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In tunnel magnetic Josephson junctions (JJs), we have provided evidence of an inverse magnetic dependence of the critical current Ic(H), i.e., a shift of the Ic(H) curves in the opposite expected direction. We have established a self-consistent method to ascribe this unconventional feature to spin polarization phenomena at the S/F interface. Through a magnetic characterization of Superconductor (S) /Ferromagnet (F) /Superconductor (S) trilayers and a magnetostatic analysis of the devices, we show that the F-stray fields fail to determine the pattern inversion. The comparative analysis with the literature supports our thesis that this phenomenon can be explained in terms of inverse proximity effect and observed with a proper choice of materials and thickness of the ferromagnetic layer involved in the junction structure. More generally, our approach proves that a careful investigation of the magnetic field pattern in magnetic JJs can lead to the detection of novel effects arising at the S/F interface.

In Magnetic Josephson Junctions (MJJs) based on Superconductor-Insulator-Superconductor-Ferromagnet-Superconductor (SIS’FS), we provide evidence of an unconventional magnetic field behavior of the critical current characterized by an inverted magnetic hysteresis, i.e., an inverted shift of the whole magnetic field pattern when sweeping the external field. By thermoremanence measurements of S/F/S trilayers, we have ruled out that this uncommon behavior could be related to the F-stray fields. In principle, this finding could have a crucial role in the design and proper functioning of scalable cryogenic memories.

The integration of superconductive nanowire logic memories and energy efficient computing Josephson logic is explored. Nanowire memories are based on the integration of switchable superconducting nanowires with a suitable magnetic material. These memories exploit the electro-thermal operation of the nanowires to efficiently store and read a magnetic state. In order to achieve proper memory operation a careful design of the nanowire assembly is necessary, as well as a proper choice of the magnetic material to be employed. At present several new superconducting logic families have been proposed, all tending to minimize the effect of losses in the digital Josephson circuits replacing resistors by adequate combinations of inductors in the logic gates. Among those, the nSQUID concept relies on the propagation of vortices along underdamped Josephson transmission lines. While, in principle, this logic family provides very low dissipation, several issues, such as speed and stability, have yet to be addressed. The current status of design and testing of n-SQUID logic gates and circuits as well as of the design and implementation of the nanowire memories is reported

Single molecules can be coupled to metallic electrodes when the latter are in the superconducting state. In such emerging hybrid molecular devices, the possibility of the Josephson effect, that is, the dissipation-less transport of Cooper-paired electrons from one electrode to the other, arises. In this theoretical study, we demonstrate that a junction formed by two superconductors linked by an annular molecule, of which benzene (phenylene group) is a prototype, can sustain indeed a supercurrent and work as a “single molecule superconducting field-effect transistor (SMoSFET)”. In this device, Cooper-paired electrons are transmitted via the molecule in the presence of quantum interference. Through the presented model, we show that the resonant nontrivial modulation of the critical current with an external gate voltage may be strongly modified by choosing a paracoupled, a metacoupled, or an orthocoupled molecular ring. These features are directly related to the possibility of obtaining a SMoSFET controlled ...

The Josephson effect in a molecular structure mimicking a benzene molecule (C6H6) placed between two nanometric superconducting electrodes is theoretically studied. In the tight bindingmodel, the Bogoliubov-de Gennes equations are solved to derive the Andreev reflection coefficient and hence the dc Josephson current. Two effects are pointed out: the quantum interference effect on the supercurrent due to the presence of two branches in the molecule and a on-resonance off -resonance state operation in the presence of an applied gate voltage. The results provide an insight on the molecular electronics devices in the case of superconducting nano-electrodes.

Recent findings by G.Pepe et al. (Appl.Phys.Lett.89, 2770, 2001) have given new insights over the dynamics of Josephson junctions subject to electronic pulse injection. These experiments can be considered important steps toward the study of non-equilibrium effects involved in junctions under very fast (e.g. laser induced) pumping signals. In the governing equations (sine-Gordon or coupled sine-Gordon depending on the system) pulse operation enters via Josephson current distribution or via bias current distribution. By means of numerical simulations here we study the dependence of the switching to resistive state on fast local depression of Josephson current. Next, using simulated electronic pulse train we show how the flip-flop operating mode found in G.Pepe et al. is a common feature of single or stacked Josephson systems without regard for the pulse waveform or the presence of the gap structure in the I-V characteristics. The effect of the length and of the magnetic field is also discussed.

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. (C) 2002 Elsevier Science B.V. All rights reserved.

The behavior of a Josephson system under pulsed operation is of utmost importance for developing reliable digital devices working at very high clock frequencies. Information about effect of thermal noise over pulsed operation is also useful to design errors free devices. Recent experiments in a system of two stacked junctions show that pulses applied to the first junction (injector) of the stack drive the Josephson biased second junction (detector) in the resistive state. The experiment was interpreted as pulsed-assisted escape toward resistive state as a consequence of an excitation of large non-linear oscillations in the detector junction that, with the contribution of thermal noise, drives out the junction from the zero voltage state. Moreover, an unstability of the resistive state toward the zero voltage state under the same pulsed operation was also noted. By means of a systematic numerical approach to the problem, we present a study of the pulsed-assisted escape using the framework of the thermal escape theory for the direct transition from zero voltage state to resistive state. We study the single junction case and a stacked configuration as examples of different systems showing pulsed-assisted escape. (C) 2002 Elsevier Science B.V. All rights reserved.