Different Single-Photon Response of Wide and Narrow Superconducting MoxSi1−x Strips

The photon count rate (PCR) of superconducting single-photon detectors made of ${\mathrm{Mo}}_{x}{\mathrm{Si}}_{1\text{\ensuremath{-}}x}$ films shaped as a 2-$\ensuremath{\mu}\mathrm{m}$-wide strip and a 115-nm-wide meander strip line is studied experimentally as a function of the dc biasing current at different values of the perpendicular magnetic field. For the wide strip, a crossover current ${I}_{\mathrm{cross}}$ is observed, below which the PCR increases with an increasing magnetic field and above which it decreases. This behavior contrasts with the narrow ${\mathrm{Mo}}_{x}{\mathrm{Si}}_{1\text{\ensuremath{-}}x}$ meander, for which no crossover current is observed, thus suggesting different photon-detection mechanisms in the wide and narrow strips. Namely, we argue that in the wide strip the absorbed photon destroys superconductivity locally via the vortex-antivortex mechanism for the emergence of resistance, while in the narrow meander superconductivity is destroyed across the whole strip line, forming a hot belt. Accordingly, the different photon-detection mechanisms associated with vortices and the hot belt determine the qualitative difference in the dependence of the PCR on the magnetic field.