Charge-carrier density and interplane coupling in ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{15}$: A Cu NMR-NQR study

We report an observation of the ${}_{}{}^{63,65}{}_{}{}^{}\mathrm{Cu}$ nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) in ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{15}$. We have measured the temperature dependence of the Cu NQR frequency and spin-lattice relaxation at all four chemically inequivalent Cu sites, and of the Cu magnetic shift at two inequivalent plane Cu sites (for the magnetic field parallel and perpendicular to the c axis). The ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{15}$ compound turns out to be a structure containing two inequivalent ${\mathrm{CuO}}_2$ planes of differing doping levels, a multilattice in which ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ blocks and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ blocks alternate. In the normal conducting state both the static and the dynamic electron spin susceptibilities of the individual planes of a double plane are governed by the same temperature dependence, which shows a behavior typical for an underdoped high-${\mathit{T}}_{\mathit{c}}$ compound. The same temperature dependence means strong coupling between these planes, with the lower limit of the coupling constant not much less than 30 meV. Although the planes are strongly coupled, their spin susceptibilities retain a distinct q dependence. The temperature variation of relaxation rate and Knight shift is described in terms of spin-gap formation or, alternatively, of frustrated phase separation. Below ${\mathit{T}}_{\mathit{c}}$, the common temperature dependence is lost, which could arise from the opening of two superconducting gaps that differ in the individual planes.