Vortex structure and dynamics in kappa-(ET)(2)Cu(NCS)(2)

We use Josephson plasma resonance (JPR) and DC torque magnetization measure ments to investigate the vortex structure and dynamics in the layered organ ic superconductor kappa-(ET)(2)Cu(NCS)(2). JPR studies probe the AC respons e of this extreme type-II superconductor for currents driven along the low conductivity axis. An observed crossover in the magnetic field and temperat ure dependence of the JPR frequency, in close proximity to the irreversibil ity line, is attributed either to a melting or a depinning transition in th e quasi-two-dimensional (Q2D) vortex structure. Angle and temperature depen dent magnetization studies reveal the rich dynamical process by which magne tic flux enters this highly anisotropic superconductor, as well as informat ion on the structure of the vortices once inside. At extremely low temperat ures (T < 100 mK), the sudden cessation of avalanching flux jumps above an angle- and temperature-dependent critical field B*(<theta>, T), is indicati ve of Q2D vortex lattice melting. Thus, using these two very different tech niques, we are able to probe the vortex phase diagram over a wide temperatu re and magnetic field range. (C) 2001 Elsevier Science B.V. All rights rese rved.