Anisotropic vortex structure in ${\mathrm{Y}}_1$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We report Bitter-pattern observations of the vortices for flux trapped in crystals of the orthorhombic high-${\mathrm{T}}_{\mathrm{c}}$ superconductor ${\mathrm{Y}}_1$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. 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.