Andreev-reflection spectroscopy of the gap in ZrB12 single crystals

We present the first results of point-contact spectroscopy (PCS) in single crystals of the boride superconductor ZrB12. 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 ZrB12, as well as an estimate of the Fermi energy. All the results indicate that ZrB12 is a conventional s-wave weak coupling superconductor.