L.A. Gurevich, I.V. Grigor'eva, N.N. Kolesnikov, M.P. Kulakov, V.A. Larkin, L.Ya. Vinnikov.
Direct observation of the vortex structure in high-T_c superconductors in tilted magnetic fields.

  Abstract:
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 Tl₂Ba₂CaCu₂O_8+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.