Marco Truccato

Control of the oxygen doping in Bi-2212 whiskers by means of their synthesis process

Direct evidence of oxygen doping control in single phase Bi2Sr2CaCu2O8+δ (Bi-2212) whiskers is reported, along with the changes in their structural properties obtained by varying the growth temperature of the synthesis process in the range from 843 to 872 °C. The as-grown whiskers were investigated by means of x-ray powder diffraction (XRPD), electrical transport measurements, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). The XRPD measurements showed that the value of the c-axis lattice parameter increases from 30.556 to 30.640 A when increasing the growth temperature, which indicates different oxygen doping levels spanning from the slightly overdoped to the nearly optimally doped regimes. Such results are also confirmed by the electrical characterizations, which revealed a typical relationship among resistivity (ρab), superconducting critical temperature (Tc), and c-axis value. The growth of CuO crystals has also been identified during this study, with a maximum yield in the range 860–864 °C, where also a slope change in the c-axis behavior has been found, implying a possible correlation between the two phenomena. Therefore, by changing the synthesis growth temperature, one can provide an effective way to tune the whisker electrical transport properties.

Structural Characterization of Multi-Quantum Wells in Electroabsorption-Modulated Lasers by using Synchrotron Radiation Micrometer-Beams

Advanced optoelectronic devices, the backbone of modern communication technology, require the monolithic integration of different functions at chip level. An example of devices fulfilling this requirement are multi-quantum well (MQW) electroabsorption-modulated lasers (EMLs) employed in long-distance, high-frequency optical fiber communications technology. Such devices are realized by using the selective area growth (SAG) technique. Optimization of the growth parameters is carried out by empirical approaches as direct structural characterization of the MQW is not possible with laboratory X-ray sources, owing to the micrometer-variation of composition and thickness inherent to the SAG technique. Micrometer-resolved X-ray (m-X-ray) beam available at third generation synchrotron radiation sources, such as the ID22 at the European Synchrotron Radiation Facility (ESRF), allowed us to directly measure the determinant structural parameters of MQW EML structures such as well and barrier widths and mismatches with a 2 mm spatial resolution. In more detail, optoelectronic devices, widely used in the generation and detection of optical signal for telecom and datacom applications, consist of different III-V semiconductors alloys deposited on suitable substrates. The deposition can be performed by different techniques that are based on the crystal rebuilding of a semiconductor used as substrate, such as metal

μ -EXAFS, μ -XRF, and μ -PL Characterization of a Multi- Quantum-Well Electroabsorption Modulated Laser Realized via Selective Area Growth

In the past few years, strong efforts have been devoted to improving the frequency of optical-fiber communications. In particular, the use of a special kind of integrated optoelectronic device called an electroabsorption modulated laser (EML) allows communication at 10 Gb s(-1) or higher over long propagation spans (up to 80 km). Such devices are realized using the selective area growth (SAG) technique and are based on a multiple quantum well (MQW) distributed-feedback laser (DFB) monolithically integrated with a MQW electroabsorption modulator (EAM). Since the variation in the chemical composition between these two structures takes place on the micrometer scale, in order to study the spatial variation of the relevant parameters of the MQW EML structures, the X-ray microbeam available at the ESRF ID22 beamline is used. The effectiveness of the SAG technique in modulating the chemical composition of the quaternary alloy is proven by a micrometer-resolved X-ray fluorescence (μ-XRF) map. Here, reported micrometer-resolved extended X-ray absorption fine structure (μ-EXAFS) spectra represent the state of the art of μ-EXAFS achievable at third-generation synchrotron radiation sources. The results are in qualitative agreement with X-ray diffraction (XRD) and micrometer-resolved photoluminescence (μ-PL) data, but a technical improvement is still crucial in order to make μ-EXAFS really quantitative on such complex heterostructures.

Doping change in the Bi-2212 superconductor directly induced by a hard X-ray nanobeam.

We describe the controlled use of a 17 keV X-ray synchrotron nanobeam to progressively change the oxygen doping level in Bi-2212 superconducting whisker-like single crystals. Our data combine structural and electrical information collected on the same crystals, showing a maximum change in the critical temperature Tc of 1.3 K and a maximum elongation of ∼1 Å in the c-axis length, compared to the as-grown conditions. Simulations of our experimental conditions by means of a finite element model exclude local heating induced by the X-ray nanobeam as a possible cause for the change in the doping level and suggest an important role of secondary electrons. These findings support the possible use of hard X-rays as a novel direct-writing, photoresist-free lithographic process for the fabrication of superconducting devices, with potential nanometric resolution and 3D capability.

CARBON INFLUENCE IN THE SYNTHESIS OF MgB2 BY A MICROWAVE METHOD

We describe a method to produce MgB2 bulk via a microwave processing, which has not been reported yet for this material. We used two experimental arrangements, which were different in the form of the graphite thermal activator responsible for the microwave absorption. The reaction products have been analyzed from the morphological, structural and electronic point of view. The results show that the critical temperature is decreased by about 4 K when the graphite can diffuse inside the reaction cell and is incorporated in the samples, even if no evidence of carbon substitution in the MgB2 lattice is given by the lattice constants. We point out that excellent conductivity features are achieved for nearly carbon-free samples.

Electrical transport effects due to oxygen content modifications in a Bi2Sr2CaCu2O8+δ superconducting whisker

We report a set of resistivity measurements along the α-axis of a Bι 2 Sr 2 CaCu 2 Ο 8 +δ microscopic superconducting whisker. The effect of the storage environment on sample ageing has been studied, considering both an air atmosphere at 273 K and a helium atmosphere at about 300 K for an overall storage time of about 100 days. It is clearly shown that the material underwent a remarkable resistivity increase of 26% at 260 K accompanied by a decrease in the critical temperature of 0.6 K during the whole ageing period. The helium atmosphere increased the average process rate by about two orders of magnitude. The present results are in agreement with previous findings on room temperature structural modifications in Bι 2 Sr 2 CaCu 2 Ο 8+δ whiskers and can be ascribed to oxygen depletion phenomena from the material.

Superconducting and hybrid systems for magnetic field shielding

In this paper we investigate and compare the shielding properties of superconducting and hybrid superconducting/ferromagnetic systems, consisting in cylindrical cups with aspect ratio height/radius close to unity. Firstly, we reproduced by finite element calculations the induction magnetic field values measured along the symmetry axis in a superconducting (MgB2) and in a hybrid configuration (MgB2/Fe) as a function of the applied magnetic field and of the position. The calculations were carried out using the vector potential formalism, taking into account simultaneously the non-linear properties of both the superconducting and the ferromagnetic material. On the basis of the good agreement between the experimental and the computed data we applied the same model to study the influence of the geometrical parameters of the ferromagnetic cup as well as of the thickness of the lateral gap between the two cups on the shielding properties of the superconducting cup. The results show that in the considered non-ideal geometry, where the edge effect in the flux penetration cannot be disregarded, the superconducting shield is always the most efficient solution at low magnetic fields. However, a partial recovery of the shielding capability of the hybrid configuration occurs if a mismatch in the open edges of the two cups is considered. On the contrary, at high magnetic fields the hybrid configurations are always the most effective. In particular, the highest shielding factor was found for solutions with the ferromagnetic cup protruding over the superconducting one

Direct-Write X-ray Nanopatterning: A Proof of Concept Josephson Device on Bi2Sr2CaCu2O8+δ Superconducting Oxide

We describe the first use of a novel photoresist-free X-ray nanopatterning technique to fabricate an electronic device. We have produced a proof-of-concept device consisting of a few Josephson junctions by irradiating microcrystals of the Bi2Sr2CaCu2O8+δ (Bi-2212) superconducting oxide with a 17.6 keV synchrotron nanobeam. Fully functional devices have been obtained by locally turning the material into a nonsuperconducting state by means of hard X-ray exposure. Nano-XRD patterns reveal that the crystallinity is substantially preserved in the irradiated areas that there is no evidence of macroscopic crystal disruption. Indications are that O ions have been removed from the crystals, which could make this technique interesting also for other oxide materials. Direct-write X-ray nanopatterning represents a promising fabrication method exploiting material/material rather than vacuum/material interfaces, with the potential for nanometric resolution, improved mechanical stability, enhanced depth of patterning, and absence of chemical contamination with respect to traditional lithographic techniques.

Electrical study of an unusual phase transformation in a Bi2Sr2Ca2Cu3O10+x whisker at room temperature

We report the observation of a phase transformation which can occur in a microscopic double phase BSCCO whisker at room temperature. We performed electrical resistivity measurements by thermally cycling the whisker in the range 78?300?K in a helium atmosphere and observed a decrease of the Bi-2223 phase amount in favour of the Bi-2212 phase, accompanied by an increase in the Tc of the Bi-2212 phase. A longer ageing of the whisker increased its resistance by about a factor of four at room temperature and caused a semiconducting behaviour at low temperature. We demonstrate that a simple electrical model can account for the experimental data and disentangle the contributions of the two phases.

17 keV-photon induced damage of Bi-2212 whiskers by synchrotron μ-beam exposure

Experimental data show that the normal state resistivity of superconducting Bi2Sr2CaCu2O8 + ? (Bi-2212) whiskers increases after a 6?h irradiation by a synchrotron ?-beam with 17?keV photons. We analyse this result on the basis of previously reported effects in Bi-2212 whiskers due to ageing or heating processes. A finite element model of the experimental setup clarifies that the heat load induced by the microbeam has to be excluded as a possible cause for the material changes. The knock on the interstitial, loosely bound, O species by secondary electrons is discussed as the most likely mechanism responsible for this effect.