Vortex decoration in untwinned and twinned YBCO single crystals. Study of the influence of the magnetic field angle and comparison with theory

The vortex lattice in untwinned and twinned YBCO single crystals has been observed in function of the magnetic field orientation, using the high-resolution Bitter decoration technique [6]. In untwinned samples and in tilted fields, a chain structure appears, which is independent of crystallographic orientation. In weekly twinned samples, pinning along the twin boundaries is still present in addition to the vortex-chain state. A quantitative analysis of these observations and comparison to theory is presented.