'Lazy Fisherman' method of vortex analysis: application to MgB2

Among the different methods of vortex study in superconductors, Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), occupies a particular place. Due to its extraordinary spatial and energy resolution STS allows the study of vortices in real space on a nanometeric scale. In practice however, the success of the observation of vortices by STS depends strongly on the surface quality. Usually highly uniform surfaces are needed to distinguish vortices. As a result of this limitation, the vortex lattice has been successfully studied only in a limited number of superconductors. Here we suggest a simple method extending the vortex analysis to 'difficult cases' of 'bad' superconductors, thin films and even granular materials. In this method the STM tip is positioned in a chosen location of the sample and the local tunneling spectroscopy is continuously performed while the magnetic field is slowly swept. The vortices entering the sample move under the tip and are detected due to the change in the shape of tunneling spectra. We show that the method allows not only to follow the vortex penetration but also to estimate essential superconducting parameters such as the penetration and the coherence lengths. Moreover, it authorizes the study of the vortex cores in the cases where scanning is difficult or impossible. (c) 2005 Elsevier Ltd. All rights reserved.