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, ρ(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 K, 460 K 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 .

We report the measurements of the temperature dependence of the resistivity, ρ(T), magnetic penetration depth, λ(T) the lower, H c1 (T), and upper, H c2 (T), critical magnetic fields, for single crystals of dodecaboride ZrB 12 , diboride ZrB 2 and thin films of diboride MgB 2 . We observe a number of deviations from conventional behavior in these materials. Although ZrB 12 behaves like a simple metal in the normal state, the resistive Debye temperature, 300 K, is three times smaller relative to that (800-1200 K) calculated from the specific heat, C(T), data. We observe predominantly quadratic temperature behavior of resistivity in ZrB 12 below 25 K, and in ZrB 2 below 100 K, indicating the possible importance of the electron-electron interaction in these borides. Superfluid density of ZrB 12 displays unconventional temperature dependence with pronounced shoulder at T/T c equal to 0.65. Contrary to conventional theories we found a linear temperature dependence of H c2 (T) for ZrB 12 from T c down to 0.35 K. We suggest that both λ(T) and H c2 (T) dependencies in ZrB 12 can be explained by two band BCS model with different superconducting gap and T c .

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We report on the synthesis and measurements of the temperature dependence of resistivity, ρ(T ), the penetration depth, λ(T ), and upper critical magnetic field, Hc2(T ), for polycrystalline samples of dodecaboride ZrB12 and diboride MgB2. We conclude that ZrB12 as well as MgB2 behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with rather low resistive Debye temperature, TR = 280 K, for ZrB12 (as compared to MgB2 with TR = 900 K). The ρ(T ) and λ(T ) dependencies of these samples reveal a superconducting transition of ZrB12 at Tc = 6.0 K. Although a clear exponential λ(T ) dependence in MgB2 thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB12 below Tc/2. These features indicate s-wave pairing state in MgB2, whereas a d-wave pairing state is possible in ZrB12. A fit to the data gives a reduced energy gap 2∆(0)/kBTc = 1.6 for MgB2 films and pellets, in good agreement with published data for 3D π -sheets of the Fermi surface. Contrary to conventional theories we found a linear temperature dependence of Hc2(T ) (Hc2(0) = 0.15 T ) for ZrB12.