Anisotropic vortex structure in Y1Ba2Cu3O7

We report Bitter-pattern observations of the vortices for flux trapped in crystals of the orthorhombic high-T/sub c/ superconductor Y1Ba2Cu3O7. For Bparallelc axis, the slight distortion of the vortex lattice yields a penetration-depth anisotropy ranging from 1.11 to 1.15 for different crystals. For Bperpendicularc, 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