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We report a systematic study of the transport properties of high critical temperature superconductor (HTS) biepitaxial Josephson junctions in the submicron range. Junction performances point to more uniform and reproducible devices and to better control of d-wave intrinsic properties. Outcomes promote novel insights into the transport mechanisms across grain boundaries and encourage further developments in the control of dissipation in HTS devices. The application of nanotechnology to HTS could be an additional tool to properly engineer the junction properties to match specific circuit design also in view of the integration into hybrid quantum circuits. [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 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. [ABSTRACT FROM AUTHOR]

Using field effect devices with side gates, we modulate the 2-dimensional electron gas hosted at the LaAlO3/SrTiO3 interface to study the temperature and doping evolution of the magnetotransport. The analysis of the data reveals different transport regimes depending on the interplay between the different (elastic, inelastic, and spin-orbit) scattering times and their temperature dependencies. We find that the spin-orbit interaction strongly affects the low-temperature transport in the normal state in a very large region of the phase diagram, extending beyond the superconducting dome. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report on the study of the phase dynamics of high-critical-temperature superconductor Josephson junctions. We realized YBa2Cu3O7-x grain boundary biepitaxial junctions in the submicron scale using low-loss substrates and analyzed their dissipation by comparing the transport measurements with Monte Carlo simulations. The behavior of the junctions can be fitted using a model based on two quality factors, which results in a frequency-dependent damping. Moreover, our devices can be designed to have Josephson energy of the order of the Coulomb energy. In this unusual energy range, phase delocalization strongly influences the device's dynamics, promoting the transition to a quantum phase diffusion regime. We study the signatures of such a transition by combining the outcomes of Monte Carlo simulations with the analysis of the device's parameters, the critical current, and the temperature behavior of the low-voltage resistance R0. [ABSTRACT FROM AUTHOR]

We describe the transport properties of mesoscopic devices based on the two dimensional electron gas (2DEG) present at the LaAlO3/SrTiO3 interface. Bridges with lateral dimensions down to 500 nm were realized using electron beam lithography. Their detailed characterization shows that processing and confinement do not alter the transport parameters of the 2DEG. The devices exhibit superconducting behavior tunable by electric field effect. In the normal state, we measured universal conductance fluctuations, signature of phase-coherent transport in small structures. The achievement of reliable lateral confinement of the 2DEG opens the way to the realization of quantum electronic devices at the LaAlO3/SrTiO3 interface. [ABSTRACT FROM AUTHOR]

Abstract: Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current–voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures. [Copyright &y& Elsevier]

Abstract: High quality YBa2Cu3O7−δ biepitaxial grain boundary junctions allowed one of the first evidences of macroscopic quantum behavior and mesoscopic transport in high critical temperature superconductors (HTS) systems. Both these studies touch delicate issues on fluctuations and give important feedback on the understanding of coherent transport of quasiparticles and of the dissipation mechanisms in HTS. [Copyright &y& Elsevier]

We study the superconducting state of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface using Josephson junctions as spectroscopic probes. The transport properties of these devices reveal the presence of two superconducting gap structures and of an unconventional superconducting π channel. These features provide evidence of an unconventional superconducting ground state, possibly related to the interplay between superconductivity and the large Rashba spin-orbit coupling in the 2DEG. [ABSTRACT FROM AUTHOR]

Abstract: We investigated transport properties of ultra-thin CaBaCuO (CBCO) films, focusing on vortex dynamics. By measuring current–voltage characteristics we found evidence that at high temperatures the vortices are thermally activated at the edges of the bridge and pushed in, causing dissipation and power-law dependence of the voltage from the current. At lower temperatures, the experimental curves change shape. We compared these data with the predicted behaviour for vortex quantum tunnelling and found a very good agreement. Measurements performed at ultra-low temperatures (down to 60mK) in a particular low noise environment provide additional evidence of the robustness of the observed effects. [Copyright &y& Elsevier]

Abstract: We report on transport measurements of Sr2RuO4–Sr3Ru2O7 eutectic crystals. In these systems, natural and clean interfaces are formed between the two phases thanks to the eutectic solidification process. The transport measurements, performed down to 300mK in a shielded environment, show a clear supercurrent flowing across the interface between Sr2RuO4 and Sr3Ru2O7, also when the contacts are placed at a distance of several hundreds of micrometers from the Sr2RuO4 domain. These results shed light on novel effects appearing in systems composed of the spin-triplet Sr2RuO4 and the metamagnet Sr3Ru2O7. [Copyright &y& Elsevier]

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 ξ < w < λ , being ξ the coherence length and λ 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 ξ < w < λ nanowires are better protected against phase slips and vortex flow. [ABSTRACT FROM AUTHOR]

Possible ferromagnetism induced in otherwise nonmagnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetism can be stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated with magnetic Ti3+ ions at the interface itself for LaAlO3/SrTiO3 and to Mn3+ ions in the overlayer for BiMnO3/SrTiO3. In LaAlO3/SrTiO3 the induced magnetism is quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in this phenomenon. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

YBaCuO nanowires were reproducibly fabricated down to widths of 50 nm. A Au/Ti cap layer on YBCO yielded high electrical performance up to temperatures above 80 K in single nanowires. Critical current density of tens of MA cm-2 at T = 4.2 K and of 10 MA cm-2 at 77 K were achieved that survive in high magnetic fields. Phase-slip processes were tuned by choosing the size of the nanochannels and the intensity of the applied external magnetic field. Data indicate that YBCO nanowires are a rather attractive system for the fabrication of efficient sensors, supporting the notion of futuristic THz devices. [ABSTRACT FROM AUTHOR]

Coplanar Al/graphene/Al junctions fabricated on the same graphene sheet deposited on silicon carbide (SiC), show robust Josephson coupling at subKelvin temperature, when the separations between the electrodes is below 400 nm. Remarkably, a hysteretic Critical State sets in when ramping an orthogonal magnetic field, with a sudden collapse of the Josephson critical current I when turning the field on, and a revival of I when inverting the sweep. Similar hysteresis can be found in granular superconducting films which may undergo the Berezinskii-Kosterlitz-Thouless transition. Here, we give quantitative arguments to prove that this odd behavior of the magnetoconductance gives evidence for an incipient Berezinskii-Kosterlitz-Thouless transition with drift and pinning of fluctuating free vortices induced by the current bias. [ABSTRACT FROM AUTHOR]

We have studied the electronic properties of the 2D electron liquid present at the LaAlO3/SrTiO3 interface in series of samples prepared at different growth temperatures. We observe that interfaces fabricated at 650°C exhibit the highest low temperature mobility (≈10000cm² V-1 s-1) and the lowest sheet carrier density (≈5 x 1012cm-2). These samples show metallic behavior and Shubnikov-de Haas oscillations in their magnetoresistance. Samples grown at higher temperatures (800-900 °C) display carrier densities in the range of ≈2 -- 5 x 1013cm-2 and mobilities of ≈1000 cm² V-1 s--1 at 4K. Reducing their carrier density by field effect to 8 x 1012cm--2 lowers their mobilities to ≈50cm²V--1 s--1 bringing the conductance to the weak-localization regime. [ABSTRACT FROM AUTHOR]

The moderately damped regime in Josephson junctions (JJ) is quite common in devices characterized by low critical currents and, therefore, by low Josephson energies. Measurements of the switching current distribution (SCD) are a direct way of determining the phase dynamics in the nontrivial case of moderate damping, which is becoming more common with advances in nanopatterning superconductors and in materials science for fabricating hybrid systems. We report measurements of SCDs, in both the thermal and quantum regimes for moderately damped YBaCuO grain boundary biepitaxial JJs. A direct transition from a phase diffusion regime to macroscopic quantum tunnelling occurs at about 130 mK. The crossover to the quantum regime is determined by the magnetic field and the phase dynamics can be described by a fully consistent set of junction parameters obtained by numerical simulation. [ABSTRACT FROM AUTHOR]

Proximity-effect and resistance magneto-fluctuation measurements in submicron Nd1.2Ba1.8Cu3Oz (NBCO) nano-loops are reported to investigate coherent charge transport in the non-superconducting state. We find an unexpected inhibition of Cooper pair transport, and a destruction of the induced superconductivity, by lowering the temperature from 6K to 250 mK. This effect is accompanied by a significant change in the conductancevoltage characteristics and the zero bias conductance response to the magnetic field, pointing to the activation of a strong pair-breaking mechanism at lower temperature. The data are discussed in the framework of mesoscopic effects specific to superconducting nanostructures, proximity effect and high temperature superconductivity. [ABSTRACT FROM AUTHOR]

The Josephson effect is a privileged access to the macroscopic quantum nature of superconductors. We 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 avoidable consequence of nanotechnology applied more and more to Josephson devices. Phase diffusion phenomena take over thermal activation in some temperature ranges also affecting the transition to macroscopic quantum tunneling, enriching the phase diagram mostly defined by the Josephson energy, the temperature and the level of dissipation. [ABSTRACT FROM AUTHOR]

An innovative fabrication procedure has been developed to obtain YBa2Cu3O7-x (YBCO) nanobridges from c-axis oriented films. The novelty regards the use of a thin titanium mask used in the patterning process. The use of the Ti makes simpler the fabrication procedure guaranteeing high quality devices, as demonstrated by transport measurements. Critical temperatures and critical current densities scale with the width of the microbridge down to 200 nm in agreement with most results available in literature. The actual properties of the devices, performances, yield and reproducibility, along with an accurate control on doping, may allow the use of these micro-bridges for nanoscale experiments. [ABSTRACT FROM AUTHOR]

Abstract: Magneto-fluctuations of the normal resistance R N have been reproducibly observed in YBa2Cu3O7− δ biepitaxial grain boundary junctions at low temperatures. We attribute them to mesoscopic transport in narrow channels across the grain boundary line, occurring even in the presence of large voltage drops. The Thouless energy appears to be the relevant energy scale. Possible implications on the understanding of coherent transport of quasiparticles in high critical temperature superconductors (HTS) and of the dissipation mechanisms are discussed. [Copyright &y& Elsevier]

Abstract: Magneto-fluctuations of the normal resistance R N have been reproducibly observed in YBa2Cu3O7−δ (HTS) biepitaxial grain boundary junctions at low temperatures. We attribute them to mesoscopic transport in narrow channels across the grain boundary line. The Thouless energy appears to be the relevant energy scale. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed. [Copyright &y& Elsevier]

Abstract: We describe the main ideas and some experimental outcomes of two experiments, both aimed to study quantum tunneling effects in high-T C structures. In the first experiment, macroscopic quantum tunneling is demonstrated in grain boundary YBaCuO Josephson junctions revealing dissipation levels lower than expected and opening novel perspectives for quantum circuitry. In the second, the low temperature dissipation is dominated by quantum tunneling of individual Pearl vortices in ultra-thin CaBaCuO systems characterized by extremely long Pearl lengths. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]