关键词: Nb meander nanowire, critical temperature, critical field, critical current

Abstract: We report the fabrication and the study of superconducting properties of ultra-thin Nb superconducting meander nanowires, which can be used as superconducting nanowire single-photon detectors (SNSPDs). The ultra-thin (about 7-nm thick) Nb films are patterned into micro-bridges, and 100-nm wide meander nanowires by using e-beam lithography (EBL). The average transition temperature (T_c) of the nanowires is about 4.8 K and the critical current density j_c is about 2.8× 10⁶ A/cm². Superconducting characteristics of the specimens at different applied magnetic fields up to 8 T (parallel or perpendicular to the specimen) are systematically investigated. The normalized temperature t (=T/T_c) dependences of the parallel critical field (H_c||) for both the micro-bridge and the meander nanowire are almost the same, following the Ginzburg and Landau (GL) formalism for ultra-thin films. However, in perpendicular field and in the vicinity of T_c (>0.95T_c), the critical field H_c⊥ of the nanowire exhibits a down-turn curvature nonlinear temperature dependence while the micro-bridge displays a linear temperature dependence. The nonlinear behavior of H_c⊥ in the nanowire is believed to be due to the fact that in the vicinity of T_c the coherence length becomes larger than the line width. Additionally, the localization of carriers in the nanowire could also contribute to the nonlinear behavior. The resistive transitions could be described by the phase-slip model for quasi-one-dimensional system. Moreover, the hysteresis in I-V curve of the meander nanowires can be illustrated by a simple model of localized normal hotspot maintained by Joule heating.

Key words: Nb meander nanowire, critical temperature, critical field, critical current