Electron Transport in Diborides: Observation of Superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB2) with different transition metals atoms (A=Zr,Nb,Ta). The temperature dependence of resistivity, ρ(T ), and ac susceptibility of these samples reveal superconducting transition of ZrB2 with Tc = 5.5 K, while NbB2 and TaB2 have been observed nonsuperconducting up to 0.37 K. Hc2 (T) is linear in temperature below Tc, leading to a rather low Hc2(0) = 0.1 T . At T close to Tc Hc2 (T) demonstrates a downward curvature. We conclude that these diborides as well as MgB2 samples behaves like a simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures: 280 K, 460 K and 440 K, for ZrB2, NbB2 and MgB2, respectively, rather than T 2 and T as previously reported for MgB2. PACS numbers: 74.70.Ad, 74.60.Ec, 72.15.Gd Typeset using REVTEX ∗e-mail: [email protected]