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 a superconducting transition of ZrB2 with T c =5.5 K, while NbB2 and TaB2 have been observed to be nonsuperconducting up to 0.37K. H c2(T) is linear in temperature below T c , leading to a rather low H c2(0)=0.1 T. At T close to T c , H c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB2 samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB2, NbB2, and MgB2, respectively, rather than T 2 and T 3, as previously reported for MgB2.