Shapiro step response in the vortex state of Bi2Sr2CaCu2O8+δ

We report the first observation of Shapiro step effect in the Josephson vortex state of Bi2Sr2CaCu2O8+delta in high magnetic field (H greater than or equal to I T). We observed phase locking between external microwave radiation and Josephson oscillation in whole stack of N intrinsic Josephson junctions at fields tilted with respect to the c-axis. Steps were observed in Josephson flux-flow (JFF) on the I-V characteristic at voltage V-st = pNh(nu)/2e, with p integer, nu = 45-142 GHz. The amplitude of both Shapiro steps and Josephson critical current increase sharply with tilting angle and then decrease. We attribute this behavior to the interaction between Josephson and pancake vortices. (C) 2003 Elsevier Science B.V. All rights reserved.