Nanostripes of hybrid superconductor/ferromagnetic (S/F) NbN/NiCu bilayers and pure superconducting NbN nanostripes have been investigated in dark count experiments. Presence of a ferromagnetic layer influences the superconducting properties of the S/F bilayer, such as the critical current density and the transient photoresponse. The observed significant decrease of the dark-count rate is discussed in terms of vortex-related fluctuation models to shed more light in the intriguing question of the basic mechanism responsible for dark counts in superconducting nanostripe single photon detectors.
Nanostripes of hybrid superconductor/ferromagnetic (S/F) NbN/NiCu bilayers and pure superconducting NbN nanostripes have been investigated in dark count experiments. Presence of a ferromagnetic layer influences the superconducting properties of the S/F bilayer, such as the critical current density and the transient photoresponse. The observed significant decrease of the dark-count rate is discussed in terms of vortex-related fluctuation models to shed more light in the intriguing question of the basic mechanism responsible for dark counts in superconducting nanostripe single photon detectors.
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.
The phenomenon of dark counts in nanostripes of different superconductor systems such as high-temperature superconducting YBa2Cu3O7-x and superconductor/ferromagnet hybrids consisting of either NbN/NiCu or YBa2Cu3O7- x/L0.7Sr0.3MnO3 bilayers have been investigated. For NbN/NiCu the rate of dark-count transients have been reduced with respect to pure NbN nanostripes and the events were dominated by a single vortex entry from the edge of the stripe. In the case of nanostripes based on YBa2Cu3O7-x, we have found that thermal activation of vortices was also, apparently, responsible for triggering dark-count signals.
Transport properties of NbN/NiCu superconductor/ferromagnet (S/F) nanostripes fabricated in both in single-wire and series-parallel, meander-type configurations are presented down to T = 4.2 K. In particular, the enhancement of the superconducting critical current has been observed at smaller widths, apparently, due to an extra pinning mechanism, arising from clustering of ferromagnetic atoms inside the thin S layer. Moreover, we observed a number of characteristic voltage steps on the nanostripe current-voltage characteristics and their nature was investigated as a function of temperature. An explanation in terms of active phase-slip phenomena has been proposed based of the time-dependent Ginzburg-Landau theory and led to an estimation of the inelastic electron-phonon relaxation time τe-ph ~ 1 ps, in agreement with the τopt = 1.2±0.3 ps value, measured by the femtosecond transient optical reflectivity spectroscopy method on the same bilayer. Transient optical properties of our superconducting S/F nano-bilayers have been also investigated and compared to those obtained for pure NbN nanostripe reference samples. Finally, electrical photoresponse signals of S/F heterostructures exposed to ultraweak pulsed (width 400 ps, repetition rate ~100 MHz) laser radiation at 850 nm wavelength exhibited the falling time of voltage responses directly dependent on the NiCu overlayer. We have also noticed that the presence of the top F layer and the resulting proximity effect reduced frequency of dark counts in our samples.
We present our studies on dark counts, observed as transient voltage pulses, in current-biased NbN superconducting single-photon detectors (SSPDs), as well as in ultrathin (~4 nm), submicrometer-width (100 to 500 nm) NbN nanobridges. The duration of these spontaneous voltage pulses varied from 250 ps to 5 ns, depending on the device geometry, with the longest pulses observed in the large kinetic-inductance SSPD structures. Dark counts were measured while the devices were completely isolated (shielded by a metallic enclosure) from the outside world, in a temperature range between 1.5 and 6 K. Evidence shows that in our two-dimensional structures the dark counts are due to the depairing of vortex-antivortex pairs caused by the applied bias current. Our results shed some light on the vortex dynamics in 2D superconductors and, from the applied point of view, on intrinsic performance of nanostructured SSPDs.
We present optical properties of nanowires consisting of both proximitized superconductor/ferromagnet bilayers of NbN/NiCu and pure superconducting NbN layers. Thin NbN/NiCu hetero-structures were deposited on MgO(100) substrates by in situ dc magnetron sputtering, while nanowires were patterned using standard electron beam lithography and reactive ion etching. Photo-response signals were recorded from 100-nm-wide, three-parallel and six-series NbN(8 nm)/NiCu nanowires by 850-nm pulsed laser irradiation (time width 400 ps, repetition rate ~ 100 MHz). For the latter configuration the fall time of voltage responses was found to be close to 1.7 ns, and it gradually increased with increasing the NiCu film thickness. Moreover, three discrete families of photo-response signals recorded for the three-parallel and six-series nanowires were also observed.
Abstract Superconducting microstrip single photon detectors (SMSPDs) received great interest since they are expected to combine the excellent performance of superconducting nanostrip single photon detectors with the possibility to cover large active areas using low-cost fabrication techniques. In this work, we fabricated SMSPDs based on NbRe to investigate the role of vortices in the dark counts events in this innovative material and in devices with micrometer size. We realized devices with different layouts, namely single microstrips and pairs of parallel microstrips. The energy barriers related to the motion of single vortices or vortex–antivortex pairs, responsible of detection events, have been determined and compared with the ones of similar devices based on different materials, such as MoSi, WSi and NbN. The analysis confirms the high potential of NbRe for the realization of superconducting single photon detectors with large areas.