S. Wanka et al., CRITICAL FIELDS AND MIXED-STATE PROPERTIES OF THE LAYERED ORGANIC SUPERCONDUCTOR KAPPA-(BEDT-TTF)(2)I-3, Physical review. B, Condensed matter, 53(14), 1996, pp. 9301-9309
We report on a systematic study of the thermodynamic superconducting p
roperties on high-quality crystals of the quasi-two-dimensional (quasi
-2D) organic superconductor kappa-(BEDT-TTF)(2)I-3. Low-field magnetiz
ation measurements served to determine the lower critical field B-c1.
Just below T-c=3.5 K, down to similar to 3 K, B-c1 perpendicular (perp
endicular to) to the highly conducting BEDT-TTF layers is found to be
strongly suppressed below the sensitivity limit of the instrumental re
solution. This might be an indication for single-vortex fluctuations e
xpected for a strongly 2D superconductor. The critical field B-c2, de
termined by ac-susceptibility measurements is found to be related to a
n irreversibility line. The temperature and frequency dependences of B
-c2 are consistent with well-established flux-creep models. The upper
critical field B-c2 perpendicular to extracted from magnetization mea
surements is consistent with specific-heat data. For both principal fi
eld orientations a crossover of the critical-field slopes \dB(c2)/dT\
towards a shallow tail close to T-c is observed. Apparent discrepancie
s of the thermodynamic critical fields for parallel and perpendicular
field orientation are found, indicating either a strongly reduced B-c1
parallel to or B-c2 parallel to.