Point-Contact Spectroscopy of Multigap Superconductors

Abstract

Point-contact spectroscopy offers a unique possibility to study the fundamental superconducting properties. Namely, the superconducting energy gap, its symmetry, multiplicity, the temperature and magnetic field dependence can be addressed usually on the scale of hundreds of nanometers. From the very beginning of the discovery of superconductivity in magnesium diboride, this technique has been applied for the investigation of the two-gap superconductivity in this compound. Very recently discovered superconducting iron pnictides are intensively studied by this technique as well. Here, we shortly review the point contact experiments leading to one of the first experimental evidences of the fact that MgB2 represents an extraordinary example of the multigap superconductivity. We show that particularly the measurements by point contacts in magnetic fields with the sample in the vortex state provide additional important informations directly in the raw data, thus not depending on a particular model used for fitting. Namely, the direct experimental evidence of the coexistence of two well-distinct superconducting energy gaps up the common transition temperature is shown. The small gap and small superconducting coupling below the BCS value characterize the π band, while the large gap and the strong coupling are found in the hole σ band. Also the carbon and aluminum doped MgB2 samples have been intensively studied. It is shown that the hole band filling effect leading to a decrease in the density of states due to electron doping by carbon and aluminum is very important. It prevails over the interband scattering introduced by doping MgB2 by these elements in the investigated doping range. This is the reason why the two gap superconductivity is preserved also in the case when T c is significantly suppressed. The effect of applied magnetic field on the point-contact spectra is also used to study the intraband scattering processes within the two bands indicating that the carbon doping enhances significantly the scattering inside the π band. This leads to a strong increase in the upper critical magnetic field, particularly at the low temperatures, with importance for practical applications. Recent point contact measurements performed on the iron pnictides also show a presence of multigap superconductivity underlying the multiband character of this new class of the high temperature superconductors.