Critical fields and mixed-state properties of the layered organic superconductor $\kappa$-${\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2$${\mathrm{I}}_3$

We report on a systematic study of the thermodynamic superconducting properties on high-quality crystals of the quasi-two-dimensional (quasi-2D) organic superconductor $\kappa$-${\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2$ ${\mathrm{I}}_3$. Low-field magnetization measurements served to determine the lower critical field $B_{c1\mathrm{}}$. Just below $T_c=3.5\mathrm{}$ K, down to ∼ 3 K, $B_{c1\mathrm{}}$ perpendicular (⊥) to the highly conducting BEDT-TTF layers is found to be strongly suppressed below the sensitivity limit of the instrumental resolution. This might be an indication for single-vortex fluctuations expected for a strongly 2D superconductor. The critical field $B_{c2\mathrm{}}^{*}$ determined by ac-susceptibility measurements is found to be related to an irreversibility line. The temperature and frequency dependences of $B_{c2\mathrm{}}^{*}$ are consistent with well-established flux-creep models. The upper critical field $B_{c2\mathrm{}\perp }$ extracted from magnetization measurements is consistent with specific-heat data. For both principal field orientations a crossover of the critical-field slopes $\left|\frac{d{B}_{c2\mathrm{}}}{\mathrm{dT}}\right|$ towards a shallow tail close to $T_c$ is observed. Apparent discrepancies of the thermodynamic critical fields for parallel and perpendicular field orientation are found, indicating either a strongly reduced $B_{c1\mathrm{}\parallel }$ or $B_{c2\mathrm{}\parallel }$.