NMR and NQR study of Ca-substituted superconducting YBa2Cu4O8.

Abstract

We report a set of nuclear quadrupole resonance (NQR) measurements including a spin-echo double-resonance (SEDOR) experiment which provides convincing evidence that the additional chain Cu NQR line in Ca-substituted ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ does stem from chain Cu sites disturbed by the ``impurity'' Ca ions. From an analysis of signal intensities we conclude that ``impurity'' Ca ions, responsible for the additional line, substitute ${\mathrm{Y}}^{3+}$. At high Ca doping, the concentration of the substituted Y sites, ${\mathit{x}}^{\ensuremath{'}}$, is appreciably smaller than the nominal Ca concentration, x. Since it is ${\mathit{x}}^{\ensuremath{'}}$ which is responsible for a direct increase of the hole charge-carrier concentration, n, effects connected with the increase of n such as the planar Cu spin-lattice relaxation rate, the NQR frequency, and the magnetic shift are only weakly dependent on x. Thus, the substantial increase of ${\mathit{T}}_{\mathit{c}}$ with x suggests that, besides the increase of n, other effects have to play a role in the ${\mathit{T}}_{\mathit{c}}$ enhancement. One such effect might be the opening of the spin pseudogap. At 150 K, both the main and additional Cu NQR lines in nominal ${\mathrm{YBa}}_{1.9}$${\mathrm{Ca}}_{0.1}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ show distinctly anomalies not seen in pure ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ that point out the Ca doping induced structural phase transition recently observed in specific-heat, elastic-neutron-scattering, and x-ray-diffraction measurements. \textcopyright{} 1996 The American Physical Society.