A. Rovelli

Functional decoupling of nanostructured areas in superconducting strips for electromagnetic detectors

We studied high temperature superconducting devices created by means of local nanostructuring with high-energy heavy-ion lithography. Linear arrays of alternate nanostructured and as-grown YBa2Cu3O7−x film regions are patterned on the microscale. The density of the induced nanostructures is tuned in such a way to locally depress the superconducting properties until a functional electromagnetic decoupling between nanostructured and as-grown microzones is achieved in suitable regions of the (B,T) phase diagram. Pilot prototypes engineered for operating as magnetic field detectors in harsh cryogenic environment are characterized by electric transport and surface morphology analysis at microscale and nanoscale. Sensitivity, linearity range, and bandwidth limits of the pilot prototype are established from electrical transport characterization. The morphological analysis reveals the salient aspects of the surface modifications occurring with high-fluence heavy-ion lithography. Radiation annealing experiments wi...

THz Detection Above 77 K in YBCO Films Patterned by Heavy-Ion Lithography

Research and application of THz technology, pursued all over the last century, are currently focused either on high-performance sources and detectors or on low-cost and rugged devices that are suitable for widespread use. Besides semiconductors, high temperature superconductors are good candidates for next generation superconducting bolometers. We employ the high energy heavy ion (HEHI) lithography for modulating the local structural and electrical properties of high temperature superconducting YBa2Cu3O7 (YBCO) films in a functional way. In this report, measurements of the photoresponse of a prototype device to electromagnetic radiation in the far infrared band (THz region) are presented. It is demonstrated that HEHI lithography is an effective tool to enhance the absorption band of the YBCO film in the infrared spectrum, enabling the THz detection of YBCO sensors, in the superconducting state and above the liquid nitrogen temperature.

NEMO: A PROJECT FOR A KM3 UNDERWATER DETECTOR FOR ASTROPHYSICAL NEUTRINOS IN THE MEDITERRANEAN SEA

The status of the project is described: the activity on long term characterization of water optical and oceanographic parameters at the Capo Passero site candidate for the Mediterranean km3 neutrino telescope; the feasibility study; the physics performances and underwater technology for the km3; the activity on NEMO Phase 1, a technological demonstrator that has been deployed at 2000 m depth 25 km offshore Catania; the realization of an underwater infrastructure at 3500 m depth at the candidate site (NEMO Phase 2).

Status and first results of the NEMO Phase-2 tower

In March 2013, the NEMO Phase 2 tower has been successfully installed in the Capo Passero site, at a depth of 3500 m and 80 km off from the southern coast of Sicily. The unfurled tower is 450 m high; it is composed of 8 mechanical floors, for a total amount of 32 PMTs and various instruments for environmental measurements. The tower positioning is achieved by an acoustic system. The tower is continuously acquiring and transmitting all the measured signals to shore. Data reduction is completely performed in the Portopalo shore station by a dedicated computing facility connected to the persistent storage system at LNS, in Catania. Results from the last 9 months of acquisition will be presented. In particular, the analyzed optical rates, showing stable and low baseline values, are compatible with the contribution mainly of 40K light emission, with a small percentage of light bursts due to bioluminescence. These features reveal the optimal nature of the Capo Passero abyssal site to host a km3-sized Neutrino Telescope.

HIGH CURRENT CARRYING NETWORKS OF JOSEPHSON JUNCTIONS IN YBCO FILMS

High-quality YBCO films present a characteristic plateau in the logarithmic plot of the critical currents versus field. This plateau can be attributed in the high-current regime to networks of defects having different sizes and distributions. In addition to the intrinsic defects, we created columnar defects across YBCO films by irradiating sets of samples with 0.25 GeV Au ions at different doses. In this paper we show that the width of the plateau is related to the distribution of defects along the grain boundaries. This distribution can be changed by means of ion irradiation. The experimental results are compared with theoretical models both in the low- and in the high-temperature regime. In the low-temperature regime, a model based on Josephson junction arrays fits optimally the experimental Jc(B) data. On the contrary, in the high-temperature regime the film behaves almost as a continuum, where the columnar defects emerge as the only effective pins. In this case we refer to a Bose-glass picture.

Localized Photoresponse of YBCO Films Patterned by Heavy-Ion Lithography

Heavy Ion Lithography was used for local tuning the superconducting properties (critical temperature and critical current) of high temperature superconducting films through the introduction of ion induced nanostructures confined in micrometric areas. These modified areas were patterned on a device layout that was designed for the wide-band bolometric detection of electromagnetic radiation. The device was biased with constant current at a specific temperature in order to obtain the localization of the electromagnetic excitation only into the modified areas. The device was tested with coherent light stimuli in the visible spectrum and the photoresponse was measured with high frequency real-time sampling. The localization of the photoexcitation was demonstrated by scanning the laser beam across the modified and as-grown areas of the device.

Nanostructuring of High‐TC Superconductors into Micro‐Sized Zones

A special apparatus was designed in order to “write”, with nanometric resolution, microsize‐confined nanostructures in oxide samples. The nanostructures are produced by high energy heavy ion irradiation that allows nanostructuring the sample over its full thickness. The properties of the nanostructured areas can be further modulated by choosing the proper energy and fluence of the incoming ion beam. We present this set‐up and different kinds of nanostructured patterns created on high temperature superconducting films. We used the magneto‐optical analysis to directly show the effect of the confined nanostructures on the micron scale. The confined nanostructured area, embedded in the virgin matrix, is demonstrated to be a fruitful element for designing a new class of devices.

Strong vortex pinning and anisotropy in Ag/BSCCO-2223 tapes caused by surface columnar defects

This paper is aimed at investigating the vortex dynamics driven by 0.25 GeV Au ion implantation in Ag/BSCCO-2223 tapes. Columnar defects are produced perpendicularly to the tape plane and only affect a surface layer of about 10% of the whole sample volume. The paper shows that this defect topology does not deteriorate the critical temperature significantly. Moreover, the irreversibility line (IL) measured with the applied field perpendicular to the tracks is nearly unaffected. On the contrary, the IL with the applied field either parallel to the tracks or moderately tilted is shifted towards higher fields and temperatures. The amount of the shift is comparable with that obtained for tracks crossing the whole sample. In a large range of magnetic fields near the dose equivalent field the IL anisotropy falls to zero. However, the ILs after surface implantation exhibit very peculiar features.

Nanostructuring superconductors by ion beams: A path towards materials engineering

The paper deals with nanostructuring of superconducting materials by means of swift heavy ion beams. The aim is to modify their structural, optical and electromagnetic properties in a controlled way, to provide possibility of making them functional for specific applications. Results are presented concerning flux pinning effects (implantation of columnar defects with nanosize cross section to enhance critical currents and irreversibility fields), confined flux-flow and vortex guidance, design of devices by locally tailoring the superconducting material properties, analysis of disorder-induced effects in multi-band superconductors. These studies were carried out on different kinds of superconducting samples, from single crystals to thin films, from superconducting oxides to magnesium diboride, to recently discovered iron-based superconductors.

Rugged superconducting detector for monitoring infrared energy sources in harsh environments

Broadband electromagnetic characterization of hot plasmas, such as in nuclear fusion reactors and related experiments, requires detecting systems that must withstand high flux of particles and electromagnetic radiations. We propose a rugged layout of a high temperature superconducting detector aimed at 3 THz collective Thomson scattering (CTS) spectroscopy in hot fusion plasma. The YBa2Cu3O7 − x superconducting film is patterned by standard photolithography and the sensing area of the device is created by means of high-energy heavy ion irradiation, in order to modify the crystal structure both of the superconducting film and of the substrate. This method diminishes process costs and resulting device fragility due to membrane or air-bridge structures that are commonly needed for MIR and FIR radiation detection. Moreover the sensing area of the device is wired by the same superconducting material and thus excellent mechanical strength is exhibited by the whole device, due to the oxide substrate. Continuous wave operation of prototype devices is demonstrated at liquid nitrogen temperature, for selected infrared spectra of broadband thermal energy sources. Several solutions, which exploit the advantages coming from the robustness of this layout in terms of intrinsic radiation hardness of the superconducting material and of the needed optical components, are analysed with reference to applications of infrared electromagnetic detectors in a tokamak machine environment.