Direct transition from quantum escape to phase diffusion regime in YBaCuO biepitaxial Josephson Junctions

Dissipation encodes 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 quantum activated running state to phase diffusion regime. The cross-over to the quantum regime is tuned by the magnetic field and dissipation is encoded in 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

Coherent transport in extremely underdoped Nd1.2Ba1.8Cu3Oz nanostructures

Proximity-effect and resistance magneto-fluctuations 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 250mK. This effect is accompanied by a significant change in the conductance-voltage characteristics and in 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.Comment: to appear on new journal of Physic

Magneto-transport study of top- and back-gated LaAlO3_3/SrTiO3_3 heterostructures

We report a detailed analysis of magneto-transport properties of top- and back-gated LaAlO$_3$/SrTiO$_3$ heterostructures. Efficient modulation in magneto-resistance, carrier density, and mobility of the two-dimensional electron liquid present at the interface is achieved by sweeping top and back gate voltages. Analyzing those changes with respect to the carrier density tuning, we observe that the back gate strongly modifies the electron mobility while the top gate mainly varies the carrier density. The evolution of the spin-orbit interaction is also followed as a function of top and back gating.Comment: 15 pages, 6 figure

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$^2$ 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

Fabricating Superconducting Interfaces between Artificially-Grown LaAlO3_3 and SrTiO3_3 Thin Films

Realization of a fully metallic two-dimensional electron gas at the interface between artificially-grown LaAlO$_3$ and SrTiO$_3$ thin films has been an exciting challenge. Here we present for the first time the successful realization of a superconducting 2DEG at interfaces between artificially-grown LaAlO$_3$ and SrTiO$_3$ thin films. Our results highlight the importance of two factors-the growth temperature and the SrTiO$_3$ termination. We use local friction force microscopy and transport measurements to determine that in normal growth conditions the absence of a robust metallic state at low temperature in the artificially-grown LaAlO$_3$/SrTiO$_3$ interface is due to the nanoscale SrO segregation occurring on the SrTiO$_3$ film surface during the growth and the associated defects in the SrTiO$_3$ film. By adopting an extremely high SrTiO$_3$ growth temperature, we demonstrate a way to realize metallic, down to the lowest temperature, and superconducting 2DEG at interfaces between LaAlO$_3$ layers and artificially-grown SrTiO$_3$ thin films. This study paves the way to the realization of functional LaAlO$_3$/SrTiO$_3$ superlattices and/or artificial LaAlO$_3$/SrTiO$_3$ interfaces on other substrates

Little-Parks effect in single YBaCuO sub-micron rings

The properties of single submicron high-temperature superconductor (HTS) rings are investigated. The Little-Parks effect is observed and is accompanied by an anomalous behavior of the magnetic dependence of the resistance, which we ascribe to non-uniform vorticity (superfluid angular momentum) within the ring arms. This effect is linked to the peculiar HTS-relationship between the values of the coherence length and the London penetration depth.Comment: 14 pages, 3 figure

Geometrical vortex lattice pinning and melting in YBaCuO submicron bridges

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

Thermal hopping and retrapping of a Brownian particle in the tilted periodic potential of a NbN/MgO/NbN Josephson junction

We report on the occurrence of multiple hopping and retrapping of a Brownian particle in a tilted washboard potential. The escape dynamic has been studied experimentally by measuring the switching current distributions as a function of temperature in a moderately damped NbN/MgO/NbN Josephson junction. At low temperatures the second moment of the distribution increases in agreement with calculations based on Kramers thermal activation regime. After a turn-over temperature T*, the shape of the distributions starts changing and width decreases with temperature. We analyze the data through fit of the switching probability and Monte Carlo simulations and we find a good agreement with a model based on a multiple retrapping process

Hybrid superconductor-semiconductor nanostructures: nanoelectronic applications, topological properties, correlation and disorder.

Obiettivo di questo progetto è progettare, realizzare e caratterizzare nanodispositivi basati su una architettura quantistica ibrida comprendente superconduttori e semiconduttori. Le nanotecnologie avanzate consentono, infatti, di unire in modo innovativo materiali superconduttori e semiconduttori, combinando le loro proprietà e realizzando nuove funzionalità eletttroniche. Lo studio di nano-dispositivi ibridi è di grande interesse sia per la fisica fondamentale che per quella applicata, e risponde a una delle più recenti e interessanti questioni della fisica della materia condensata: la possibilità di misurare e manipolare eccitazioni complesse di particelle ed elettroni. Le strutture ibride proposte in questo progetto saranno prevalentemente basate su superconduttori non-convenzionali, in particolare superconduttori ad alta temperature critica (HTS), e barriere composte da nanofili semiconduttivi (InAs, InSb, InP) oppure flake di grafene o isolanti topologici (Bi2Te3 e Sb2Te3). Inoltre studieremo l'accoppiamento con il gas di elettroni bidimensionale che si realizza all'interfaccia tra LaAlO3/SrTiO3. Le nanostrutture superconduttore-semiconduttore sono un passo fondamentale per l'implementazione del transistor superconduttivo e sono molto promettenti per varie applicazioni, che vanno dalla spintronica di bassa potenza ai sensori ultra-sensibili nel regime dei THz, nonché all'elettronica quantistica. Il progetto si prefigge di 1) comprendere come la coerenza superconduttiva si propaga in barriere quasi unidimensionali e attraverso interfacce di nuova generazione composte da superconduttori non-convenzionali e nanostrutture; 2) individuare, sia teoricamente che sperimentalmente, evidenze della creazione di fermioni di Majorana e stati topologicamente protetti, e comprenderne la stabilità in presenza di correlazioni, disordine e condizioni di non-equilibrio. Questi risultati rappresenterebbero un importante traguardo nella fisica dello stato solido. Il nostro progetto sfrutterà in modo sinergico le competenze di tre gruppi di giovani ricercatori italiani che condivideranno le loro competenze per lavorare in questo campo nuovo e inesplorato, in continua collaborazione con prestigiosi centri di ricerca europei e mondiali. L'unità sperimentale fa riferimento al Dipartimento di Fisica, Università degli studi di Napoli “Federico II” - NA e si occuperà delle misure di trasporto basse temperature. I campioni saranno realizzati in collaborazione con il CNR-NANO di Pisa continuando una fruttuosa collaborazione già attiva da alcuni anni. L'unità sperimentale sarà supportata nell'interpretazione, nell'analisi e nella progettazione degli esperimenti da due unità teoriche: ISC-CNR Roma e SPIN-CNR Genova. Le competenze complementari dei ricercatori coinvolti garantiranno rapidi progressi nel campo. In particolare l'unità di Napoli metterà a disposizione infrastrutture per caratterizzazione in trasporto di campioni a temperature ultra-basse (fino a 15mK) e il suo bagaglio di conoscenze nel campo della superconduttività debole. L'unità ISC-CNR si concentrerà sugli aspetti topologici, sulla fisica dei Fermioni di Majorana e su effetti di disordine e correlazione. L'unità SPIN-CNR supporterà il progetto grazie alle proprie competenze in trasporto di non-equilibrio ed effetti a molti corpi in sistemi elettronici fortemente correlati