Abstract
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.
Highlights
We have shown in previous work [17, 23,24,25] that sharp and elongated nanotips or nanoprotrusions on the sample surface resulting from the tip–sample indentations remain superconducting for magnetic fields much larger than the bulk critical field of lead, depending on their sharpness and dimensions compared with the bulk coherence length and penetration depth, while the larger bulk, flat or blunt parts of the Pb tip and sample have become normal
We show the evolution of the conductance curves versus magnetic field in a situation where no ‘anomalous’ or topological superconducting state is expected to happen
As we increase the external magnetic field, once the bulk critical field of lead is crossed, we observe a sharp transition from SS spectroscopic curves below Hc, to NS like curves, with no indication of zero bias current
Summary
Length scales of the superconducting condensate which are closely related to the Fermi wavelength in each type of material Another important aspect is related to the requirement of a small number of quantum electronic modes involved in the experimental object. If we use a metal, the condition of having a small number (of the order of one) of quantum modes, or channels, is only achieved if the diameter of the nanowire where topological superconductivity will be induced is of the order of a few atoms [15] This small (atomic) dimension of the nanowire implies that the effective superconducting coherence length will be strongly reduced. It might be possible to tune the parameters of the device by changing the bias voltage, a detailed analysis of this topic lies outside the scope of this work
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