Location of gap nodes in the organic superconductors κ-(ET) 2Cu(NCS)2 and κ-(ET)2Cu[N (CN) 2]Br determined by magnetocalorimetry

We report specific-heat measurements of the organic superconductors κ- (ET) 2 Cu (NCS) 2 and κ- (ET) 2 Cu [N (CN) 2] Br. When the magnetic field is rotated in the highly conducting planes at low temperature (T0.4K), we observe clear oscillations of specific heat which have a strong fourfold component. The observed strong field and temperature dependence of this fourfold component identifies it as originating from nodes in the superconducting energy gap which point along the in-plane crystal axes (dxy symmetry).