Evidence for Single-gap Superconductivity in Mg(B_{1-x}C_x)_2 Single Crystals with x=0.132 from Point-Contact Spectroscopy

We report the results of the first directional point-contact measurements in Mg(B_{1-x}C_{x})_2 single crystals with 0.047 <= x <= 0.132. The two-gap superconductivity typical of MgB_2 persists up to x=0.105. In this region, the values of the gaps Delta_{sigma} and Delta_{pi} were determined by fitting the Andreev-reflection conductance curves with a two-band Blonder-Tinkham-Klapwijk (BTK) model, and confirmed by the single-band BTK fit of the sigma- and pi-band conductances, separated by means of a magnetic field. At x=0.132, when T_{c}=19 K, we clearly observed for the first time the merging of the two gaps into one of amplitude Delta~3 meV.Comment: 5 pages, 5 figures. One figure and one panel added; text and discussion update

Directional point-contact spectroscopy of MgB2 single crystals in magnetic fields: two-band superconductivity and critical fields

The results of the first directional point-contact measurements in MgB2 single crystals, in the presence of magnetic fields up to 9 T either parallel or perpendicular to the ab planes, are presented. By applying suitable magnetic fields, we separated the partial contributions of the sigma and pi bands to the total Andreev-reflection conductance. Their fit with the BTK model allowed a very accurate determination of the temperature dependency of the gaps (Delta_sigma and Delta_pi), that resulted in close agreement with the predictions of the two-band models for MgB2. We also obtained, for the first time with point-contact spectroscopy, the temperature dependence of the (anisotropic) upper critical field of the sigma band and of the (isotropic) upper critical field of the pi band.Comment: 2 pages, 2 figures, proceedings of M2S-HTSC-VII conference, Rio de Janeiro (May 2003

Evidence for Single-gap Superconductivity in Mg(B1-xCx)2 Single Crystals with x=0.132 from Point-Contact Spectroscopy

We report the results of the first directional point-contact measurements in MgsB1−xCxd2 single crystals with 0.047<x<0.132. The two-gap superconductivity typical of MgB2 persists up to x=0.105. In this region, the values of the gaps Ds and Dp were determined by fitting the Andreev-reflection conductance curves with a two-band Blonder-Tinkham-Klapwijk sBTKd model, and confirmed by the single-band BTK fit of the s- and p-band conductances, separated by means of a magnetic field. At x=0.132, when Tc=19 K, we clearly observed the merging of the two gaps into one of amplitude D=3 meV

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