We report Bitter-pattern observations of the vortices for flux trapped in crystals of the orthorhombic high-Tc superconductor YiBa2Cu307. For Bile 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_Lc, 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.
Using the high-resolution Bitter-pattern technique, we have studied the vortex structure in crystals of the high-TV superconductor, YBa2Cu3C>7, for fields parallel to the c axis. We describe the vortex pinning by twin boundaries and show that a vortex lattice develops in twin-free regions. But the results also show directly that vortices are pinned at low temperatures even in crystals which are essentially defect free. We believe this pinning must be intrinsic to the new superconductors.
We report fracture toughness measurements on single crystals of YBa2Cu3Ox, the phase responsible for superconductivity above liquid-nitrogen temperatures. Indentation crack length measurements on the (010) orthorhombic crystal growth faces revealed the (100) and (001) planes as preferred fracture planes. The toughness of these planes is Kc=1.1±0.3 MPa m1/2, and the hardness H=8.7±2.4 GPa. The observed growth of both radial and lateral cracks in ambient air suggests that these crystals are susceptible to moisture-enhanced nonequilibrium crack propagation.
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