Phase diagram of optimally dopedYBa2Cu3O7−δ: Effect of oxygen stoichiometry

Abstract

Influence of the oxygen stoichiometry on the phase diagram of clean ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals has been studied in the region of optimal doping $(0.05<\ensuremath{\delta}<0.09).$ Temperature and angular dependencies of magnetoresistance were analyzed for fields of up to 23 T. At low fields a sharp resistivity kink, which is a signature of the first order melting transition, was observed in all the samples studied. This kink, however, disappears when B exceeds the multicritical field ${B}_{\mathrm{mc}}.$ It was found that ${B}_{\mathrm{mc}}$ strongly increases with decreasing \ensuremath{\delta}. We have demonstrated that variations in the oxygen stoichiometry in the range of $0.05<\ensuremath{\delta}<0.09$ do not have a significant effect on intrinsic parameters (such as anisotropy, coherence length, and penetration depth) but lead to a strong increase in entanglement of the vortex lines. We relate the dependence of ${B}_{\mathrm{mc}}$ on \ensuremath{\delta} with entanglement of the vortex lines, which is promoted by their interaction with oxygen vacancies.