L. Longobardi

High critical current density and scaling of phase-slip processes in YBaCuO nanowires

YBaCuO nanowires were reproducibly fabricated down to widths of 50xa0nm. A Au/Ti cap layer on YBCO yielded high electrical performance up to temperatures above 80xa0K in single nanowires. Critical current density of tens of MAxa0cm−2 at Txa0=xa04.2xa0K and of 10xa0MAxa0cm−2 at 77xa0K 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.

Direct Transition from Quantum Escape to a Phase Diffusion Regime in YBaCuO Biepitaxial Josephson Junctions

Dissipation encodes the interaction of a quantum system with the environment and regulates the activation regimes of a Brownian particle. We have engineered grain boundary biepitaxial YBaCuO junctions to drive a direct transition from a quantum activated running state to a phase diffusion regime. The crossover to the quantum regime is tuned by the magnetic field and dissipation is described by a fully consistent set of junction parameters. To unravel phase dynamics in moderately damped systems is of general interest for advances in the comprehension of retrapping phenomena and in view of quantum hybrid technology.

High quality factor HTS Josephson junctions on low loss substrates

We have extended the off-axis biepitaxial technique to produce YBCO grain boundary junctions on low loss substrates. Excellent transport properties have been reproducibly found, with remarkable values of the quality factor IcRn (with Ic the critical current and Rn the normal state resistance) above 10 mV, far higher than the values commonly reported in the literature for high temperature superconductor (HTS) based Josephson junctions. The outcomes are consistent with a picture of a more uniform grain boundary region along the current path. This work supports a possible implementation of grain boundary junctions for various applications including terahertz sensors and HTS quantum circuits in the presence of microwaves.

Quantum crossover in moderately damped epitaxial NbN/MgO/NbN junctions with low critical current density

High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d=1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm

Breakdown of the escape dynamics in Josephson junctions

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Resolving the effects of frequency-dependent damping and quantum phase diffusion in YBa2Cu3O7−xJosephson junctions

have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN.

Escape dynamics in moderately damped Josephson junctions (Review Article)

We have identified anomalous behavior of the escape rate out of the zero-voltage state in Josephson junctions with a high critical current density J(c). For this study we have employed YBa2Cu3O7-x grain boundary junctions, which span a wide range of J(c) 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.

Geometrical Vortex Lattice Pinning and Melting in YBaCuO Submicron Bridges.

We report on the study of the phase dynamics of high critical temperature superconductor Josephson junctions. We realized YBa

Macroscopic quantum tunneling and retrapping processes in moderately damped YBaCuO Josephson junctions

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What happens in Josephson junctions at high critical current densities

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