Josephson effect in MgB2 break junctions.

We present the first observation of the dc and ac Josephson effect in MgB2 break junctions. The junctions, obtained at 4.2 K in high-quality, high-density polycrystalline metallic MgB2 samples, show a nonhysteretic dc Josephson effect. By irradiating the junctions with microwaves we observe clear Shapiro steps spaced by the ideal ΔV value. The temperature dependence of the dc Josephson current and the dependence of the height of the steps on the microwave power are obtained. These results directly prove the existence of pairs with charge 2e in MgB2 and give evidence of the superconductor-normal metal-superconductor weak link character of these junctions.