Superconducting ultra-narrow aluminum nanowires

A well known theorem of Mermin and Wagner explicitly states that Superconductivity in one or two dimensions is not possible. However, experimentally, it is well known that there are wires (1D) and films (2D) which superconduct. The natural question then is: which parameters control the behavior of these wires and films? Since the fabrication of films is much easier than the fabrication of wires, 2D systems have been studied in much more detail from an experimental and theoretical point of view. In 1D systems recent theoretical works have suggested the existence of a critical length scale, the wire width: in wires less than 10 nm wide, the superconductivity is extinguished. Unfortunately, the difficulties inherent to the fabrication of such thin wires have slowed down the research in this field. Just a few years ago, new experimental techniques emerged which have been applied to this problem; the authors did not find any clear evidence of the role of this particular length. What was found, instead, was that the extinction of superconductivity appears when the resistance per superconducting coherence length is greater than a fraction of the quantum of superconducting resistance (R q = h/4e2). In order to study this physical phenomenon, and possibly many others, we have developed a new technique for fabricating ultra narrow wires. Our technique relies on Molecular Beam Epitaxy (MBE) grown substrate in order to fabricate wires as wide (in principle) as a few atomic planes and whose length can be made (almost) arbitrarily long. In particular, wires 8 nm wide and as long as 100 μm have been fabricated. These wires were made of aluminum, which is the material closest to the predictions of the BCS theory in its simplest form. Their transition in magnetic field has shown deviations from LAMH theory, based on classical Thermal Activated Phase Slip, but compatible with more recent theories of Quantum Phase Slip. Moreover, we have observed, in the Current-Voltage characteristics, a stepped structure, which is probably due to Quantum Phase Slips, induced by the magnetic field