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 .

Using a set of micro Hall probes for the detection of the local induction, the inhomogeneous Condon domain structure has been directly observed at the surface of a pure silver single crystal under strong Landau quantization in magnetic fields up to 10 T. The inhomogeneous induction occurs in the theoretically predicted part of the Condon domain phase diagram. Information about size, shape, and orientation of the domains is obtained by analyzing Hall probes placed along and across the long sample axis and by tilting the sample. On a beryllium surface the induction inhomogeneity is almost absent although the expected induction splitting here is at least 10 times higher than in silver.

We describe a novel single-coil mutual inductance technique for measuring the magnetic penetration depth of superconductors at 2-4 MHz as a function of temperature in the 4-100 K range. We combine a single-coil configuration with a high-stability marginal oscillator; this enables us to measure the absolute value of on both bulk samples and thin films with very high resolution (␦ϭ10 pm͒ and a precision of 30 nm. As example of application, we report measurements on NbTi bulk samples and Nb films. This contactless technique is suited for probing the superconducting properties of samples over large surfaces.

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