1,060 research outputs found
Josephson effect in MgB_2 break junctions
We present the first observation of the DC and AC Josephson effect in MgB_2
break junctions. The junctions, obtained at 4.2 K in high-quality, high-density
polycrystalline metallic MgB_2 samples, show a non-hysteretic DC Josephson
effect. By irradiating the junctions with microwaves we observe clear Shapiro
steps spaced by the ideal value. The temperature dependence of the
DC Josephson current and the dependence of the height of the steps on the
microwave power are obtained. These results are a direct prove for the
existence of pairs with charge 2e in this new metallic superconductor and give
evidence of the superconductor-normal metal-superconductor weak link character
of these junctions.Comment: 4 RevTEX pages, 4 eps figure
A Meta-Heuristic Optimization Procedure for the Identification of the Nonlinear Model Parameters of Hydraulic Dampers Based on Experimental Dataset of Real Working Conditions
Hydralic dampers are widely implemented in railway vehicle suspension stages, especially in high-speed passenger trains. They are designed to be mounted in different positions to improve comfort, stability, and safety performances. Numerical simulations are often used to assist the design and optimization of these components. Unfortunately, hydraulic dampers are highly nonlinear due to the complex fluid dynamic phenomena taking place inside the chambers and through the by-pass orifices. This requires accurate damper models to be developed to estimate the influence of the nonlinearities of such components during the dynamic performances of the whole vehicle. This work aims at presenting a new parametric damper model based on a nonlinear lumped element approach. Moreover, a new model tuning procedure will be introduced. Differently from the typical sinusoidal characterization cycles, this routine is based on experimental tests of real working conditions. The set of optimal model parameters will be found through a metaheuristic iterative approach able to minimize the differences between numerical and experimental damper forces. The performances of the optimal model will be compared with the ones of the most common Maxwell model generally implemented in railway multibody software programs
Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements
In a type-II superconductor the generation of higher harmonics in the
magnetic response to an alternating magnetic field is a consequence of the
non-linearity in the I-V relationship. The shape of the current-voltage (I-V)
curve is determined by the current dependence of the thermal activation energy
U(J) and is thus related to the dynamical regimes governing the vortex motion.
In order to investigate the vortex dynamics in MgB2 bulk superconductors we
have studied the fundamental (chi1) and third (chi3) harmonics of the ac
magnetic susceptibility. Measurements have been performed as a function of the
temperature and the dc magnetic field, up to 9 T, for various frequencies and
amplitudes of the ac field. We show that the analysis of the behaviour in
frequency of chi3(T) and chi3(B) curves can provide clear information about the
non-linearity in different regions of the I-V characteristic. By comparing the
experimental curves with numerical simulations of the non-linear diffusion
equation for the magnetic field we are able to resolve the crossover between a
dissipative regime governed by flux creep and one dominated by flux flow
phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova
Science Publishers, Inc., NY, 2003
Editorial: Critical Perspectives on Replicability in Work/Organizational Psychology Research
Organization studies is emerging as an applied and performative science (Alvesson, 2020), aimed
at enhancing academic-practitioner collaboration, and going beyond narrowly circumscribed areas
of study to pursue ground-breaking research. Much of the research in this field is context-specific
and therefore requires a shift in the mode of knowledge production
Dust remobilization in fusion plasmas under steady state conditions
The first combined experimental and theoretical studies of dust
remobilization by plasma forces are reported. The main theoretical aspects of
remobilization in fusion devices under steady state conditions are analyzed. In
particular, the dominant role of adhesive forces is highlighted and generic
remobilization conditions - direct lift-up, sliding, rolling - are formulated.
A novel experimental technique is proposed, based on controlled adhesion of
dust grains on tungsten samples combined with detailed mapping of the dust
deposition profile prior and post plasma exposure. Proof-of-principle
experiments in the TEXTOR tokamak and the EXTRAP-T2R reversed-field pinch are
presented. The versatile environment of the linear device Pilot-PSI allowed for
experiments with different magnetic field topologies and varying plasma
conditions that were complemented with camera observations.Comment: 16 pages, 11 figures, 3 table
Temperature and junction-type dependency of Andreev reflection in MgB2
We studied the voltage and temperature dependency of the dynamic conductance
of normal metal-MgB2 junctions obtained either with the point-contact technique
(with Au and Pt tips) or by making Ag-paint spots on the surface of
high-quality single-crystal-like MgB2 samples. The fit of the conductance
curves with the generalized BTK model gives evidence of pure s-wave gap
symmetry. The temperature dependency of the gap, measured in Ag-paint junctions
(dirty limit), follows the standard BCS curve with 2Delta/kTc = 3.3. In
out-of-plane, high-pressure point contacts we obtained almost ideal Andreev
reflection characteristics showing a single small s-wave gap Delta = 2.6 +/-
0.2 (clean limit). These results support the two-gap model of
superconductivity, the presence of a modified layer at the surface of the
crystals and an important and non-conventional role of the impurities in MgB2.Comment: 5 pages, 4 eps figures, SNS 2001 conferenc
Blue straggler stars in dwarf spheroidal galaxies
Blue straggler star (BSS) candidates have been observed in all old dwarf spheroidal galaxies (dSphs), however whether or not they are authentic BSSs or young stars has been a point of debate. To both address this issue and obtain a better understanding of the formation of BSSs in different environments, we have analysed a sample of BSS candidates in two nearby Galactic dSphs, Draco and Ursa Minor. We have determined their radial and luminosity distributions from wide field multicolour imaging data extending beyond the tidal radii of both galaxies. BSS candidates are uniformly distributed through the host galaxy, whereas a young population is expected to show a more clumpy distribution. Furthermore, the observed radial distribution of BSSs, normalized to both red giant branch (RGB) and horizontal branch (HB) stars, is almost flat, with a slight decrease towards the centre. Such a distribution is at odds with the predictions for a young stellar population, which should be more concentrated. Instead, it is consistent with model predictions for BSS formation by mass transfer in binaries (MT-BSSs). Such results, although not decisive, suggest that these candidates are indeed BSSs and that MT-BSSs form in the same way in Draco and Ursa Minor as in globular clusters. This favours the conclusion that Draco and Ursa Minor are truly ‘fossil' galaxies, where star formation ceased completely more than 8 billion years ag
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