Superconductivity and weak electron-phonon coupling in transition metal diborides

H. Rosner1, S.-L. Drechsler2, G. Fuchs2, K.-H. Müller2, G. Behr2, H. Eschrig2, R. Heid3, E. Forzani4, K. Winzer4, Y. Naidyuk5 und O. Otani6
1Max Planck Institut für Chemische Physik fester Stoffe
2Institut für Festkörper und Werkstoffforschung Dresden
3FZ Karlsruhe
4Universität Göttingen
5NAS Kharkiv, Ukraine
6NIMS Tsukuba, Japan

After the surprising discovery of superconductivity at 40K in MgB2 an intense search for other superconducting diborides was started. Several contradicting reports about the superconductivity in TB2 (T = transition metal) have been published so far, and the discussion is not settled yet. Combining extensive electronic structure and model calculations with various experiments (specific heat, resistivity, susceptibility, de-Haas-van-Alphen oscillations, point contacts) we investigate the electronic structure and the electron-phonon coupling l in various TB2 with focus on ZrB2. Based on our calculations, we provide a clear microscopic picture for the rather weak electron-phonon coupling in these systems and the absence of conventional superconductivity in agreement with the empirically determined values from thermodynamic and transport measurements. Open problems are briefly discussed.