We present the first observation of the DC and AC Josephson effect in MgB_2 break junctions. The junctions, obtained at 4.2 K in high-quality, high-density polycrystalline metallic MgB_2 samples, show a non-hysteretic 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 are a direct prove for the existence of pairs with charge 2e in this new metallic superconductor and give evidence of the superconductor-normal metal-superconductor weak link character of these junctions.
@article{arxiv.cond-mat/0105241,
title = {Josephson effect in MgB_2 break junctions},
author = {R. S. Gonnelli and A. Calzolari and D. Daghero and G. A. Ummarino and V. A. Stepanov and G. Giunchi and S. Ceresara and G. Ripamonti},
journal= {arXiv preprint arXiv:cond-mat/0105241},
year = {2009}
}