Dynamics of fluxons in Y1−Ba2−Cu3−O7 single crystals

Evidence is given that in Y1−Ba2−Cu3−O7 single crystals a critical state is developed on increasing (or decreasing) the applied fieldH 0, only whenH 0 is parallel to the crystalc-axis. An «irreversibility» line in the (H, T)-plane can be drawn where the irreversibility of the second-harmonic response sets in.