Abstract
Structural and superconducting properties of high-quality niobium nanofilms with different thicknesses are investigated on silicon oxide (SiO2) and sapphire substrates. The role played by the different substrates and the superconducting properties of the Nb films are discussed based on the defectivity of the films and on the presence of an interfacial oxide layer between the Nb film and the substrate. The X-ray absorption spectroscopy is employed to uncover the structure of the interfacial layer. We show that this interfacial layer leads to a strong proximity effect, especially in films deposited on a SiO2 substrate, altering the superconducting properties of the Nb films. Our results establish that the critical temperature is determined by an interplay between quantum-size effects, due to the reduction of the Nb film thicknesses, and proximity effects. The detailed investigation here provides reference characterizations and has direct and important implications for the fabrication of superconducting devices based on Nb nanofilms.
Highlights
Superconductivity has been recently shown to survive even in extremely confined nanostructures such as metal monolayers [1]
While the superconducting properties of the Nb film is influenced by the lattice parameters of the deposited film as well the grain sizes in the polycrystalline film [27,28], the decrease of the film resistivity in thicker films may suggest a gradual reduction of the film defectivity via atomic rearrangement or increase of the grain size [29,30,31]
An additional effect that could play a role in the suppression of Nb critical temperature, for decreasing thickness, is based on changes in the density of states (DOS) due to the quantization of the electronic motion, while the electron-phonon coupling responsible for Cooper pairing remains almost unaffected by reducing the thickness, as found in Ref. [32]
Summary
Superconductivity has been recently shown to survive even in extremely confined nanostructures such as metal monolayers [1]. The proximity effect occurs when a superconductor is placed in contact with non-superconducting materials. It is shown that the substrate induced surface states can play a significant role in electronic properties of nanostructures [16,17]. A combined electrical and structural study on the presence and role of the proximity effect induced by the substrate on the superconducting properties of Nb ultra-thin films is lacking. In this work we report an extended experimental investigation of the superconducting to normal state transition in Nb nanofilms, with a thickness in the range 9 nm to 80 nm, deposited on SiO2 and Al2O3 substrates. The main microscopic parameters characterizing the normal and the superconducting state of these films were investigated based on recent models. Our study establishes the existence of an interplay between quantum-size and proximity effects at the substrate interface [19,20]
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