-65 nuclear quadrupole resona nce (NQR) and nuclear magnetic resonance (NMR) in Y2Ba4CurO15. We have measured the temperature dependence of the Cu NQR frequency and spin- lattice relaxation at all four chemically inequivalent Cu sites, and o f the Cu magnetic shift at two inequivalent plane Cu sites (for the ma gnetic field parallel and perpendicular to the c axis). The Y2Ba4Cu7O1 5 compound turns out to be a structure containing two inequivalent CuO 2 Planes of differing doping levels, a multilattice in which YBa2Cu4O8 blocks and YBa2Cu3O7, blocks alternate. In the normal conducting stat e both the static and the dynamic electron spin susceptibilities of th e individual planes of a double plane are governed by the same tempera ture dependence, which shows a behavior typical for an underdoped high -T(c) compound. The same temperature dependence means strong coupling between these planes, with the lower limit of the coupling constant no t much less than 30 meV. Although the planes are strongly coupled, the ir spin susceptibilities retain a distinct q dependence. The temperatu re variation of relaxation rate and Knight shift is described in terms of spin-gap formation or, alternatively, of frustrated phase separati on. Below T(c), the common temperature dependence is lost, which could arise from the opening of two superconducting gaps that differ in the individual planes.