Lockin transition, irreversibility field, and Josephson vortex penetrationdepth in kappa-(ET)(2)Cu[N(CN)(2)]Br

Penetration depth measurements in K-(ET)(2)Cu[N(CN)(2)]Br are reported for fields H up to 1 T in magnitude and arbitrary orientation. For H nearly par allel to the conducting planes, the vortex penetration depth is nonmonotoni c with field, indicating a lockin effect. Qualitatively similar behavior is observed in BSCCO. The data indicate a much stronger interaction between v ortex kinks than predicted. In the field range over which vortex kinks are first formed, the response is hysteretic with an irreversibility field equa l to the lower critical field. Both fields show positive curvature with tem perature, suggesting relaxation over surface barriers. Josephson vortices e xhibit single vortex pinning below 300 Oe. Between 300 and 1500 Oe, the Jos ephson vortex penetration depth changes dramatically, becoming proportional to both H and to the zero field interplane penetration depth, lambda(perpe ndicular to)(T)- lambda(perpendicular to)(0). (C) 1999 Published by Elsevie r Science B.V. All rights reserved.