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, \rho(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-Gr\"uneisen 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^3 as previously reported for MgB2.