Geometry-Driven Dimensional Crossover from Weak Localization to Superconducting Fluctuations in Nanostructured NbN Networks

Abstract

Nanofabrication of disordered superconductors provides a powerful approach to realize multichannel electron and Cooper-pair transport, ultimately leading to a globally phase-coherent superconducting state in the three-dimensional (3D) or quasi-two-dimensional (2D) limit. In such nanostructured systems, the transport mechanism is governed not by the actual film thickness but by the geometry of the fabricated weak links. By confining electron motion to dimensions smaller than the film thickness, the weak-link spacing becomes the critical length scale that dictates quantum transport. In this study, we investigate the recovery of superconducting coherence in an NbN thin film patterned by focused ion beam (FIB) milling into a hexagonal array of nanoholes with diameters of ~1 μm and weak-link spacing of ~80 nm. This spacing defines the effective conduction path for electrons and Cooper pairs. The transport behavior evolves under the combined influence of magnetic field and temperature, where superconducting fluctuations emerge as quantum-interference–driven weak localization is suppressed at the crossover temperature Tpeak=12.9T. These findings demonstrate that the dynamics of electrons, quasiparticles, and Cooper pairs are dictated primarily by the weak-link spacing, revealing a geometry-controlled crossover between weak localization and superconducting fluctuations similar to that observed in true 2D systems. This work establishes a controlled platform for tuning quantum interference and fluctuation phenomena in disordered superconductors and provides new opportunities for exploring vortex dynamics, Bose-metal crossover, and engineered quantum phase transitions, as well as for developing advanced quantum-coherent devices such as phase-slip–based superconducting networks.