Vortex lattice and vortex dynamics inYBa2Cu4O8fromY89NMR

Abstract

${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR spectra in oxygen-deficient, pure and calcium-doped ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ powdered samples below ${\mathit{T}}_{\mathit{c}}$ are presented. The square root of the derived second moment of the field distribution inside the vortex lattice at low temperatures appears to be linearly related to ${\mathit{T}}_{\mathit{c}}$. The values derived for 〈Δ${\mathit{B}}^2$〉, however, are remarkably different from the ones obtained from ${\mathrm{\mu }}^{+}$SR. At higher temperatures a narrowing of the lines is observed and a crossover between two different temperature dependences of the vortex dynamics is evidenced.