Upper critical field of κ-(BEDT-TTF)2X (X=Cu[N(CN)2]Br, Cu(NCS)2) in magnetic fields parallel to the layer

The upper critical field Bc2 of κ-(BEDT-TTF)2X (X=Cu[N(CN)2]Br, Cu(NCS)2) was investigated in magnetic fields applied parallel to the superconducting plane with use of pulsed magnetic fields. The Bc2 of both salts far exceeds the BCS Pauli paramagnetic limit. The Bc2 of X=Cu[N(CN)2]Br increases in the low temperature region with a monotonically decreasing slope, while that of X=Cu(NCS)2 exhibits an upturn below 4.2 K. The different behaviour can be explained by the difference in disorder.