Anisotropic vortex structure in Y1Ba2Cu3O7

We report Bitter-pattern observations of the vortices for flux trapped in crystals of the orthorhombic high-Tc superconductor Y1Ba2Cu3O7. For B c axis, the slight distortion of the vortex lattice yields a penetration-depth anisotropy ranging from 1.11 to 1.15 for different crystals. For B c, the case of strong anisotropy, we observe chains of oval vortices. When they are not aligned by defects, the chains appear to undulate rather than adopt a crystalline habit. From the vortex spacing we deduce an anisotropy of 5.5± 1. © 1989 The American Physical Society.