We have used a millimeter-wave magneto-optical technique to study the angle dependence of the high-frequency conductivity of the molecular superconductor $\kappa -\left(\mathrm{BEDT}-\mathrm{TTF}{)}_2\mathrm{Cu}\right(\mathrm{NCS}{)}_2$. The data strongly suggest that the superconducting gap has nodes directed along the $\mathbf{b}$ and $\mathbf{c}$ directions of the crystal, in agreement with recent theoretical predictions. This supports the idea that the superconductivity in $\kappa -\left(\mathrm{BEDT}-\mathrm{TTF}{)}_2\mathrm{Cu}\right(\mathrm{NCS}{)}_2$ is $d$ wave in nature, and is mediated by spin fluctuations.

• Received 28 June 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.3041