Partially-ordered vortex lattices in the high-field, low-temperature mixed state of quasi two-dimensional organic superconductors

We report the results of high-field, low-temperature MuSR measurements of the quasi two-dimensional organic superconductors k{appa}-(ET)2Cu(NCS)2 and k{appa}-(ET)2Cu[N(CN)2]Br. The MuSR lineshapes for these compounds indicate the existence of partially-ordered vortex lattice phases in the high magnetic field regime, up to 2.5 T for the former compound and 4 T for the latter compound. The observed sharp loss of order is found to be consistent with a vortex-lattice melting transition that is predicted by numerical simulations of weakly coupled layers of pancake vortices. It is argued that the robustness of the partially-ordered vortex lattice phases could be due to strong flux-line pinning by a dilute ensemble of defects.