C-13 NMR-STUDY OF A QUASI-2-DIMENSIONAL ORGANIC SUPERCONDUCTOR K-(ET)2CU[N(CN)2]BR

We present C-13 NMR measurements carried out under pressure and betwee n 10 and 300 K on a single crystal of the organic superconductor kappa -(ET)2Cu[N(CN)2]Br. Under pressure, the large variation of the Knight shift is interpreted as the presence of electronic correlations. We es timate the U/W ratio to be of order unity. As regards temperature, we report a large enhancement of 1/T1 T and a drop of Knight shift near 5 0 K. These anomalies tend to disappear under pressure, all properties becoming more usual for a metal. These results are seen to be similar to those observed in the high-T(c) superconductors. We propose that ne sting properties of the Fermi surface can qualitatively explain this b ehaviour.