Josephson effect in the cuprates: microscopic implications

In the tunnel limit, the current-phase relation of Josephson junctions can be expanded as $I(\phi)=I_1\sin\phi+I_2\sin 2\phi$. Standard BCS theory predicts that $I_1R_N\sim\Delta/e$ and $I_2/I_1\sim D$, where $R_N$ is the resistance of the junction in the normal state, $\Delta$ is the superconducting gap, and $D\ll 1$ is the junction transparency. In the cuprates, the experimental value of $I_1R_N$ ($I_2/I_1$) is much smaller (larger) than the BCS prediction. We argue that both peculiarities of the cuprates can be explained by postulating quantum fluctuations of the pairing symmetry.