Fluctuation effects and mixed-state properties of the layered organic superconductors κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$ and κ-(BEDT-TTF${)}_2$Cu[N(CN${)}_2$]Br

We report on dc-magnetization measurements on high-quality single crystals of the organic superconductors κ-(BEDT-TTF${)}_2$ Cu(NCS${)}_2$ and κ-(BEDT-TTF${)}_2$Cu[N(CN${)}_2$]Br for fields parallel and perpendicular to the conducting planes. For H perpendicular to the planes, we find clear indications for strong superconducting fluctuations, i.e., a pronounced rounding of the transition with increasing field and field-induced diamagnetism above ${\mathit{T}}_{\mathit{c}}$. Careful experiments near ${\mathit{T}}_{\mathit{c}}$ reveal the existence of a field-independent magnetization at ${\mathit{M}}^{\mathrm{*}}$(${\mathit{T}}^{\mathrm{*}}$,H) for H≤10 kOe. The anisotropy of superconducting parameters such as the upper critical field and coherence length, along with a scaling form of the high-field magnetization data demonstrate the highly anisotropic character for the present superconductors. Over an extended range in the H-T plane the mixed-state properties are governed by an entirely reversible vortex motion. The lower boundary of this range, the so-called irreversibility line, is found to be anisotropic and, for H perpendicular to the planes, to follow an exponential T dependence. Cooling below the irreversibility line is accompanied by the occurrence of magnetic relaxation effects, indicating a metastable vortex state.