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Superconducting nanostrip single photon detectors have emerged as the highest performing single-photon detectors; however, the possibility to use superconducting microstrip as single photon detectors is very appealing both to use them as larger areas detectors and for an easier technology in the manufacturing. The aim of this work is to test the photoresponse in liquid helium dewar of 9 nm thick MoSi covered with a very thin (2 nm) layer of Al, in two different configurations: nanomeanders and microstrips. We demonstrate that MoSi/Al microstrips can work as photodetectors also at T = 4.2 K. We also compare the dark count rate of the microstrip and the nanowire, confirming the lower noise for the microstrips also at 4.2 K.

The coexistence between ferromagnetic ordering and superconducting transport in tunnel ferromagnetic Josephson junctions (SFS JJs) accounts for a wide range of unconventional physical properties. The integration of both insulating ferromagnets or multi-layered insulator-ferromagnet barriers allows to combine ferromagnetic switching properties with peculiar low quasiparticle dissipation, which could enhance the capabilities of SFS JJs as active elements in quantum circuits. Here we show that split-transmon qubits based on tunnel ferromagnetic JJs realize an ideal playground to study noise fluctuations in ferromagnetic Josephson devices. By considering the transport properties of measured Al-based tunnel SFS JJs, we report on a theoretical study of the competition between intrinsic magnetization fluctuations in the barrier and quasiparticles dissipation, thus providing specific operation regimes to identify and disentangle the two noise sources, depending on the peculiar properties of the F layer and F/S interface.

The competition between superconducting and ferromagnetic orderings in Josephson devices has promoted fundamental and applicative studies of high impact for superconducting digital technology, cryogenic memories, and spintronics, where the possibility of switching between different magnetic states is a crucial advantage. Here, we report on fabrication and characterization of very high quality tunnel ferromagnetic Josephson junctions (JJs) with aluminum electrodes, demonstrating hysteretic behavior of the magnetic field pattern preserving typical transport properties of Al junctions, underdamped behavior, and very low quasiparticle dissipation. The proposed technology paves the way for the possible implementation of Al tunnel-ferromagnetic JJs in superconducting quantum circuits, toward alternative approaches based on digital control of the Josephson device.

We report on the magnetotransport properties of a spin-polarized two-dimensional electron system (2DES) formed in LaAlO3(LAO)/EuTiO3/SrTiO3 (STO) heterostructures. We show that, at low temperature, the 2DES magnetoconductance exhibits weak antilocalization corrections related to Rashba spin-orbit scattering, in analogy with the LAO/STO 2DES. However, the characteristic spin-orbit scattering field decreases substantially for carrier density higher than 1.9×1013cm-2. We attribute this behavior to the masking effect of ferromagnetism, which sets in at the same carrier density and at a temperature below 10 K. Our work shows that, while weak antilocalization corrections to the magnetoconductance are strongly reduced by the emergence of ferromagnetism, they persist in a large part of the phase diagram of a spin-polarized oxide 2DES.

Recently it has been proven that the magnetic response of magnetic nano-objects such as nanoparticles, nanobeads, and small cluster of molecules can be effectively measured by using a Superconducting Quantum Interference Device (SQUID) with a small sensitive area. Here, we present a high sensitivity nanoSQUID based on deep submicrometer Josephson tunnel junctions fabricated by a Focused Ion Beam (FIB) sculpting method. The nanosensor consists of a niobium superconducting loop (0.4 × 1.0 ?m^2) interrupted by two sandwich nanojunctions (Nb/Al-AlOx/Nb) having an area of about (300 × 300) nm^2. An experimental investigation of the main characteristics of such nanodevice as a function of the temperature is presented.

Superconducting hybrid junctions are revealing a variety of novel effects. Some of them are due to the special layout of these devices, which often use a coplanar configuration with relatively large barrier channels and the possibility of hosting Pearl vortices. A Josephson junction with a quasi ideal two-dimensional barrier has been realized by growing graphene on SiC with Al electrodes. Chemical Vapor Deposition offers centimeter size monolayer areas where it is possible to realize a comparative analysis of different devices with nominally the same barrier. In samples with a graphene gap below 400 nm, we have found evidence of Josephson coherence in presence of an incipient Berezinskii-Kosterlitz-Thouless transition. When the magnetic field is cycled, a remarkable hysteretic collapse and revival of the Josephson supercurrent occurs. Similar hysteresis are found in granular systems and are usually justified within the Bean Critical State model (CSM). We show that the CSM, with appropriate account for the low dimensional geometry, can partly explain the odd features measured in these junctions., Comment: 13 pages, 9 figures

Since the discovery of high-temperature superconductors (HTSs), most efforts of researchers have been focused on the fabrication of superconducting devices capable of immobilizing vortices, hence of operating at enhanced temperatures and magnetic fields. Recent findings that geometric restrictions may induce self-arresting hypervortices recovering the dissipation-free state at high fields and temperatures made superconducting strips a mainstream of superconductivity studies. Here we report on the geometrical melting of the vortex lattice in a wide YBCO submicron bridge preceded by magnetoresistance (MR) oscillations fingerprinting the underlying regular vortex structure. Combined magnetoresistance measurements and numerical simulations unambiguously relate the resistance oscillations to the penetration of vortex rows with intermediate geometrical pinning and uncover the details of geometrical melting. Our findings offer a reliable and reproducible pathway for controlling vortices in geometrically restricted nanodevices and introduce a novel technique of geometrical spectroscopy, inferring detailed information of the structure of the vortex system through a combined use of MR curves and large-scale simulations.

We describe a simple mechanical approach for low-density polyethylene film coating by multilayer graphene. The technique is based on the exfoliation of nanocrystalline graphite (few-layer graphene) by application of shear stress and allows to obtain thin graphene layers on the plastic substrate. We report on the temperature dependence of electrical resistance behaviors in films of different thickness. The experimental results suggest that the semiconducting behavior observed at low temperature can be described in the framework of the Efros-Shklovskii variable-range-hopping model. The obtained films exhibit good electrical conductivity and transparency in the visible spectral region. PACS 72.80.Vp; 78.67.Wj; 78.66.Qn; 85.40.Hp

High critical temperature superconductors (HTS) based devices can have impact in the study of the phase dynamics of Josephson junctions (JJs) thanks to the wide range of junction parameters they offer and to their unconventional properties. Measurements of current-voltage characteristics and of switching current distributions constitute a direct way to classify different regimes of the phase dynamics and of the transport, also in nontrivial case of the moderately damped regime (MDR). MDR is going to be more and more common in JJs with advances in nanopatterning superconductors and synthesizing novel hybrid systems. Distinctive signatures of macroscopic quantum tunneling and of thermal activation in presence of different tunable levels of dissipation have been detected in YBCO grain boundary JJs. Experimental data are supported by Monte Carlo simulations of the phase dynamics, in a wide range of temperatures and dissipation levels. This allows us to quantify dissipation in the MDR and partially reconstruct a phase diagram as guideline for a wide range of moderately damped systems. (C) 2014 Elsevier B.V. All rights reserved.

The edge states of Topological Insulators (TI) are protected against backscattering, thanks to the topological properties arising from their band structure. Coupling a TI to a superconductor (S) can induce unconventional effects, including the creation of Majorana bound states (MBS). The fabrication of coplanar hybrid devices is a fundamental step to pave the way to the understanding of proximity effects in topologically non-trivial systems, and to a large variety of experiments aimed at the possible detection of MBS. We discuss the feasibility and some relevant properties of Al-Bi2Se3-Al coplanar proximity devices. Special attention is devoted to the design of the junction, aimed at enhancing the coupling between the electrodes and the TI.

The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F) is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunneling in Josephson junctions with GdN ferromagnetic insulator barriers. We prove a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions which demonstrate a clear signature of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits., Comment: 16 pages, 4 figures

We report on $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-x} $ biepitaxial grain boundary Josephson junctions (JJs) demonstrating significant tunability of their circuital parameters. We show how the enhanced control of the junction properties, and particularly of the capacitance, leads to advances in the understanding of phase dynamics of high-critical-temperature superconductor JJs and potentially of their intrinsic dissipation mechanisms.

We have identified anomalous behavior of the escape rate out of the zero-voltage state in Josephson junctions with a high critical current density Jc. For this study we have employed YBa2Cu3O7-x grain boundary junctions, which span a wide range of Jc and have appropriate electrodynamical parameters. Such high Jc junctions, when hysteretic, do not switch from the superconducting to the normal state following the expected stochastic Josephson distribution, despite having standard Josephson properties such as a Fraunhofer magnetic field pattern. The switching current distributions (SCDs) are consistent with nonequilibrium dynamics taking place on a local rather than a global scale. This means that macroscopic quantum phenomena seem to be practically unattainable for high Jc junctions. We argue that SCDs are an accurate means to measure nonequilibrium effects. This transition from global to local dynamics is of relevance for all kinds of weak links, including the emergent family of nanohybrid Josephson junctions. Therefore caution should be applied in the use of such junctions in, for instance, the search for Majorana fermions., Comment: 8 pages, 4 figures

We investigate a new architecture for the implementation of Josephson junctions combining high critical temperature superconductors (HTS) and semiconductor (Sm) nanowires (NWs). The devices are obtained starting from pre-patterned YBaCuO (YBCO) electrodes and assembling suspended InAs-NWs bridges between nearby superconductive banks. Such a non-standard approach is necessary in the case of HTS since growth conditions for cuprates are incompatible with thermal stability of the Sm nanostructures. We investigate the device behavior as a function of the length L of the InAs bridge. For L > 200 nm, all junctions display a tendency to insulation at low temperatures. For L ∼ 200 nm, a metallic-like behavior is obtained, signaling the possible onset of a superconductive transition of the junction. Technological limitations on the width of the gap separating the YBCO banks currently prevent a further scaling. Our results suggest that a further reduction of the size of the trench between the YBCO electrodes can lead to functional YBCO/InAs-NWs/YBCO Josephson junctions.

We have developed a fabrication process that allows us to realize pure YBCO nanowires displaying robust superconductivity at widths w as low as 160 nm. We can modify the process in order to maintain a Au protective layer. This allows us to scale our nanowires even further to widths as low as 50 nm. We have studied how the presence of vortices and the occurrence of phase slips affect the transport properties of nanowires in the width range xi < w < lambda, being xi the coherence length and lambda the magnetic penetration depth. Magnetoresistance curves present features which are related to the effect of screening currents. Vortex entry barrier is found to scale with the width. Our findings confirm that for widths xi < w < lambda nanowires are better protected against phase slips and vortex flow. (C) 2014 Elsevier B.V. All rights reserved.

We investigate the phase dynamics of moderately damped NbN/AlN/NbN Josephson junctions and we present experimental results on detailed aspects of phase diffusion processes. We measure both single escape and multiple escape and retrapping events obtaining a crossover temperature T* from Kramers to phase diffusion switching. We observe a clear dependence of the crossover temperature T* by the bias current ramp rate, while the damping factor Q remains the same. The measured effect is in strong agreement with theoretical predictions reported by Fenton and Warburton. © 2014 AIP Publishing LLC.

The moderately damped regime in a Josephson junction (JJ) is quite common in devices characterized by low critical currents and therefore by low Josephson energies. Measurements of switching current distribution (SCD) are a direct way of discriminating the phase dynamics also in the nontrivial case of moderate damping, which is going to be more and more common with advances in nanopatterning superconductors and in materials science finalized to build hybrid systems. We report on measurements of SCDs, both in thermal and quantum regime, on moderately damped YBaCuO grain boundary biepitaxial JJs. A direct transition from phase diffusion regime to macroscopic quantum tunnelling occurs at about 130 mK. The crossover to the quantum regime is tuned by the magnetic field and phase dynamics is described by a fully consistent set of junction parameters derived through numerical simulations.

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.

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