A. S. Siri

Large transport critical currents in unsintered MgB2 superconducting tapes

The powder-in-tube process has been employed to fabricate tape-like conductors with a strong metallic sheath and based on the newly discovered MgB2 superconducting phase. Long superconducting tapes have been prepared by packing reacted MgB2 powders inside pure Ag, Cu, and Ni tubes which are then cold worked by drawing and rolling. Such tapes have shown transport properties as good as bulk MgB2 samples sintered in high pressure and high temperature conditions. At 4.2 K, the highest critical current density of 105 A/cm2 has been achieved on nickel-sheathed single-filament conductors. A direct correlation between the sheath tensile strength and the critical current of the unsintered tape has been observed.

Multistate memory devices based on free-standing VO2/TiO2 microstructures driven by Joule self-heating.

Two-terminal multistate memory elements based on VO(2)/TiO(2) thin film microcantilevers are reported. Volatile and non-volatile multiple resistance states are programmed by current pulses at temperatures within the hysteretic region of the metal-insulator transition of VO(2). The memory mechanism is based on current-induced creation of metallic clusters by self-heating of micrometric suspended regions and resistive reading via percolation.

Fabrication of submicron-scale SrTiO3−δ devices by an atomic force microscope

By applying a negative voltage to the conducting tip of an atomic force microscope, we modify on submicron-scale semiconducting oxygen deficient SrTiO3−δ thin films grown on LaAlO3 substrates. In comparison with the as-grown film, the modified regions present different electrical and structural properties, which can be exploited to realize submicrometer circuits. After a discussion on the mechanisms of the process, we report a prototype of a SrTiO3−δ-based sidegate field-effect transistor, showing a 4% modulation of channel resistivity with gate voltages up to 40 V.

SrTiO3-based metal–insulator–semiconductor heterostructures

We explored the feasibility of employing strontium titanate (SrTiO3) as semiconducting material in field-effect metal–insulator–semiconductor epitaxial heterostructures. This idea was suggested by the observation of a dramatic effect of the oxygen deficiency on SrTiO3−δ transport properties, which brings about metallic behavior with low-temperature mobility values comparable with those commonly found for silicon. By pulsed-laser deposition, we realized patterned field-effect devices, showing a resistance enhancement up to 90%. This promising result could open perspectives for crystalline-oxide electronics.

High quality epitaxial FeSe0.5Te0.5 thin films grown on SrTiO3 substrates by pulsed laser deposition

Superconducting epitaxial FeSe0.5Te0.5 thin films are prepared on SrTiO3(001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy are studied with different experimental techniques: x-rays diffraction, reflection high energy electron diffraction, scanning tunneling microscopy and atomic force microscopy. The substrate temperature during the deposition is found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate, without evidence of any other orientation. Moreover, such films exhibit a metallic behavior over the whole measured temperature range and the critical temperature is above 17?K, which is higher than the target value.Superconducting epitaxial FeSe0.5Te0.5 thin films were prepared on SrTiO3 (001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy were studied with different experimental techniques: X-rays diffraction, reflection high energy electron diffraction, scanning tunnelling microscopy and atomic force microscopy. The substrate temperature during the deposition was found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained in the optimal conditions show an epitaxial growth with c axis perpendicular to the film surface and the a and b axis parallel to the substrates one, without the evidence of any other orientation. Moreover, such films show a metallic behavior over the whole measured temperature range and critical temperature above 17K, which is higher than the target one.

Fabrication and properties of monofilamentary MgB2 superconducting tapes

Nickel-sheathed MgB2 tapes were fabricated by means of the powder-in-tube method with a monocore configuration of the superconductor. The metallic tubes were filled with commercial reacted powders and cold-worked, respectively, by groove rolling, drawing, and rolling to flat tapes of about 4 mm in width and 0.35 mm in thickness. Portions of the manufactured conductors were heat-treated in an argon atmosphere at temperatures up to 940 °C. Conductors were characterized by transport measurements to determine the magnetic field dependence of the critical current density, the irreversibility line, as well as to establish an initial relationship between the thermo-mechanical treatment and the current carrying capacity of the manufactured MgB2 samples. Our optimized samples for the low field operation carry a critical current density of at least 4 × 105 A cm−2 at 4.2 K, 1 T. The main outcome of this study, however, is that the optimal heat treatment condition changes if the MgB2 conductor has to be employed respectively for a low- or high-field operation. Finally, the first successful demonstration of the current carrying capability of longer MgB2 tapes is also reported in this paper.

Effects of neutron irradiation on polycrystalline Mg 11 B 2

We studied the influence of the disorder introduced in polycrystalline MgB2 samples by neutron irradiation. To circumvent self shielding effects due to the strong interaction between thermal neutrons and 10B we employed isotopically enriched 11B which contains 40 times less 10B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1-2x1018 cm-2. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behaviour has been carried out to identify what scattering and pinning mechanisms come into play. Finally the correlation between some characteristic lengths and the transition widths is analysed.

Synthesis, crystal structure, microstructure, transport and magnetic properties of SmFeAsO and SmFeAs(O0.93F0.07)

SmFeAsO and the isostructural superconducting SmFeAs(O0.93F0.07) phase were prepared and characterized by means of Rietveld refinement of x-ray powder diffraction data, scanning electron microscope observation, transmission electron microscope analysis, and resistivity and magnetization measurements. Sintering treatment strongly improves the grain connectivity, but, on the other hand, induces a competition between the thermodynamic stability of the oxy-pnictide and Sm2O3, thus affecting the sample purity. In the pristine sample both magnetization and resistivity measurements clearly indicate that two different sources of magnetism are present: the former related to Fe ordering at T~140 K and the latter due to the Sm ions that order antiferromagnetically at low temperature. The feature at 140 K disappears in the F-substituted sample, and a superconducting transition appears, at low temperatures. The magnetoresistivity curves of the F-substituted sample probably indicate very high critical field values.

Magnetic properties of PbFe2/3W1/3O3-PbTiO3 solid solutions

The magnetic properties of the ferroic (1−x)PbFe2/3W1/3O3–xPbTiO3 system with various compositions in the range x∈(0,0.50) were investigated. A magnetodielectric coupling is proved by an anomaly of the magnetic hysteresis loop found in the Curie range of temperatures for the relaxor PbFe2/3W1/3O3 ceramic. Two types of superexchange magnetic interactions and a succession of magnetic transitions with Neel temperatures dependent on the composition were found in the system. The present results confirm the hypothesis of a relationship between the magnetic properties and the nanopolar ordering in the relaxor–ferroelectric system.

Modulation of resistance switching in Au/Nb:SrTiO3 Schottky junctions by ambient oxygen

We investigated the room-temperature current-voltage characteristics of Au/Nb:SrTiO3 Schottky junctions under various atmospheres and working pressures. We observed that oxygen partial pressure reversibly modulates junction response, briefly individual specimens behave as high-quality rectifiers in oxygen-rich atmospheres and as bipolar resistive switches in vacuum and inert gases. A two orders of magnitude modulation of resistance switching characterizes samples with the highest content of interfacial oxygen vacancies. We attribute this behavior to oxygen ionosorption and chemical oxidation at the metal-oxide interface. Our results are relevant to oxide devices displaying resistive switching at ambient-exposed interfaces, and might be exploited for gas detection purposes.