A vortex structure consisting of vortex chains embedded into the flux-line lattice (FLL) has been observed in ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals using the Bitter decoration technique. This structure formed in magnetic fields applied at a high angle, between 50° and 80°, with respect to the c^ axis. Although the vortex chains–FLL mixture looked similar to the vortex structure observed earlier in the highly anisotropic ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$, its evolution with the tilt angle and the applied field demonstrated that it represents an essentially different vortex state which, unlike ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$, originates from the attractive vortex-vortex interaction. The coexistence of the vortex chains with the FLL is attributed to an incomplete transition from the isotropic vortex lattice to the pure chain state during the sample cooling, owing to pinning.

• Received 17 September 1993

DOI:https://doi.org/10.1103/PhysRevB.48.16865