Current Induced Resistive State in Fe(Se,Te) Superconducting Nanostrips

We study the current-voltage characteristics of Fe(Se,Te) thin films deposited on CaF2 substrates in form of nanostrips (width w ~ λ, λ the London penetration length). In view of a possible application of these materials to superconductive electronics and micro-electronics we focus on transport properties in small magnetic field, the one generated by the bias current. From the characteristics taken at different temperatures we derive estimates for the pinning potential U and the pinning potential range δ for the magnetic flux lines (vortices). Since the sample lines are very narrow, the classical creep flow model provides a sufficiently accurate interpretation of the data only when the attractive interaction between magnetic flux lines of opposite sign is taken into account. The observed voltages and the induced depression of the critical current of the nanostrips are compatible with the presence of a low number ([Formula: see text]) magnetic field lines at the equilibrium, a strongly inhomogeneous current density distribution at the two ends of the strips and a reduced Bean Livingston barrier. In particular, we argue that the sharp corners defining the bridge geometry represent points of easy magnetic flux lines injection. The results are relevant for creep flow analysis in superconducting Fe(Se,Te) nanostrips

Competition between intrinsic and extrinsic effects in the quenching of the superconducting state in FeSeTe thin films

We report the first experimental observation of the quenching of the superconducting state in current-voltage characteristics of an iron-based superconductor, namely, in FeSeTe thin films. Based on available theoretical models, our analysis suggests the presence of an intrinsic flux-flow electronic instability along with non-negligible extrinsic thermal effects. The coexistence and competition of these two mechanisms classify the observed instability as halfway between those of low-temperature and of high-temperature superconductors, where thermal effects are respectively largely negligible or predominant.Comment: 7 pages, 5 figures, fixed typo

Stability mechanisms of high current transport in iron-chalcogenides superconducting films

The improvement in the fabrication techniques of iron-based superconductors have made these materials real competitors of high temperature superconductors and MgB$_2$. In particular, iron-chalcogenides have proved to be the most promising for the realization of high current carrying tapes. But their use on a large scale cannot be achieved without the understanding of the current stability mechanisms in these compounds. Indeed, we have recently observed the presence of flux flow instabilities features in Fe(Se,Te) thin films grown on CaF$_2$. Here we present the results of current-voltage characterizations at different temperatures and applied magnetic fields on Fe(Se,Te) microbridges grown on CaF$_2$. These results will be analyzed from the point of view of the most validated models with the aim to identify the nature of the flux flow instabilities features (i.e., thermal or electronic), in order to give a further advance to the high current carrying capability of iron-chalcogenide superconductors.Comment: 4 pages, 3 figure

High-energy ball milling and synthesi temperature study to improve superconducting properties of MgB2 ex-situ tapes and wires

MgB2 monofilamentary nickel-sheated tapes and wires were fabricated by means of the ex-situ powder-in-tube method using either high-energy ball milled and low temperature synthesized powders. All sample were sintered at 920 C in Ar flow. The milling time and the revolution speed were tuned in order to maximize the critical current density in field (Jc): the maximum Jc value of 6 x 10e4 A/cm2 at 5 K and 4 T was obtained corresponding to the tape prepared with powders milled for 144h at 180rpm. Vorious synthesis temperature were also investigated (730-900 C) finding a best Jc value for the wire prepared with powders synthesized at 745 C. We speculate that this optimal temperature is due to the fluidifying effect of unreacted magnesium content before the sintering process which could better connect the grains

Performance of Magnetic-Superconductor Non-Contact Harmonic Drive for Cryogenic Space Applications

Harmonic drives are profusely used in aerospace mainly because of their compactness and large reduction ratio. However, their use in cryogenic environments is still a challenge. Lubrication and fatigue are non-trivial issues under these conditions. The objective of the Magnetic-Superconductor Cryogenic Non-contact Harmonic Drive (MAGDRIVE) project, funded by the EU Space FP7, is to design, build, and test a new concept of MAGDRIVE. Non-contact interactions among magnets, soft magnetic materials, and superconductors are efficiently used to provide a high reduction ratio gear that smoothly and naturally operates at cryogenic environments. The limiting elements of conventional harmonic drives (teeth, flexspline, and ball bearings) are substituted by contactless mechanical components (magnetic gear and superconducting magnetic bearings). The absence of contact between moving parts prevents wear, lubricants are no longer required, and the operational lifetime is greatly increased. This is the first mechanical reducer in mechanical engineering history without any contact between moving parts. In this paper, the test results of a −1:20 inverse reduction ratio MAGDRIVE prototype are reported. In these tests, successful operation at 40 K and 10−3 Pa was demonstrated for more than 1.5 million input cycles. A maximum torque of 3 N·m and an efficiency of 80% were demonstrated. The maximum tested input speed was 3000 rpm, six times the previous existing record for harmonic drives at cryogenic temperature

A Dirichlet process mixture model for automatic (18)F-FDG PET image segmentation: Validation study on phantoms and on lung and esophageal lesions

The aim of this study was to implement a Dirichlet process mixture (DPM) model for automatic tumor edge identification on (18)F-fluorodeoxyglucose positron emission tomography ((18)F-FDG PET) images by optimizing the parameters on which the algorithm depends, to validate it experimentally, and to test its robustness