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 YBa2Cu4O8 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 Y3+. At high Ca doping, the concentration of the substituted Y sites, x′, is appreciably smaller than the nominal Ca concentration, x. Since it is x′ 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 Tc with x suggests that, besides the increase of n, other effects have to play a role in the Tc 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 YBa1.9Ca0.1Cu4O8 show distinctly anomalies not seen in pure YBa2Cu4O8 that point out the Ca doping induced structural phase transition recently observed in specific-heat, elastic-neutron-scattering, and x-ray-diffraction measurements. © 1996 The American Physical Society.

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