Electron Transport in Diborides: Observation of Superconductivity in ZrB$_2$

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB$_2$) 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 ZrB$_2$ with $T_c=5.5\,$K, while NbB$_2$ and TaB$_2$ have been observed nonsuperconducting up to $0.37$ K. $H_{c2}(T)$ 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 MgB$_2$ samples behaves like a simple metals in the normal state with usual Bloch–Grüneisen temperature dependence of resistivity and with Debye temperatures: $280$, $460$ and $440$ K, for ZrB$_2$, NbB$_2$ and MgB$_2$, respectively, rather than $T^2$ and $T^3$ as previously reported for MgB$_2$.