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Abstract:
We report on dc-magnetization measurements on high-quality single
crystals of the organic superconductors kappa-(BEDT-TTF)2 Cu(NCS)2 and
kappa-(BEDT-TTF)2Cu[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 T(c). Careful experiments near T(c) reveal the
existence of a field-independent magnetization at M*(T*,H) for H
less-than-or-equal-to 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.
