L. Parlato

Superconducting device with transistor-like properties including large current amplification

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.

Magnetic properties of annular Josephson junctions for radiation detection: Experimental results

We report on experimental results about Josephson junctions for radiation detection with an annular geometry. The observed suppression of the critical Josephson current and the presence of one single resonance step in the current–voltage characteristic, as predicted by the theory, strongly support the idea that this geometry could be very convenient for radiation detection. The results were obtained when one single magnetic fluxon was trapped in the central hole, and they represent the first experimental confirmation of the theoretical predictions in the case of annular junctions with finite ring width.We report on experimental results about Josephson junctions for radiation detection with an annular geometry. The observed suppression of the critical Josephson current and the presence of one single resonance step in the current–voltage characteristic, as predicted by the theory, strongly support the idea that this geometry could be very convenient for radiation detection. The results were obtained when one single magnetic fluxon was trapped in the central hole, and they represent the first experimental confirmation of the theoretical predictions in the case of annular junctions with finite ring width.

Influence of a NbN overlayer on Nb/Al–AlOx/Nb high quality Josephson tunnel junctions for x‐ray detection

Nb/Al–AlOx/Nb–NbN Josephson junctions for use as x‐ray detectors have been fabricated. The NbN overlayer has been used to realize the trapping of quasiparticles in the Nb top layer with an increase of the charge collection efficiency. The temperature dependence of quasiparticle and Josephson critical current have been investigated and compared with that without a NbN overlayer, showing low leakage currents, about 200 pA (V=0.5 mV, T=0.67 K, A=20×20 μm2), and high dynamical resistances in the subgap region. Preliminary measurements under 6 keV x‐ray irradiation have shown an increased value of the maximum collected charge.

High-quality superconducting Josephson junctions on LiNbO3 electro-optical crystals

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=0.7ΔIgR2mV (where ΔIg is the current jump at the sum gap voltage and R2mV the static resistance at V=2mV) up to 61 mV has been measured at T=4.2K. The very low contribution of subgap leakage currents is demonstrated by the BCS-like quasiparticle current measured at V=0.1mV down to T=1.9K.

Annular Josephson junctions for radiation detection: fabrication and investigation of the magnetic behaviour

We have fabricated and characterised superconducting tunnel junctions with annular geometry for radiation detection. The advantage of the annular geometry is that, once the trapping has occurred the magnetic eld can be switched o! and there is no need to further sustain any parallel magnetic eld 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}< characteristic when magnetic #uxons were trapped in the central hole of the junction. ( 2000 Elsevier Science B.V. All rights reserved.

Transport and optical properties of epitaxial Nd1.83Ce0.17Cu04−δthin films

Electrical transport measurements and transient optical pump-probe experiments have been performed on epitaxial films of the electron doped Nd1.83Ce0.17Cu4−δ compound for studying the non-equilibrium carrier dynamics in this material. Samples have been grown on (001)-oriented SrTiO3 substrates by dc sputtering in a mixed atmosphere of both Ar and O2. X-ray diffraction analysis and scanning electron microscope equipped with a wavelength dispersive spectroscopy detector have been used to characterize the structure and the composition of the thin films. Time-resolved femtosecond pump-and-probe spectroscopy has been also carried out on our samples in the temperature range 4.2K-300 K.

Non equilibrium superconductivity by using quasi-particle injection in stacked tunnel junctions

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 tunneling 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 non-equilibrium in such proximity structures makes these results important toward a better understanding of the underlying physics in high resolution particle spectrometers.

Femtosecond spectroscopy in a nearly optimally doped Fe-based superconductors FeSe0.5Te0.5and Ba(Fe1−xCox)2As2/Fe thin film

Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 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.

Femtosecond time-resolved differential reflectively measurements in Ga1-xMnxAs epitaxial thin films

Magnetic semiconductors such as Ga1-xMnxAs have attracted a great interest in the last years due to their high potential as advanced-performance materials in optical detection and in novel spintronic devices. The carrier dynamics and the nonlinear optical response in low-temperature-grown GaAs/Ga1-xMnxAs heterostructures represent an interesting topic much less explored than their electronic transport and/or structural studies. We report our optical investigations of Ga1-xMnxAs films, grown with different Mn concentrations and subject to annealing conditions, by time-resolved, femtosecond pump-probe, differential reflectivity measurements. The analysis of the carrier relaxation times at low temperatures is presented and discussed according to nonequilibrium theories for electron scattering in magnetic materials.

Time-resolved optical characterization of proximized nano-bilayers for ultrafast photodetector applications

Time-resolved transient pump-probe spectroscopy measurements on proximized ferromagnet/superconductor (F/S) hybrid structures are presented. We focused our attention on both low and high critical temperature superconductors such as Nb and YBCO, while for F the weak-ferromagnetic alloy Ni0.48Cu0.52 has been used. Dynamics of the electron-phonon relaxation process has been investigated as a function of both the temperature and the F-film thickness. In the case of NiCu/Nb bilayers the presence of a thin F overlayer reduces the slow bolometric component of the photoresponse, while in YBCO-based structures faster relaxation times were measured. F/S nanobilayers are very attractive toward the development of novel hybrid superconducting photodetectors.