Thermal fluctuations in hybrid superconductor/ferromagnetic
NbN/NiCu bilayers, as well as in pure superconducting NbN, two-dimensional (2D), nanostripes, have been investigated in order to understand the origin of dark counts in superconducting nanostripes when operated as single-photon detectors in the temperature range from
4.2to8K. In 2D superconductors, the dynamics of vortex motion play a significant role in the formation of a transient normal state, leading to dark-count events in current-biased nanostripes. By introducing a weak ferromagnetic overlayer on top of pure NbN, we managed to control the vortex dynamics, which subsequently enabled us to differentiate between several proposed theoretical models. In particular, a $6\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{nm}\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{thick}$ NiCu film grown on top of $8\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{nm}\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{thick}$ NbN nanostripes led to an enhanced critical current density in the resulting nanostructure, as well as significantly lowered fluctuation rates, as compared to pure NbN structures, leading to reduced dark counts. The enhancement of pinning in
NbN/NiCu bilayers provided evidence that thermal excitations of single vortices (vortex hopping) near the edge of a 2D nanostripe were the dominant mechanism of the observed dark-count transients. On the other hand, in pure NbN the leading source of thermal fluctuations was the current-assisted thermal unbinding of vortex-antivortex pairs.
One of the highest performing single-photon detectors in the visible and near-infrared regions is the superconducting nanostrip photon detector (SNSPD or SSPD), which usually uses NbN or NbTiN as the superconductor. Using other superconductors may significantly improve, for example, the operating temperature and count rate characteristics. This paper briefly reviews the current state of the potential, characteristics, thin film growth, and nanofabrication process of SNSPD using various superconductors.