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The possible use of high-temperature superconductors (HTS) for realizing superconducting nanowire single-photon detectors is a challenging, but also promising, aim because of their ultrafast electron relaxation times and high operating temperatures. The state-of-the-art HTS nanowires with a 50-nm thickness and widths down to 130 nm have been fabricated and tested under a 1550-nm wavelength laser irradiation. Experimental results presenting both the amplitude and rise times of the photoresponse signals as a function of the normalized detector bias current, measured in a wide temperature range, are discussed. The presence of two distinct regimes in the photoresponse temperature dependence is clearly evidenced, indicating that there are two different response mechanisms responsible for the HTS photoresponse mechanisms.

We find an unambiguous relationship between disorder-driven features in the temperature dependence of the resistance and the behavior, as functions of the temperature, of the parameters necessary to describe some of the relaxation processes in the photoinduced differential time-resolved reflectivity of three samples of ${\text{Nd}}_{1.83}{\text{Ce}}_{0.17}{\text{CuO}}_{4\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}}$. The latter, sharing the same $\text{Ce}$ content, have been fabricated and annealed ad hoc in order to differ only for the degree of disorder, mainly related to oxygen content and location, and, consequently, for the temperature dependence of the resistance: Two of them present a minimum in the resistance and behave as a superconductor and as a metal, respectively; the third behaves as an insulator. The systematic coherence between the resistance and the relaxation parameters behaviors in temperature for all three samples is absolutely remarkable and shows that pump-probe measurements can be extremely sensitive to disorder as it drives the emergence of new excitations and of the related relaxation channels as in this paradigmatic case.

By using nanolithography and a soft etching procedure, we have realized YBa2Cu3O7-x/La0.7Sr0.3MnO3 (YBCO/LSMO) nanowires, with cross sections down to 100 x 50 nm(2) that ensure the cover age of areas up to 10 x 30 mu m(2). The LSMO layer acts as a capping for YBCO, minimizing the degradation of the superconducting properties taking place during the patterning; moreover, as a ferromagnetic manganite, it is expected to accelerate the relaxation dynamics of quasiparticles in YBCO, making such a system potentially attractive for applications in superconducting ultrafast optoelectronics. The reproducibility of the values of the critical current densities measured in different devices with the same geometry makes our nanowires ideal candidates for photoresponse experiments. First measurements have shown a satisfactory photoresponse from YBCO/LSMO devices.

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.

In this work an eddy current technique based on a HTS-SQUID (superconducting quantum interference device) is used to evaluate the relation between the magnetic field response and the damage process of one of the most promising fibre metallic laminates known as GLARE®. In particular the magnetic field response due to impact loading at energies ranging from 5 to 36 J has been detected. The magnetic field signal from GLARE® specimens in the presence of mechanical damaging is discussed according to a simple electromagnetic model.

A systematic characterization of Co-doped BaFe2As2(Ba-122) thin films has been carried out. Two samples were available, one grown on CaF2substrate and the other on MgO with an Fe buffer layer. The goal was to investigate films’ magnetic and superconducting properties, their reciprocal interplay, and the role played by the Fe buffer layer in modifying them. Morphological characterization and Energy Dispersive X-ray analyses on the Fe-buffered sample demonstrate the presence of diffused Fe close to the Co-doped Ba-122 outer surface as well as irregular holes in the overlying superconducting film. These results account for hysteresis loops obtained with magneto-optic Kerr effect measurements and observed at both room and low temperatures. The magnetic pattern was visualized by magneto-optical imaging with an indicator film. Moreover, we investigated the onset of superconductivity through a measure of the superconducting energy gap. The latter is strictly related to the decay time of the excitation produced by an ultrashort laser pulse and has been determined in a pump-probe transient reflectivity experiment. A comparison of results relative to Co-doped Ba-122 thin films with and without Fe buffer layer is finally reported.

A superconducting transistor with large current gain and bandwidth at low temperatures would have many applications. We have fabricated and tested a new three-terminal superconducting device with transistor-like properties at an operating temperature of 4.2 K. It is based on a stacked double tunnel junction structure where the intermediate film is a bilayer of superconducting Nb and an Al quasiparticle trap which can work 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, while large current gains of more than 50 are observed when the Al is in the normal state. The device shows a high degree of unidirectionality. The results can be explained on the basis of the recently proposed QUAsiparticle TRApping TRANsistor, which should have wide applications in detection systems operating at low temperatures.

We have fabricated and characterised superconducting tunnel junctions with annular geometry for radiat ion detection. The advantage of the annular geometry is that, once the trapping has occurred the magnetic field can be switched off and there is no need to further sustain any parallel magnetic field during the detector working time. In agreement with our previous theoretical prediction we have observed the suppression of the Josephson critical current and the presence of one single resonance step in the I-V characteristic when magnetic fluxons were trapped in the: central hole of the junction. (C) 2000 Elsevier Science B.V. All rights reserved.

Superconducting Tunnel Junctions (STJ) are promising devices as a next generation of nuclear and optical detectors. Non-equilibrium experiments can provide useful information on the behaviour of a perturbed superconductor. Among possible nonequilibrium sources, we used quasi-particle injection in three-terminal double tunnel junction devices and a laser pumping source. We investigated the effect of perturbed conditions on sub-harmonic gap structures, which are present in I-V characteristics of high quality Nb-based STJs. Experimental results concerning the change of the energy gap measured at V=2/spl Delta//2e show features which can not be completely explained in terms of simple heating effects.

An experimental study of quasi-particle trapping in proximised Nb/Al layers with different Al thicknesses is presented using symmetrical 3-terminal stacked double tunnel junctions. The experimental tunnelling efficiency of such devices is measured at temperatures down to 1.2 K for different injection current rates. The role of Al layers in the intermediate electrode is discussed in terms of effective non equilibrium processes. The agreement with existing theories on nonequilibrium in such proximity structures makes these results important toward a better understanding of the underlying physics in high resolution particle spectrometers.

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 femtosecond pump-probe spectroscopy studies of time-resolved optical reflectivity of all-oxide YBa2Cu3O7/La0.7Sr0.3MnO3 (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 T-C, 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 T-C and above (up to similar 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. DOI: 10.1103/PhysRevB.87.134514

Femtosecond spectroscopy has been used to investigate the quasiparticle relaxation times in a nearly optimally doped chalcogenide superconducting FeSe0.5Te0.5 thin film. It is characterized by an enhancement of the critical temperature Tc = 19.1 K by strain. Experimental results indicate the existence of a temperature dependence of the quasiparticle recombination time τR in the superconducting state where an energy gap Δ(T) is present. A fitting procedure based on a BCS-like assumption for τR gives Δ0 = 4.1 ± 0.4 meV for the superconducting energy gap extrapolated at T = 0, which corresponds to 2Δ0/kBTc = 4.9 ± 0.5. Accordingly, an estimation of the characteristic phonon energy and the electron–phonon coupling constant λ is reported and discussed. The obtained value λ ≈ 0.6 is in good agreement with other similar determinations obtained by different techniques and numerical approaches. The small value of λ confirms that a phonon-mediated coupling cannot be the only mechanism leading to the formation of the superconducting state in FeSeTe films.

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.

Among superconducting materials used in developing high performance nanowire detectors, NbN demonstrated to have unique characteristics in terms of fast response time, quantum efficiency and photon-number resolving capability. We investigated the role of proximity effect on superconducting NbN thin films covered by a weak ferromagnetic NiCu thin layer. Hetero-structures NbN/NiCu have been deposited by DC magnetron sputtering on MgO substrates without any heating. Characterization in terms of morphological, transport, and ultra-fast optical properties has been done. Superconducting nanowires with meander-type geometries (48 micron lengths and

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.

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 quasiparticle 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.

Three-terminal superconducting tunnel devices offer a powerful tool to investigate nonequilibrium phenomena in superconductors. We study the role of the proximity effect in a Nb/Al bilayer forming the intermediate electrode of a stacked structure configuration. The spatial profile of the pair potential along the common electrode and the effective density of states near the detector tunnel barrier are determined by combining the deconvolution of the current-voltage (I-V) injector characteristics and the numerical solution of the microscopic proximity model equations. The Josephson current variation following a steady-state injection was measured down to 1.5 K. Results are discussed in the framework of a simple nonequilibrium model, which leads also to an estimate of the effective relaxation time limiting quasiparticle scattering processes in the Nb/Al bilayer.

Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe 0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained.

The efficiency of the charge collecting mechanism of a radiation detector based on superconducting tunnel junctions depends strongly on the competition between tunneling and losses of quasiparticles in the electrodes of the junction. In this article we discuss the loss of quasiparticles, both in the bulk and at the edges of the electrodes. The quasiparticle loss taking place during their diffusion in the junction limits the amount of charge carriers tunneling and thereby the energy resolution of the detector. A charge amplification process caused by multiple tunneling of quasiparticles is beneficial to higher signal to noise ratios. Such multiple tunneling is always present in the case of junctions based on symmetrical electrodes. In this article we present a theory for the detector response which takes into account both multiple tunneling and the diffusion of quasiparticles. A model is presented for the spectral response of the detector based on four parameters one of which is the degree of back-tunneling present in the device. Analytical solutions of the model for rectangular and circular junctions are discussed. The predictions of the theory are compared with an experimental pulse height spectrum obtained on a Nb/Al2O3/Nb junction. © 1999 American Institute of Physics.

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 ?IgR2 mV (where ?Ig 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.

superconducting thin films by Mo75Re25 single-target magnetron sputtering at various temperatures on sapphire substrates were prepared. Sharp superconductive transitions andT c values above 10 K were achieved, and the deposited films showed a good metallic behavior. According to structural analyses (EDS and X-rays), all the samples have the same stoichiometry as the target and, even at low deposition temperatures, they exhibit theA15 phase. From measured residual resistivities and superconducting critical temperatures the theoretical BCS surface resistances were estimated and compared to those o Mo60Re40 films, presented in a previous work. Both these alloys, in fact, being characterized by low surface resistances and high critical fields, are good candidates as coating films in r.f. cavities.

Coherent passage of Cooper pairs in a Josephson junction (JJ) above the liquid nitrogen temperature has been the first impressive revolutionary effect induced by high critical temperature superconductors (HTS) in the domain of the study of Josephson effect (JE). But this has been only the start. A d-wave order parameter has lead to significant novel insights in the physics of the JE turning into a device the notion of a ?-junction. Spontaneous currents in a frustrated geometry, Andreev bound states, long-range proximity effect have rapidly become standard terms in the study of the JE, standing as a reference bench for conventional systems based on low critical temperature superconductors (LTS) and inspiring analogies for junctions based on novel superconductors discovered in the meantime. The extreme richness of the physics of HTS JJs has not been adequately supported by the expected impact in the applications, the main reason lying in the complexity of these materials and in the consequent unsatisfactory yield and reproducibility of the performances of the JJs within the required limits. The continuous progress in material science, and specifically in the realization of oxide multi-layers, and in nanotechnologies applied to superconductors, accompanied by the advances in a better understanding of the properties of HTS and of HTS devices, has as a matter of fact opened possible novel scenarios and interest in the field. We intend to give a brief overview on interesting new problems concerning HTS JJs of inspiration also for other systems. We also review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation, which characterize a large majority of low critical current Josephson devices and are therefore an unavoidable consequence of nanotechnology applied more and more to Josephson devices.

The moderately damped regime (MDR) in a Josephson junction (JJ) is quite distinct from the well-known case of underdamped systems and quite common in devices characterized by low critical currents. Measurements of switching current distribution (SCD) are a direct way of discriminating the phase dynamics and the transport also in nontrivial cases of moderate damping. MDR is going to be more and more common with advances in nanopatterning superconductors and in materials science finalized to build hybrid devices. We report on measurements of SCDs, both in the thermal and quantum regime, on moderately damped NbN/MgO/NbN JJs. A transition from the Thermal Activation (TA) regime to Macroscopic Quantum Tunnelling (MQT) occurs at about 90 mK, while above 1.6 K a phase diffusive dynamics is clearly observed. The combined analysis of these regimes through numerical simulations allowed us to extract the fundamental parameters, which completely characterize the phase dynamics of such moderately damped JJs.

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 (Δ R / 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(21 nm)/Nb(70 nm) samples.