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J C (T) dependence for a 400-nm-wide NbN(6 nm)/NiCu(6 nm) mircobridge. The inset shows R(T) dependences near T C for both NbN(6 nm)/NiCu(6 nm) and NbN(6 nm) films.
Source publication
Performance of superconducting single-photon detectors based on resistive hotspot formation in nanostripes upon optical photon absorption depends strongly on the critical current density J C of the fabricated nanostructure. Utilization of an ultrathin, weak-ferromagnet cap layer on the top of a superconducting film enhances of the structure's J C d...
Contexts in source publication
Context 1
... bilayers were patterned into microbridges using optical lithography and reactive ion etching. The bridge width varied from 400 nm to 2 µm and the length was 40 µm. All transport measurements were done in a four-probe configuration. Figure 2 shows typical current-voltage (I-V) characteristics for a 400-nm-wide NbN(6 nm)/NiCu(6 nm) bridge, while Fig. 3 shows the corresponding J C dependence on temperature T for this bridge. The inset in Fig. 3 presents a comparison between resistive temperature transitions R(T) of the NbN/NiCu tested structure and a reference 6-nm-thick NbN microbridge. Analyzing the data presented in Figs. 2 and 3, we note that I-V curves are very typical for a long ...
Context 2
... The bridge width varied from 400 nm to 2 µm and the length was 40 µm. All transport measurements were done in a four-probe configuration. Figure 2 shows typical current-voltage (I-V) characteristics for a 400-nm-wide NbN(6 nm)/NiCu(6 nm) bridge, while Fig. 3 shows the corresponding J C dependence on temperature T for this bridge. The inset in Fig. 3 presents a comparison between resistive temperature transitions R(T) of the NbN/NiCu tested structure and a reference 6-nm-thick NbN microbridge. Analyzing the data presented in Figs. 2 and 3, we note that I-V curves are very typical for a long and narrow superconducting constriction, with well-defined transition between the ...
Context 3
... for a long and narrow superconducting constriction, with well-defined transition between the superconducting and resistive states at low temperatures. The sample's J C reaches a respective 1 MA/cm 2 value at low temperatures, and T C is appox. 12 K. The latter value is about 2 K suppressed in comparison to the pure NbN reference sample (inset in Fig. 3). Temperature (K) 10 ...
Citations
... In the normal state, however, the S film has a vanishing MR, while the S/F bilayer exhibits a negative MR over a wide temperature range that increases towards lower temperatures. The negative MR appears compatible with a Curie temperature of approximately 30 K found in our reference NiCu films [10], but its temperature dependence follows the theory [11] for spin-fluctuation induced negative MR only qualitatively. ...
... At low temperatures, ε < 0.05, the paraconductivity drops dramatically below both the 2D-AL and 1D-AL predictions. Again, we invoke the influence of ferromagnetic domains in the NiCu cap layer and can fit our experimental results to Eq. 2, assuming |BFM| = 0.5 T and ξ(0) = 4.7 nm, shown as a full line in Fig. 2. The |BFM| used in this fit appears to be larger than in previous magnetization measurements on 6-nm-thick NiCu films and NbTiN(6 nm)/NiCu(6 nm) bilayers [10]. However, the overall sample magnetization with the magnetic field oriented parallel to the film surface in these measurements allows that the local perpendicular magnetization of individual domains might be larger. ...
Thermodynamic fluctuations of the superconducting order parameter in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) thin bilayers patterned to microbridges are investigated. Plain NbN and NbTiN films served as reference materials for the analyses. The S/F bilayers show a negative magnetoresistance up to almost room temperature, while the signature of fluctuations is similar to that in the plain films, demonstrating the relevance of both ferromagnetic and superconducting phenomena in the S/F bilayers. The paraconductivity is reduced below theoretical predictions, in particular in the NbTiN/NiCu bilayers.
Thermodynamic fluctuations of the superconducting order parameter in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) thin bilayers patterned to microbridges are investigated. Plain NbN and NbTiN films served as reference materials for the analyses. The samples were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the superconducting films were coated with NiCu overlays, using co-sputtering. The positive magnetoresistance of the superconducting single layers is very small in the normal state but with a sharp upturn close to the superconducting transition, a familiar signature of superconducting fluctuations. The fluctuation-enhanced conductivity (paraconductivity) of the NbN and NbTiN single layer films is slightly larger than the prediction of the parameter-free Aslamazov-Larkin theory for order-parameter fluctuations in two-dimensional superconductors. The addition of a ferromagnetic top layer, however, changes the magnetotransport properties significantly. The S/F bilayers show a negative magnetoresistance up to almost room temperature, while the signature of fluctuations is similar to that in the plain films, demonstrating the relevance of both ferromagnetic and superconducting effects in the S/F bilayers. The paraconductivity is reduced below theoretical predictions, in particular in the NbTiN/NiCu bilayers. Such suppression of the fluctuation amplitude in S/F bilayers could be favorable to reduce dark counts in superconducting photon detectors and lead the way to enhance their performance.
