Images of the vortex chain state in untwinned YBa2Cu3O7- delta crystals.

A vortex lattice consisting of a pinstripe pattern of vortex chains has been observed in untwinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals. This state is present when the field is tilted away from the c^ axis, and is oriented with the chains confined to the plane defined by B and c^. The chains arise from the attractive part of the vortex-vortex interaction in this geometry as described within the ansiotropic London equations. For Bg${\mathrm{\ensuremath{\Phi}}}_{0}$/2\ensuremath{\pi}\ensuremath{\lambda}\ifmmode \tilde{}\else \~{}\fi{} $^{2}$ the chains merge into an oriented vortex lattice. From the fitted value of \ensuremath{\lambda}\ifmmode \tilde{}\else \~{}\fi{}, we can conclude that the microscopic irreversibility temperature is T/${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}0.92 for B\ensuremath{\sim}20 G.