Josephson plasma resonance and anomalous hysteresis in the vortex lock-in state in Bi₂Sr₂CaCuO_8+δ

We present data on the Josesphson plasma resonance in Bi₂Sr₂CaCuO_8+δ in a field nominally aligned with the CuO₂ planes (the tilt angle θ < 0.25°). At temperatures between 15 and 40 K, we find that the resonance field observed in sweep-up field scans is always smaller than that in sweep-down scans. The hysteresis has a sign opposite to that usually encountered in isotropic superconductors. A second unusual aspect of the hysteresis is that the turning field (maximum or minimum field in a particular scan cycle) influences the value of one of the resonance fields. We propose an explanation in which the critical field for vortex pancake nucleation and the existence of the vortex lock-in state generate a hysteresis loop in the B_y-B_z plane. The model accounts for the anomalous sign as well as other features observed. The experiment clarifies plasma resonance experiments performed close to alignment at low temperatures. We discuss the importance of zero-field cooling the sample in resonance measurements in this geometry.