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

SQUID magnetometer operating at 37K based on nanobridges in epitaxial MgB2 thin films

Superconducting quantum interference devices SQUIDs and magnetometers are fabricated from nanoconstrictions in epitaxial MgB2 films. The nanobridges are contained within single-crystalline grains, resulting in clean transport, a large critical current density of 5107 A/cm2 at 4.2 K, and stable SQUID voltage modulation up to 38.8 K. The magnetometer is realized with an inductively coupled pickup loop, giving rise to a field sensitivity of 1 pT Hz¿1/2 down to 1 Hz. The device properties are governed by the two-band superconducting nature of MgB2, posing, however, no problems to a successful development of boride magnetic field sensing devices. The MgB2 zero-temperature London penetration depth is measured to be 62 nm, close to theoretical predictions