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We experimentally investigate the sensing properties of an open cavity operating in the THz regime and realized by employing as mirrors two thin vanadium dioxide (VO

The catalytic and electro-optical properties of TiO2-based materials depend on the defectivity of their electronic structures and in turn on the synthesis conditions. It is however less clear how t...

The incorporation of graphene-related materials as nanofiller can produce multifunctional foams with enhanced specific properties and density reduction. Herein we report on the preparation of microcellular thermoplastic polyurethane/graphene foams by batch foaming. Solution blending was first adopted to disperse graphene nanoplatelets (GNP) in the elastomeric matrix. Then, a foaming process based on the use of supercritical CO2 was adopted to produce the microcellular TPU/GNP composite foams with graphene content up to 1 wt%. The EMI shielding behaviour of the TPU/GNP foams has been assessed in the THz range, and has revealed their potential in comparison with other graphene-filled foams presented in the literature, that exhibit similar specific shielding effectiveness but at much higher content of graphene-related materials (10-30 wt%).

Realistic lumped-element circuits describing split-ring resonator metasurfaces and their complementary forms are presented. To consider the polarization dependence, we exploit the symmetry of the meta-atoms with respect to the impinging electric field. Our approach enables to describe the circuital architectures including the effect of Ohmic dissipation in real metasurfaces. Lumped-element models describes in a simple way features like enhanced transmission and deep absorption that are observed in metasurfaces and complementary metasurfaces based on split-ring resonators.

We experimentally investigate the sensing properties of an open cavity operating in the THz regime and realized by employing as mirrors two thin vanadium dioxide (VO2) films grown on silicon parallel plates and separated by a variable length. The phase transition of VO2 is used to control the behavior of the system between two different responses: a high transmission mode to the incident radiation (VO2 in the insulating state) and a high sensitivity to tiny changes in the cavity refractive index (VO2 in the conducting state). In the first state, the low loss regime enables to adjust the cavity length and easily optimize the resonances due to the Fabry-Perot (FP) effect in the Si plates and in the cavity volume. The activation of the metallic-like state instead, by damping the FP oscillations in the plates, promotes the onset of a comb-like spectrum that can be exploited as a versatile tool for accurate sensing applications. Using both an analytical model and full-wave simulations, we estimate the device response to variation in the refractive index of the cavity volume, showing that the proposed structure can achieve sensitivity values among the highest reported for THz sensors.

A novel, to the best of our knowledge, methodology based on the combination of experimental measurements and simulations of the wave transmission through a metasurface at different angles is presented, enabling us to identify the fundamental and first high-order mode of spoof surface plasmon polaritons (SSPPs) excited in the terahertz regime. The approach offers a new way, an alternative to standard near field imaging, to trace out the presence of SSPPs on a metal-dielectric interface.

Coding metasurfaces, composed of only two types of elements arranged according to a binary code, are attracting a steadily increasing interest in many application scenarios. In this study, we apply this concept to attain diffuse scattering at THz frequencies. Building up on previously derived theoretical results, we carry out a suboptimal metasurface design based on a simple, deterministic and computationally inexpensive algorithm that can be applied to arbitrarily large structures. For experimental validation, we fabricate and characterize three prototypes working at 1 THz, which, in accordance with numerical predictions, exhibit significant reductions of the radar cross-section, with reasonably good frequency and angular stability. Besides the radar-signature control, our results may also find potentially interesting applications to diffusive imaging, computational imaging, and (scaled to optical wavelengths) photovoltaics.

Different spectroscopic techniques have been applied to fluorine doped ZnO powders prepared through hydrothermal synthesis, to discern the effective capability of F atoms to improve ZnO conductivity. From XRD analysis, no lattice distortion was observed up to F doping at 5 at. % concentration. Photoluminescence measurements and electron paramagnetic resonance data show that F atoms tend to occupy oxygen vacancies, inducing the onset of luminescence centers. The resulting doping effect consists into the increment of localized charge, as also proved via THz spectroscopy, where the Drude-Smith model has been applied to extract quantitative information on the electrodynamic parameters of ZnO:F samples. Results show that F doping does not produce any substantial change of plasma frequency but only the enhancement of scattering rate due to an increase of grain boundary density. Our measurements are in agreement with theoretical calculations asserting that the energy required to excite donor levels is on the order of 0.7 eV, and therefore, the doping mechanism is ineffective at room temperature.

We report on fs laser structuring and graphitization of diamond and experimental characterization of its THz response. A full characterization of graphitized, conductive layer generated by laser irradiation is carried out by performing scanning-electron microscopy, Raman spectroscopy and electrical measurements. The transmittance of the laser textured diamond samples, both with the graphitic overlayer and after selective oxidizing etching, is analyzed in the (0.25 ÷ 6.0) THz spectral range. A significant selective absorption of the graphitized overlayer towards polarized THz radiation is demonstrated, which is associated to the formation of graphitic laser induced periodic surface structures. This anisotropy allows conceiving compact passive metasurfaces based on conductive/dielectric patterns on the diamond plate surface for the development of robust, lightweight and broadband THz optical components.

Resistive-wall impedance constitutes a significant percentage of the total beam-coupling impedance budget of an accelerator. A number of different reduction techniques have been proposed during the years depending on the specific applications, ranging from higher order modes damping to solutions entailing high electrical-conductivity coatings of the pipe. This article investigates the use of metamaterial-based absorbers for sensibly reducing or nearly canceling the beam-coupling impedance. We design and fabricate subwavelength 2-D metallic resonant structures based on the split-ring resonator (SRR) geometry that can be employed as mode dampers in accelerating structures. A number of prototypes have been fabricated and measured in a “test model” pillbox cavity. Experimental results, combined with full-wave electromagnetic simulations, prove the efficiency of the SRR-based metamaterials. This article opens up to the possibility of considering metamaterials as a valid alternative to other devices for impedance mitigation in experimental setups commonly operating along a particle beamline, such as accelerating cavities or collimators, and more in general for the development of filters with a large out-of-band signal rejection in specific applications.

The electromagnetic interference (EMI) shielding mechanisms of graphene/PMMA multilayered structures are experimentally investigated by using time domain spectroscopy (TDS) in the THz range. Stacked plates of similar thickness (~ 5μm), starting from a single layer up to 100 layers, were produced by a novel approach combining ultra-thin polymer casting and wet deposition techniques. These nano laminates show enhanced electrical conductivity (~100 S/cm) and superior specific shielding effectiveness (~2.104 dB cm2 g-1).

We study the sensitivity of two different metagrids to the presence of biological fluids on the surface. When a THz beam impinges on each metagrid capped with a dielectric substrate, peaks in transmission related to the onset of high order surface plasmon polaritons appear and allow to estimate the properties of a guest fluid filling the partition volume with a very high sensitivity, comparable with record literature values.

Superconductors are essential in many present and future technologies, from large-scale devices for medical imaging, accelerators, or fusion experiments to ultra-low-power superconducting electronics. However, their potential applicability, and particularly that of high-temperature superconductors (HTS), is severely affected by limited performances at large magnetic fields and high temperatures, where their use is most needed. One of the main reasons for these limitations is the presence of quantized vortices, whose movements result in losses, internal noise, and reduced performances. The conventional strategy to overcome the flow of vortices is to pin them along artificial defects. Here, we theoretically and experimentally demonstrate that critical-current density in high-temperature superconductors can reach unprecedented high values at high fields and temperatures by preventing vortex entry. By tailoring the geometry, that is, reducing the width, W, of nanowire-patterned HTS films, the range of the Meissner state, for which no vortices are present, is extended up to very large applied field values, on the order of ∼1 T. Current densities on the order of the depairing current can be sustained under high fields for a wide range of temperatures. Results may be relevant both for devising new conductors carrying depairing-current values at high temperatures and large magnetic fields and for reducing flux noise in sensors and quantum systems., We acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence (SEV-2015-0496), COACHSUPENERGY project (MAT2014-51778-C2-1-R), and SuMaTe (RTI2018-095853-B-C21) cofinanced by the European Regional Development Fund, and project MAT2016-79426-P (Agencia Estatal de Investigacion/Fondo ́ Europeo de Desarrollo Regional). We also thank support from the European Union for NANOCOHYBRI project (Cost Action CA 16218) and from the Catalan Government projects SGR-2017-1519 and 2017-SGR-105. A.S. acknowledges a grant from ICREA Academia, funded by the Generalitat de Catalunya. V.R. Acknowledges EU H2020 research and innovation programme (Marie Sklodowska-Curie IF grant agreement OXWALD 838693). We acknowledge the use of ICTS-CNM facilities and ICMAB scientific and technical services. We thank Alexey V. Pan for fruitful discussions.

We report on the realization of high Q metasurfaces operating in the THz frequency range by femtosecond laser ablation applied to a nanometric metallic layer over a silicon substrate. Two different fabrication methods are used to develop periodic patterns whose basic elements are in form of an array of through-holes or metallic islands. The response of the resulting structures is characterized using a time-domain spectrometer in the frequency range 0.3–1.5 THz. The experimental findings are compared with the predictions of full wave electromagnetic simulations. The fairly good agreement between simulation predictions and experimental findings evidences that the proposed approach can offer a facile way to the elaboration of THz metasurfaces.

We describe an experimental strategy for the use of Terahertz (THz) metasurfaces as a platform for label-free wide range detection of the dielectric function in biological fluids. Specifically, we propose a metagrid (MG), opportunely infiltrated with a fluid and then capped, as the reference structure for sensing experiments with a high reproducibility character. By combining experiments and full-wave simulations of the transmission T of such a structure, we introduce a reliable set up where the volume of the involved analyte in each unit cell is precisely determined. The unavoidable decrease in the quality factor of the intrinsic resonances due to the lossy fluid and cap layer is circumvented using an appropriate transformation of T that amplifies the change in the MG intrinsic resonances, improving in such a way the sensor sensitivity to values close to the experimental limits. The transformed signal features delta-like peaks enabling an easy readout of frequency positions at resonances.

The transmission response of metallo-dielectric grid metasurfaces is experimentally investigated through Terahertz Time Domain Spectroscopy and the corresponding effective dielectric function is retrieved. Using a lumped element model we can determine the dependence of the effective plasma frequency (the transition frequency) on the metasurface filling factor F. The change of the transition frequency vs. F spans over one order of magnitude and sets the threshold between the metamaterial (homogeneous) and the photonic crystal (diffraction-like) regime, ruling the onset of two different Surface Plasmon Polaritons, spoof and high order. Field symmetry and spatial extension of such excitations are investigated for the possible applications of THz grid metasurfaces in bio- and chemical sensing and sub-wavelength imaging.

Resistive-wall impedance constitutes a significant percentage of the total beam-coupling impedance budget of an accelerator. Under extreme conditions, like large power handling and ultrahigh vacuum, metamaterial-based absorbers can represent a valid alternative to other approaches for impedance mitigation in specific accelerator components, like resonant (parasitic) cavities or collimators operating along the beam line. We design sub-wavelength 2D metallic resonant structures based on split rings or on 3D hyperbolic tapered waveguide arrays that can be employed as mode dampers in accelerating structures. A number of prototypes are fabricated and then measured in a "test model" pillbox cavity.

High-resolution focused ion beam lithography has been used to fabricate YBa2Cu3O7-x (YBCO) wires with nanometric lateral dimensions. In the present work, we investigate Flux-flow instabilities in nanowires of different widths, showing sudden voltage switching jumps from the superconducting to the normal state. We present an extensive study on the temperature and field dependence of the switching characteristics which reveal that voltage jumps become less abrupt as the temperature increases, and disappear at the vortex-liquid state. On the contrary, the current distribution at the critical point becomes narrower at high temperatures. Sharp voltage switchings very close to the critical current density can be obtained by reducing the width of the nanowires, making them very appealing for practical applications.

The Terahertz region covers the electromagnetic spectrum incorporating the advantages of both microwaves and infrared light waves. Many different techniques have been proposed to manipulate THz waves. Among them, metamaterials and metasurfaces have proven to have a high potential to engineer new type of functional THz devices. A recent and promising approach is based on coding metasurfaces, consisting of two or more basic ("digital") unit cells with out-of-phase response, which can be used to achieve diffuse scattering. Here, with special focus on the scattering-signature reduction, the physical mechanism is elucidated via a theoretical study and the relevant scaling-laws validated using THz Time Domain Spectroscopy.

This paper describes a reliable, handy, and inexpensive measurement system to characterize, in the sub-THz frequency range, the coating materials used in the beam pipes of accelerators. The method is based on time domain measurements of an electromagnetic wave passing through a waveguide having a thin central slab, where the coating material is deposited on both sides. Two horn antennas are integrated on both sides of the device to optimize the signal collection and detection. This novel technique is tested on slabs coated with a nonevaporable getter (NEG) and allows to evaluate the surface impedance in the frequency range from 0.1 to 0.3 THz.

High-resolution focused ion beam lithography has been used to fabricate YBa2Cu3O7-x (YBCO) wires with nanometric lateral dimensions. In the present work, we investigate Flux-flow instabilities in nanowires of different widths, showing sudden voltage switching jumps from the superconducting to the normal state. We present an extensive study on the temperature and field dependence of the switching characteristics which reveal that voltage jumps become less abrupt as the temperature increases, and disappear at the vortex-liquid state. On the contrary, the current distribution at the critical point becomes narrower at high temperatures. Sharp voltage switchings very close to the critical current density can be obtained by reducing the width of the nanowires, making them very appealing for practical applications., We acknowledge financial support fromthe SpanishMinistry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496), CONSOLIDER Excellence Network (MAT2015-68994-REDC), COACHSUPENERGY project (MAT2014-51778-C2-1-R, co-financed by the European Regional Development Fund). We also are grateful for support from the European Union for ULTRASUPERTAPE project (ERC-2014-ADG-669504), NANOCOHYBRI project (Cost Action CA 16218), and from the Catalan Government with SGR-2017 and Xarmae. We acknowledge the use of ICTS-CNM facilities and ICMAB scientific and technical services.

The paper aims at experimentally assessing the metrological performance of a compressive sampling (CS)-based terahertz (THz) imaging system, an emerging technology for carrying out non-destructive tests of materials in order to detect defects and flaws. Differently from systems based on traditional raster scan, the exploitation of CS allows, in fact, obtaining images of interest in a reduced observation interval to the detriment of accuracy. In literature, several studies have faced the topic at hand showing results obtained in simulated and real conditions. On the same way, the authors have decided to focus their attention on the experimental investigation of the impact of some uncertainty sources on the final image reconstruction realizing a metrological characterization that could be used in different fields as for example aerospace. It is so possible to provide a preliminary roadmap capable of driving experimenters in the definition of the main features of the measurement setup to achieve a defined goal in term of reconstruction quality.

We investigate a hybrid re-configurable three dimensional metamaterial based on liquid crystal as tuning element in order to build novel devices operating in the terahertz range. The proposed metadevice is an array of meta-atoms consisting of split ring resonators having suspended conducting cantilevers in the gap region. Adding a “third dimension” to a standard planar device plays a dual role: (i) enhance the tunability of the overall structure, exploiting the birefringence of the liquid crystal at its best, and (ii) improve the field confinement and therefore the ability of the metadevice to efficiently steer the THz signal. We describe the design, electromagnetic simulation, fabrication and experimental characterization of this new class of tunable metamaterials under an externally applied small voltage. By infiltrating tiny quantities of a nematic liquid crystal in the structure, we induce a frequency shift in the resonant response of the order of 7–8% in terms of bandwidth and about two orders of magnitude change in the signal absorption. We discuss how such a hybrid structure can be exploited for the development of a THz spatial light modulator.

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.

Electromagnetic properties of a photonic crystal slab in the shape of a two dimensional periodic lattice of polydimethylsiloxane (PDMS) rods with square symmetry is investigated through the use of THz time domain spectroscopy. The fit of the transmission through a full wave simulation give insights on the guided mechanism responsible for the onset of a resonance at about ƒ 0 = 1.57THz. The device can be exploited for enhanced sensing experiments of different organic/biological substances as supported by full wave simulations.

Superconducting nanowires currently attract great interest due to their application in single-photon detectors and quantum-computing circuits. In this context, it is of fundamental importance to understand the detrimental fluctuations of the superconducting order parameter as the wire width shrinks. In this paper, we use controlled electromigration to narrow down aluminium nanoconstrictions. We demonstrate that a transition from thermally assisted phase slips to quantum phase slips takes place when the cross section becomes less than ∼150 nm2. In the regime dominated by quantum phase slips the nanowire loses its capacity to carry current without dissipation, even at the lowest possible temperature. We also show that the constrictions exhibit a negative magnetoresistance at low-magnetic fields, which can be attributed to the suppression of superconductivity in the contact leads. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic-size contacts., Nanostructured superconductors allow dissipationless electrical transport to be exploited in technologically relevant devices. Here, the authors follow how detrimental fluctuations of the superconducting order parameter evolve in Al atomic contacts as their width is controlled by electromigration.

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.

Over the past few years, the scientific community, both in academia and industry, witnessed a continuous and persistent effort in order to close the unexplored portion of the electromagnetic spectrum called THz gap, ranging from 100 GHz to 10 THz. Thanks to its properties, the THz technology finds application in diverse dual-use areas, spanning from chemical to pharmaceutical, biology and environmental sciences, and everyday applications within a broad range of industries including security, cultural heritage, manufacturing, and aerospace sectors. After a brief description of the common architecture of THz system, we focus here on Time Domain Spectroscopy (TDS-THz) THz system, we provide here some examples of dual-use THz applications, such as spectroscopic measurements of unknown objects or contactless and non-destructive analysis of different materials by spatial imaging operations.

We present the experimental characterization of mantle cloaks designed so as to minimize the electromagnetic scattering of moderately-sized dielectric and conducting cylinders at S-band microwave frequencies. Our experimental setup is based on a parallel-plate waveguide system, which emulates a two-dimensional plane-wave scattering scenario and allows the collection of near-field maps as well as more quantitative assessments in terms of global scattering observables (e.g., total scattering width). Our results, in fairly good agreement with full-wave numerical simulations, provide a further illustration of the mantle- cloak mechanism, including its frequency-sensitivity and confirm its effectiveness both in restoring the near-field impinging wavefront around the scatterer and in significantly reducing the overall scattering.

We present a simple model for a patch antenna based on metamaterial concepts operating in the microwave region with a bandwidth of 1.9–2.6 GHz. The small size and the relatively large operational band make this device exploitable for electromagnetic energy harvesting in urban environments. Describing the antenna by an equivalent circuit composed of a transmission line having right/left hand properties with a RC load, we can properly account for the phase dynamics in correspondence of resonances characterized by a specific handedness. The model is validated measuring the response of the reflection scattering parameter S1,1 for a copper prototype fabricated on a FR4 printed circuit board.

The terahertz (THz) frequency region is often defined as the last unexplored area of the electromagnetic spectrum. Over the past few years, the full access has been the objective of intense research efforts. Progress in this area has played an important role in opening up the possibility of using THz electromagnetic radiation (T-waves) in science and in real-world applications. T-waves are not perceptible by the human eye, are not ionizing, and have the ability to cross many non-conducting materials such as paper, fabrics, wood, plastic, and organic tissues. Moreover, the use of THz radiation allows non-destructive analysis of the materials under investigation both by study of their "fingerprint" via spectroscopic measurements and by high-resolution spatial imaging operations, exploiting the see-through capability of T-waves. Such technology can be applied in diverse areas, spanning from biology to chemical, pharmaceutical, environmental sciences, etc. In this chapter, we will present the typical architecture of measurement systems based on the THz technology, detailing what are the parameters that define their performance, the measurement methods, and the related errors and uncertainty, and focusing at the end on the use of time-domain spectroscopy for the evaluation of different material properties in this specific frequency region.

Graphene-like (GL) layers and eumelanin-based graphene-like (EUGL) hybrids have been investigated through THz time domain spectroscopy. The interest in these materials lies on their peculiar chemical-physical properties: the former are conductive water stable materials, whereas the latter are biocompatible materials with good conductive and adhesive properties. Both exhibit promising optoelectronic and bioelectronic applications. We measured mixtures of GL layers or EUGL hybrids with KBr, shaped in pellets with uniform thickness, in order to circumvent problems related to sample inhomogeneity and roughness. A mean field theory was applied to extract direct information on permittivity and conductivity. Data have been carefully fitted through the Drude-Smith theory, confirming the conductive nature of the hybrid materials. The results show that EUGL hybrid-based devices can be promising for the next generation of printable bio-circuits.

The paper deals with the problem of early detecting cracks in composite materials for avionic applications. In particular, the authors present a THz imaging system that exploits compressive sampling (CS) to detect submillimeter cracks with a reduced measurement burden. Traditional methods for THz imaging usually involve raster scan of the issue of interest by means of highly collimated radiations and the corresponding image is achieved by measuring the received THz power in different positions (pixels) of the desired image. As it can be expected, the higher the required resolution, the longer the measurement time. On the contrary, two different approaches for THz imaging (namely, continuous wave and time domain spectroscopy) combined with a proper CS solution are used to assure as good results as those granted by traditional raster scan; a proper set of masks (each of which characterized by a specific random pattern) are defined to the purpose. A number of tests conducted on simulated data highlighted the promising performance of the proposed method thus suggesting its implementation in an actual measurement setup.

We investigate new classes of artificial materials exhibiting unconventional properties in order to build novel devices operating in the Terahertz regime. The design, fabrication and electromagnetic characterization of tunable metamaterials with unit cells based on Split Ring Resonators are presented. By incorporation of a nematic liquid crystal in the structure, we observe a frequency shift in the resonant response over 10% in bandwidth and more than 10 dB change in the signal absorption. We discuss how such a hybrid structure can be exploited for the development of a THz spatial light modulator.

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.

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., to appear on new journal of Physics

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/cm2 at T = 4.2 K and of 10 MA/cm2 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 rather attractive system for the fabrication of efficient sensors, supporting the notion of futuristic THz devices., 8 pages, 3 figures. Accepted for publication in Superconductor Science and Technology

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.

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. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624471]

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 I(c)R(n) (with I(c) the critical current and R(n) 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.