We measured the specific heat, the magnetization, and the magnetoresistance of a single crystal of ZrB 12 , which is superconducting below T c Х 6 K. The specific heat in zero field shows a BCS-type superconducting transition. The normal-to superconducting-state transition changes from first order ͑with a latent heat͒ to second order ͑without latent heat͒ with increasing magnetic field, indicating that the pure compound is a low-, type-II/1 superconductor in the classification of Auer and Ullmaier ͓Phys. Rev. B 7, 136 ͑1973͔͒. This behavior is confirmed by magnetization measurements. The H-T phase diagram based on specific-heat and magnetization data yields H c2 ͑0͒ = 550 G for the bulk upper critical field, whereas the critical field defined by vanishing resistance is a surface critical field H c3 ͑0͒ϳ1000 G.

We present the first results of point-contact spectroscopy (PCS) in single crystals of the boride superconductor ZrB 12 . The material was first characterized by AC susceptibility and resistivity measurements that allowed us to determine several quantities of physical interest. Then we performed point-contact measurements that gave conductance curves with clear Andreev-reflection features typical of an s-wave single-gap superconductor. By measuring these conductance curves at various temperatures and in the presence of magnetic fields, we obtained the temperature dependence of the gap and of the upper critical field of ZrB 12 , as well as an estimate of the Fermi energy. All the results indicate that ZrB 12 is a conventional s-wave weak coupling superconductor.

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