Suppression of superconductivity in epitaxial MgB2 ultrathin films

Abstract

MgB2 ultrathin films have potential to make sensitive superconducting devices such as superconducting single-photon detectors working at relatively high temperatures. We have grown epitaxial MgB2 films in thicknesses ranging from about 40 nm to 6 nm by using the hybrid physical-chemical vapor deposition method and performed electrical transport measurements to study the thickness dependence of the superconducting critical temperature Tc. With reducing film thickness d, although a weak depression of the Tc has been observed, which could be attributed to an increase of disorder (interband impurity scattering) in the film, the Tc retains close to the bulk value of MgB2 (39 K), being about 35 K in the film of 6 nm thick. We show that this result, beneficial to the application of MgB2 ultrathin films and in accordance with recent theoretical calculations, is in contrast to previous findings in MgB2 films prepared by other methods such as co-evaporation and molecular-beam epitaxy, where a severe Tc suppression has been observed with Tc about one third of the bulk value in films of ∼5 nm thick. We discuss this apparent discrepancy in experiments and suggest that, towards the ultrathin limit, the different degrees of Tc suppression displayed in currently obtained MgB2 films by various techniques may arise from the different levels of disorder present in the film or different extents of proximity effect at the film surface or film-substrate interface.

Fabrication of superconducting nanowires from ultrathin MgB2 films via focused ion beam milling

“…Note that it also indicates a good uniformity of the precursor ultrathin film, which provided the basis for us to obtain MgB 2 nanowires showing narrow superconducting transitions. Moreover, it is noted that the T c 's of the fabricated MgB 2 microstrips and nanowires are quite similar to those of our previously reported MgB 2 ultrathin films, 17,25 which, unlike the present study, were not covered by any protective layers on top of the films. This fact suggests that the superconductivity of the MgB 2 microstrips and nanowires have not been apparently affected by the possible proximity effect 26 between the microstrips or nanowires and the metallic Ti/Au layers on top of them.…”

“…This fact suggests that the superconductivity of the MgB 2 microstrips and nanowires have not been apparently affected by the possible proximity effect 26 between the microstrips or nanowires and the metallic Ti/Au layers on top of them. 25 To further characterize the superconductivity of the nanowires, we have performed the IVCs measurements to determine the critical current density J c as a function of temperature. Here, we have chosen to measure the J c 's of the fabricated nanowires in 10 µm length as they may help to better indicate the homogeneity and current-carrying capability of the nanowires.…”

Shape resonances and shell effects in thin-film multiband superconductors

“…Even though the scattering rates in Ref. [19] are attributed to the film granularity, tunneling into the substrate is expected to also contribute to the level broadening. In any case, decoherence of any form is effectively modelled by a finite τ so our results are, at least qualitatively, applicable to more general situations.…”

“…Here we use recent experimental results [19] for MgB 2 and assume a linear dependence for the level broadening with the film thickness. We note that both the energy spectrum and the wavefunctions inside the film are modified by tunneling into the substrate.…”

Orbital effect on the in‐plane critical field in free‐standing superconducting nanofilms

“…Recently, the enhancement of the superconducting critical temperature with respect to the bulk limit has been reported for interfaces and heterostuctures based on cuprates (), iron pnictides (), and LaAlO/SrTiO (). Moreover, the experimental reports on the growth techniques of high quality MgB films of thicknesses less than 10 nm entailed a series of theoretical papers describing the quantum size effect in multiband thin film superconductors .…”