Copper NQR and NMR in the superconductor YBa2Cu4O8+x

https://doi.org/10.1016/0921-4534(89)91304-XGet rights and content

Abstract

The frequencies νQ, the linewidth and the spin-lattice and spin-spin relaxation times T1 and T2, respectively, of the 63Cu nuclear quadrupole resonance (NQR) signals for chain and planar copper sites in YBa2Cu4O8 were measured between 6 and 750 K; in addition nuclear magnetic resonance (NMR) spectra were taken at 150 and 300 K. The results are compared with data known for the related structure YBa2Cu3O7. Above Tc the tensors of the electric field gradient and the anisotropic Knight shift at the copper sites differ in a subtle way from those of YBa2Cu3O7. At the Cu2 site, the derivative of νQ with respect to temperature changes its sign around 200 K. No indication for magnetic ordering at the Cu sites was found. Below Tc there is no evidence for BCS type relaxation and the Cu2 Tc rates vary like exp(TT0) and the Cu1 rates like Tn with n≈1.35. Above Tc the Cu1 rates suggest the presence of quasiparticles (and phonon interactions) while the Cu2 rates can perhaps be explained by a model of Monien et al. postulating the coupling of Cu nuclei to itinerant interacting quasiparticles.

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