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 NbN thin film patterned by focused ion beam (FIB) milling into a hexagonal array of nanoholes with diameters (d) of ∼1 μm and weak-link spacing (s) of ∼81 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, and emerges as quantum-interference-driven weak localization (WL), which coexists with superconducting fluctuations (SCF) at the crossover temperature T_peak = 12.9 K, and later gets suppressed by the SCF below T_peak. 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 WL and SCF, 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.