Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain

High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by Pulsed Laser Ablation on different substrates. By varying the film thickness, Tc up to 21K were observed, significantly larger than the bulk value. Structural analyses indicated that the a axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe-(Se,Te) bond length and angle, suggesting the possibility of further enhancement

Initial Penetration of an Elastic Axially Symmetric Indenter into a Rigid-Perfectly-Plastic Half-Space

This paper is concerned with the axially symmetric plastic flow of a rigid perfectly-plastic nonhardening halfspace. The initial penetration of the elastic indenter is studied based on Haar and von Karman hypothesis. The analytical distribution of contact stress and the approximate penetration depth of the indenter are obtained

Temperature- and doping-dependent nanoscale Schottky barrier height at the Au/Nb:SrTiO3interface

We use ballistic electron emission microscopy to investigate prototypical Au/Nb-doped SrTiO3(NSTO) Schottky barrier diodes for different temperatures and doping levels. To this end, ultrathin Au overlayers are thermally evaporated onto TiO2-terminated NSTO single crystal substrates. We show that at room temperature, regardless of the nominal doping, rectification is controlled by a spatially inhomogeneous Schottky barrier height (SBH), which varies on a length scale of tens of nanometers according to a Gaussian distribution with a mean value of 1.29-1.34 eV and the standard deviation in the range of 80-100 meV. At lower temperatures, however, doping effects become relevant. In particular, junctions with a low Nb content of 0.01 and 0.05 wt. % show an 3c300 meV decrease in the mean SBH from room temperature to 80 K, which can be explained by an electrostatic analysis assuming a temperature-dependent dielectric permittivity for NSTO. In contrast, this model fails to predict the weaker temperature dependence of SBH for junctions based on 0.5 wt. % NSTO. Our nanoscale investigation demands to reassess conventional models for the NSTO polarizability in high-intensity electric fields. Furthermore, it contributes to the comprehension and prediction of transport in metal/SrTiO3junctions and devices

Characterization of Magnetic Steels for the FCC-ee Magnet Prototypes

At the European Organization for the Nuclear Research (CERN), several efforts were combined for a preliminary design of a new accelerator, the Future Circular Collider (FCC), a 100-TeV double-ring hadron collider to be installed in a 100-km tunnel. As potential intermediate step, a high-luminosity lepton collider called FCC-ee is foreseen with more than 9,000 magnets. This paper provides an insight into the magnetic properties of the steels, potentially considered for the new dipole magnets, with nominal field of only 56 mT. The influence of the properties of these steels on the magnet transfer function has been assessed analytically using an equivalent reluctance network to model the first 1-m long dipole prototypes. The analytical results were validated experimentally. The proposed approach can be a useful tool for traceability and quality control during the series production

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

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.Comment: 10 pages including 4 figure

Field quality of the LHC dipole magnets in operating conditions

We report here the main results of the field measurements performed so far on the pre-series LHC superconducting dipoles at superfluid helium temperature. After discussing the results at injection and collision conditions, we focus on the non-linear contributions at high field, on the contribution of superconductor magnetization at injection, and on ramp rate effects. The statistics accumulated on the first magnets of the production verify the hypotheses that have been used to design the correctors scheme for the LHC. In particular high field saturation is in line with the expectations, although a small systematic deformation due to Lorentz forces affects both sextupole and decapole terms. The decay at injection and snap-back at beginning of beam acceleration require careful characterization

Checking the Polarity of Superconducting Multipole LHC Magnets

This paper describes the design and operation of the âﾜPolarity Checkerâ, a scanning probe designed to check multipole field order, type and polarity of superconducting LHC magnets. First we introduce the measurement method, based on the harmonic analysis of the radial field component picked up by a rotating Hall sensor at different current levels. Then we describe the hardware and the software of the system, which features automatic powering, data acquisition and treatment, discussing the achieved sensitivity and performance. Finally we provide a summary of the test results on the first 505 cryoassemblies, showing how the system was usefully employed to detect some potentially harmful connection errors

Dissipation of vibration in rough contact

The relationship which links the normal vibration occurring during the sliding of rough surfaces and the nominal contact area is investigated. Two regimes are found. In the first one, the vibrational level does not depend on the contact area, while in the second one, it is propor- tional to the contact area. A theoretical model is proposed. It is based on the assumption that the vibrational level results from a competition between two processes of vibration damping, the internal damping of the material and the contact damping occurring at the interface