OBSERVATION OF THE AC JOSEPHSON EFFECT INSIDE COPPER-OXIDE BASED SUPERCONDUCTORS

We have measured the current-voltage characteristic of a point contact on a sample of Sr-doped La//2CuO//4. Josephson junction characteristics are observed even when the metal tip is nonsuperconducting, a result that indicates that the junction is inside the material under the tip. Under microwave irradiation, we observe well-defined Shapiro steps which yield the usual value h/2e for the coupling constant between the quantum phase and the electromagnetic field. Preliminary measurements on samples of Y//1Ba//2Cu//3O//9// minus //x also yield h/2e. We conclude that in these newly discovered materials, the superconducting state is formed of paired electrons.