Order-disorder transition in the vortex lattice induced by anisotropy of the YBaCuO superconductor

Pinning force F-p variation with the magnetic field H and the angle alpha = angle H, ab is investigated in crystals having different concentrations of point defects n(pd). Comparison between the functions F-p(H) and F-p(alpha) strongly suggests that the minimum position in the F-p(alpha)-curve corresponds to the angle alpha(OD), which separates the ordered vortex lattice (VL) formed at alpha < alpha(OD) and the disordered VL formed at alpha > alpha(OD). This conclusion is confirmed by variation in the E(J)-curve shape in the vicinity of the angle alpha(OD), and by hysteresis behavior of the E(J)-curves measured for increased and decreased transport current at angles alpha > alpha(OD). This order-disorder transition arises, because the elastic energy of the disordered VL decreases with an increased alpha, while the pinning energy is independent of alpha.