Localization and interaction effects in ultrathin epitaxial NbN superconducting films

Area of Science:

• Condensed Matter Physics
• Materials Science

Background:

• Superconductivity in thin films exhibits unique quantum phenomena.
• Understanding the superconducting-insulator transition is crucial for novel electronic devices.

Purpose of the Study:

• Investigate the thickness-dependent superconducting properties of epitaxial NbN films.
• Determine the theoretical framework governing the suppression of the superconducting transition temperature (T(c)).

Main Methods:

• Epitaxial growth of NbN films with thicknesses ranging from 2.0 nm to 20.5 nm.
• Measurement of superconducting transition temperature (T(c)) and sheet resistance (R(sq)).
• Fitting experimental data to theoretical models, including localization theory and the dirty boson model.

Main Results:

• A monotonic decrease in T(c) was observed with increasing inverse thickness (1/d).
• The suppression of T(c) closely followed the Finkel'stein formula from localization theory.
• The critical sheet resistance (R(c)) for the superconducting-insulator transition was significantly lower than predicted by the dirty boson model.

Conclusions:

• The observed depression of T(c) in NbN films is governed by localization effects.
• The dirty boson model does not accurately describe the superconducting-insulator transition in this system.
• Localization theory provides a more suitable explanation for the observed phenomena in thin NbN films.