Effect of disorder in MgB2 thin films

We report on scanning tunneling spectroscopy studies of magnesium diboride (MgB2) thin films grown by different techniques. The films have critical temperatures ranging between 28 and 41 K with very different upper critical fields. We find that the superconducting gap associated with the sigma band decreases almost linearly with decreasing critical temperature while the gap associated with the pi band is only very weakly affected in the range of critical temperatures above 30 K. In the sample with the lowest critical temperature (28 K) we observe a small increase of the pi gap that can only be explained in terms of an increase in the interband scattering. The tunneling data was analyzed in the framework of the two-band model. The magnetic-field-dependent tunneling spectra and the upper critical field measurements of these disordered samples can be consistently explained in terms of an increase of disorder that mostly affects the pi band in samples with reduced critical temperatures

Paraconductivity of MgB2 thin films

Paraconductivity of MgB2 has been measured in homogeneous thin films grown by Hybrid physical chemical vapor deposition. In order to reduce the possible effects of spatial inhomogeneities, stripes of different widths were cut on the films. We measured several samples with different resistivity values; after subtracting the normal state resistivity, paraconductivity appears to be of the same order of magnitude in all the samples. The dependence on the reduced temperature epsilon = ln(T/Tc) is discussed and compared with the existing models. (c) 2007 Elsevier B.V. All rights reserved

Effects of Neutron Irradiation on Magnesium Diboride Thin Films

The effect of disorder on normal state and super-conducting properties is studied in a series of neutron irradiated magnesium diboride thin films. Irradiation increases residual resistivity by more than two orders of magnitude and progressively decreases critical temperature down to the complete suppression of superconductivity. In this disorder-controlled set of samples, we analyze magnetoresistivity, extracting the scattering times in each band, and upper critical field. H-c2 is extremely high and only weakly depends on resistivity at low irradiation level. We propose a simple explanation for this puzzling topic, and we discuss the importance of reaching high magnetic field to correctly extrapolate H-c2 (0)