High-resolution specific-heat measurements on a small (0.2 mg) single crystal of κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$ in magnetic fields up to 5 T reveal a jump ΔC at ${\mathrm{T}}_{\mathrm{c}}$. Comparison with γ=34 mJ/mol ${\mathrm{K}}^2$ derived from Pauli paramagnetism yields ΔC/γ${\mathrm{T}}_{\mathrm{c}}$=1.50±0.15, within experimental error of the BCS value of 1.43. Application of a magnetic field broadens the transition but depresses it only slightly: -${\mathrm{H}}_{\mathrm{c}2\mathrm{}}^{\prime }$=16 T/K for H parallel to the planes, 0.75 T/K perpendicular. These very large values are, respectively, factors of 7 and 4 times higher than reported from magnetoresistance data, suggesting that flux-flow resistance obscures ${\mathrm{T}}_{\mathrm{c}}$ in magnetoresistance measurements, much as it does in the ceramic high-${\mathrm{T}}_{\mathrm{c}}$ materials. No evidence is found for the presence of an anomaly near 4 K, as observed in proton NMR relaxation rates.

• Received 27 November 1989

DOI:https://doi.org/10.1103/PhysRevB.41.4808