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Detection of single infrared photons in superconducting microstrips of 4 nm thick disordered Nb0.15Re0.85 has been investigated. Microstrips with a critical temperature of 5.15 K and widths from 1.0 to 2.5 μm have been fabricated by optical lithography. We demonstrate single photon detection sensitivity at 1.5 μm wavelength at a temperature of 1.79 K. By investigating the detection process at this temperature, we find that the current bias threshold is at 21% of the depairing current. This threshold is similar to what should be observed in typical amorphous superconductors, which confirms that ultrathin disordered Nb0.15Re0.85 is an interesting material for superconducting microstrip single photon detectors that operate above 1 K. [ABSTRACT FROM AUTHOR]

Ultrathin microstrips based on polycrystalline NbRe films were investigated in order to preliminarily test the suitability of this material for the realization of superconducting single-photon detectors. The voltage-current characteristics measured on these samples show clear hysteresis. This is a fundamental ingredient for investigating single-photon detection as well as single vortex fluctuation phenomena in 2D NbRe-based devices. [ABSTRACT FROM AUTHOR]

The impressive advances in material science and nanotechnology are more and more promoting the use of exotic barriers and/or superconductors, thus paving the way to new families of Josephson junctions. Semiconducting, ferromagnetic, topological insulator and graphene barriers are leading to unconventional and anomalous aspects of the Josephson coupling, which might be useful to respond to some issues on key problems of solid state physics. However, the complexity of the layout and of the competing physical processes occurring in the junctions is posing novel questions on the interpretation of their phenomenology. We classify some significant behaviors of hybrid and unconventional junctions in terms of their first imprinting, i.e., current-voltage curves, and propose a phenomenological approach to describe some features of junctions characterized by relatively high critical current densities Jc. Accurate arguments on the distribution of switching currents will provide quantitative criteria to understand physical processes occurring in high-Jc junctions. These notions are universal and apply to all kinds of junctions. [ABSTRACT FROM AUTHOR]

We present femtosecond pump-probe spectroscopy studies of time-resolved optical reflectivity of all- oxide YBa2Cu3O7/La07Sr0.3MnO3 (YBCO/LSMO) superconductor/ferromagnet (S/F) bilayers consisting of a 100-nm-thick YBCO base layer and either 10 or 35 nm LSMO cap thickness. At temperatures far below the YBCO superconducting transition Tc, samples with a 10 nm F overlayer show a photoresponse that is similar to, but faster than, pure-YBCO, 100-nm-thick reference samples, while close to Tc and above (up to ∼ 160 K) we observe a signature of both the electronic and spin response that cannot be interpreted as an incoherent sum of contributions from the two layers. The photoresponse of the S/F structures with the 35-nm LSMO layer always qualitatively follows that of the pure LSMO, but with a shorter relaxation time. In all cases, the YBCO/LSMO nonequilibrium dynamics can be modeled using a generalized multitemperature model, which is a superposition of the dynamics of the three-temperature models that are used to describe the superconductor and ferromagnet subsystems, respectively. The long term of the photoresponse signal of the S/F bilayer can be well fitted with the two characteristic relaxation times. [ABSTRACT FROM AUTHOR]

Abstract: Weak ferromagnet/superconductor (F/S) proximity bilayers have been fabricated and characterized for basic physics studies concerning the ultrafast carrier dynamics in layered materials. The normalized reflectivity change (ΔR/R) as a function of the time delay between the pump and the probe laser beams has been measured in F/S heterostructures formed by a low critical temperature superconductor (Nb) with a NiCu overlayer, and a high T c superconductor (YBCO) covered by Au/NiCu and manganite (LSMO) overlayers. The attention is mainly focused to the investigation of nonequilibrium excitation dynamics inside different bilayers in the low temperature region. The presence of the weak ferromagnetic overlayer produces faster optical relaxation times with respect to sole superconducting films. The results are promising in view of potential applications as ultrafast kinetic inductance superconducting photodetectors as confirmed by preliminary time-resolved photoimpedance experiments on both Nb and NiCu(21nm)/Nb(70nm) samples. [Copyright &y& Elsevier]

Ferromagnet/superconductor (F/S) bilayers composed of ultrathin metallic films demonstrate new proximity effects and show a peculiar behavior of the superconductor-order parameter. In terms of applications, F films provide additional "anchoring" of the S film vortex lattice, resulting in increased critical current. By changing composition and the thickness ratio of the F/S bilayer, one can modify the quasiparticle dynamics through an effective enhancement or suppression of the quasiparticle lifetime under nonequilibrium conditions. The latter effect should lead to "custom-designed" superconducting detectors, with the sensitivity and photoresponse speed tailored to a given application. We present fabrication details of nanometer-thickness NiCu/Nb bilayers of different F/S ratios. The NiCu/Nb structures have been implemented to fabricate Josephson junctions and tested using femtosecond optical spectroscopy. The time-resolved photoresponse experiments allowed us to measure the quasi-particle relaxation dynamics in our bilayers. In addition, the photoresponse temperature dependence studies revealed the spatial evolution of the superconductor-order parameter across the bilayer. Comparison between the bilayer data and the single-material data is also presented. [ABSTRACT FROM AUTHOR]

We present experimental results concerning both the fabrication and the characterization of superconducting tunnel junctions based on Nb/NiCu superconductor/ferromagnet (S/F) bilayers. Josephson Junctions have been characterized down to T = 1.4 K in terms of current-voltage I-V characteristics, Josephson critical current vs magnetic field, role of external magnetic field. Proximity parameters of the bilayer, such as the coherence length and the exchange field energy of the F metal, have been estimated by means of a numerical deconvolution of the I-V data the electronic density of states on both sides of the S/F bilayer. Results have been compared with theoretical predictions from a proximity model for S/F bilayers in the dirty limit in the framework of Usadel equations for the S and F layer, respectively. [ABSTRACT FROM AUTHOR]

Experimental investigations on the proximity effect in Nb/Al bilayers by using Nb/AlxOy/Nb/Al/AlxOy/Nb structures with different Al thickness are reported. Three-terminal stacked devices were fabricated by using a new process based on a whole-wafer deposition of the multilayer without any vacuum breaking in order to ensure a high control of the interfaces. The quasiparticle density of states at both sides of the bilayer and the effective interface coupling parameters were obtained by a deconvolution of the I–V data. They were compared with numerical calculations based on the microscopic model proposed by Golubov et al. [Phys. Rev. B 51 (1995) 1073] for the proximity effect in spatially inhomogeneous bilayers. The investigation has been also performed in the presence of steady-state injection at different quasiparticle energies. [Copyright &y& Elsevier]

Since many years, superconductive tunnel junction devices have been widely investigated in the context of energy spectroscopy. High-resolution detectors for energy spectroscopy have been realized based on the single particle tunneling. On the other hand, the Josephson effect offers a very powerful tool for the design of proper detector configurations and an intrinsic potential for fast-response detectors to the radiation interaction. The present paper deals with this last issue discussing the underlying physical aspects and potential performance of such devices. [Copyright &y& Elsevier]

Discusses the fabrication and testing of a superconducting device with transistor-like properties. Applications of superconducting transistors with large current gain and bandwidth at low temperature; Tunnel junction structure; Current amplification factors.

Interest in superconducting transistors is due to some potential benefits of these devices, such as the possibility to have large current gain and bandwidth at low temperatures. In this work we present experimental results concerning with a new superconducting 3-terminal device showing true transistor-like properties at T=4.2K. It is a stacked double tunnel junction structure where the intermediate film is a bilayer of superconducting Nb with an Al quasiparticle trap which can be either in the superconducting or in the normal metal state. Current amplification factors of up to 2.0 are observed at a temperature of 4.2 K when the Al is superconducting. When the Al is in the normal state, large current gains of more than 50 are observed. The operation is highly directional, i.e. practically no effect is observed when the roles of the injector and detector junctions are reversed. The results are explained according to the recently proposed QUAsiparticle TRApping TRANnsistor model. [ABSTRACT FROM AUTHOR]

A novel class of sensors is constituted by radiation detectors based on superconducting tunnel junctions, which could lead in the next future to an advanced instrumentation with an interesting fall out for applications. So far, this area is restricted to LTc superconductors. Among them, niobium has the highest critical temperature and is therefore less demanding in terms of operational temperature. The best performances achieved so far in pure Nb-based junction are far away from the expected theoretical limit. The reason of this discrepancy could be attributed to various problems. Quasiparticle diffusion and edge losses seem to be responsible for the broadening of the signal amplitude spectra so often observed and reported in literature. In this paper we report on efforts to investigate the response to X-Ray of Nb-based superconducting tunnel junctions in terms of a model which includes the quasiparticle diffusion and the edge losses. Our model takes into account the contribution from both the electrodes of the junction and well reproduces the experimental data. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

We present measurements of ferromagnet/superconductor (NiCu/NbN) and plain superconducting (NbN) nanostripes with the linewidth ranging from 150 to 300 nm. The NiCu (3 nm)/NbN (8 nm) bilayers, as compared to NbN (8 nm), showed a up to six times increase in their critical current density, reaching at 4.2 K the values of 5.5 MA/cm2 for a 150 nm wide nanostripe meander and 12.1 MA/cm2 for a 300 nm one. We also observed six-time sensitivity enhancement when the 150 nm wide NiCu/NbN nanostripe was used as an optical detector. The strong critical current enhancement is explained by the vortex pinning strength and density increase in NiCu/NbN bilayers and confirmed by approximately tenfold increase in the vortex polarizability factor. [ABSTRACT FROM AUTHOR]

The attainment of high-quality Josephson junctions on electro-optical substrates constitutes a key step toward the fabrication of functional superconducting opto-electronics. We report on the fabrication of high-quality all-refractory niobium superconducting tunnel junctions deposited on X-cut LiNbO3 single crystals. A high quality factor Vm=0.7 ΔIgR2 mV (where ΔIg is the current jump at the sum gap voltage and R2 mV the static resistance at V=2 mV) up to 61 mV has been measured at T=4.2 K. The very low contribution of subgap leakage currents is demonstrated by the BCS-like quasiparticle current measured at V=0.1 mV down to T=1.9 K. [ABSTRACT FROM AUTHOR]

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-nm-thick NiCu film grown on top of 8-nm-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. [ABSTRACT FROM AUTHOR]

We have fabricated and studied a stacked superconducting double tunnel junction device with transistor-like properties. The intermediate electrode is a bilayer consisting of a Nb film together with an Al film that acts as a quasiparticle trap. Large current gains of more than 50 are observed at 4.2 K when the Al layer is normal. The operation is highly directional. Results are explained on the basis of trapping of quasiparticles from a superconductor into a normal metal, together with a conversion of relaxation energy into electronic excitations. Similar devices should have wide applications in low-temperature measurement and detection systems. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The study of the flux-flow instability in superconducting materials has recently gained renewed attention due to its potential implications for the use of the analyzed materials as micrometer-sized superconducting detectors for single photons. The values of the quasiparticle relaxation time (τ E) measured for these detectors are affected by pinning properties. Here, we report electric transport properties of NbReN microstrips of different quality. For the strip characterized by high resistivity, and large critical currents and pinning, we estimate a value of τ E that is almost two orders of magnitude larger compared to that of another strip with a smaller value for the critical current, for which we measure τ E ∼ 12 ps. This low value is comparable to those reported in the literature for microstrips made of other highly-disordered superconductors. Our results suggest that NbReN microstrips have great potential for the realization of superconducting single-photon detectors, depending on further optimization of their fabrication process and the superconducting properties affected by it. • Flux-flow in superconductors gains attention for single-photon detectors. • τ E in NbReN microstrips suggests competitive detector performance. • Optimization of device geometry and fabrication can enhance performance. [ABSTRACT FROM AUTHOR]