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 partial
ly-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.
The phenomenon of magnetic quantum oscillations in the superconducting state poses several questions that still defy satisfactory answers. A key controversial issue concerns the additional damping observed in the vortex state. Here, we show results o
f mu SR, dHvA, and SQUID magnetization measurements on borocarbide superconductors, indicating that a sharp drop observed in the dHvA amplitude just below H_{c2} is correlated with enhanced disorder of the vortex lattice in the peak-effect region, which significantly enhances quasiparticle scattering by the pair potential.
