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We report on several low-temperature experiments supporting the presence of Majorana fermions in superconducting lead nanowires fabricated with a scanning tunneling microscope (STM). These nanowires are the connecting bridges between the STM tip and the sample resulting from indentation-retraction processes. We show here that by a controlled tuning of the nanowire region, in which superconductivity is confined by applied magnetic fields, the conductance curves obtained in these situations are indicative of topological superconductivity and Majorana fermions. The most prominent feature of this behavior is the emergence of a zero bias peak in the conductance curves, superimposed on a background characteristic of the conductance between a normal metal and a superconductor in the Andreev regime. The zero bias peak emerges in some nanowires when a magnetic field larger than the lead bulk critical field is applied. This field drives one of the electrodes into the normal state while the other, the tip, remains superconducting on its apex. Meanwhile a topological superconducting state appears in the connecting nanowire of nanometric size. © IOP Publishing and Deutsche Physikalische Gesellschaft.

This paper is based on field data collected during a basic survey of mining exploration carried out by the authors for the Instituto Geologico y Minero de España. The investigated area belongs to the southeastern part of the Ordenes Complex and is mainly composed of metabasites and ultramafic rocks. Inside the area, three great ultramafic bodies can be mainly considered. They are more or less parallel and trend in a NNE direction, being separated by zoisite-bearing amphibolites. Asbestos showings are numerous in the eastern ultramafic body, which forms the Careon range, the length of the fibra ranging up to 14 mm. Only two minor asbestos occurrences have been found in the central body, and, finally, a few asbestos veinlets have been seen in the western ultramafic body, with fibres not reaching 1 mm in length. Chrysotile asbestos mineralization in the area always belongs to the ‘cross-fibre’ type. Brief descriptions of asbestos occurrences and geological framework are made, and hypotheses about asbestos genesis in the area are finally set up.