DIRECT OBSERVATION OF THE VORTEX STRUCTURE IN HIGH-TC SUPERCONDUCTORS IN TILTED MAGNETIC-FIELDS .1. FLUX-LINE LATTICE IN TL2BA2CACU2O8+X SINGLE-CRYSTALS

A high-resolution Bitter technique has been used to observe an effect of tilting the external magnetic field on the flux line lattice (FLL) in Tl2Ba2CaCu2O8+x single crystals. The FLL was found to remain isotropic. with average vortex spacing in the FLL corresponding to the normal component of the external field; no vortex chains or any distortion of the FLL were observed. in contrast to what the theory predicted. The reason is believed to be that the formation of the equilibrium vortex state (vortex chains) was prevented by strong pinning, revealed for our crystals by the -M(t) curves.