1,346 research outputs found
Electrostatic tailoring of magnetic interference in quantum point contact ballistic Josephson junctions
The magneto-electrostatic tailoring of the supercurrent in quantum point
contact ballistic Josephson junctions is demonstrated. An etched InAs-based
heterostructure is laterally contacted to superconducting niobium leads and the
existence of two etched side gates permits, in combination with the application
of a perpendicular magnetic field, to modify continuously the magnetic
interference pattern by depleting the weak link. For wider junctions the
supercurrent presents a Fraunhofer-like interference pattern with periodicity
h/2e whereas by shrinking electrostatically the weak link, the periodicity
evolves continuously to a monotonic decay. These devices represent novel
tunable structures that might lead to the study of the elusive Majorana
fermions.Comment: 4.5 pages, 4 color figure
Energy Efficiency Measures for an Electrical Material Industry
The main goal of the present paper is to present the study of energy efficiency measures for an electrical material industry. The high-energy consumption of this kind of industry lead companies to search for solutions that allow increasing the energy efficiency in their installations and in the processes, promoting the reduction of the energy consumption and costs. In this context, the objectives of this study emerged, which resulted from the main needs identified by the company. Therefore, the work was divided into four parts. The first one, was the study of replacing the existing lighting (mainly mercury vapor lamps by led); another study conducted was the replacement of the electric motors in injection machines by new and more efficient ones; the third one was the installation of a photovoltaic solar system (for self-consumption) and, finally, the project of a new power converter station for the company. The energy efficiency studies carried out are based on: a careful analysis of the data provided by the company; all the measurements done; the consumption profiles that have been drawn; an extensive market research (with the purpose of finding the most efficient solutions for each case); and in already existing and proven calculation methodologies, leading, in this way, to a greater reliability of the obtained results. These studies showed to which extent the implementation of the various measures presented are economically viable, their impact on the reduction of energy consumption and the annual savings achieved.This work was supported by Coimbra Institute of Engineering – Polytechnic Institute of Coimbra and by projects: UID/MULTI/00308/2019 (supported by the Portuguese ‘Fundação para a Ciência e a Tecnologia’: FCT – Foundation for Science and Technology) and by the European Regional Development Fund through the COMPETE 2020 Programme, FCT Portuguese Foundation for Science and Technology within project T4ENERTEC (POCI-01- 0145-FEDER-029820)
Circularly Polarized Resonant Rayleigh Scattering and Skyrmions in the = 1 Quantum Hall Ferromagnet
We use the circularly polarized resonant Rayleigh scattering (RRS) to study
the quantum Hall ferromagnet at = 1. At this filling factor we observe a
right handed copolarized RRS which probes the Skyrmion spin texture of the
electrons in the photoexcited grounds state. The resonant scattering is not
present in the left handed copolarization, and this can be related to the
correlation between Skymionic effects, screening and spin wave excitations.
These results evidence that RRS is a valid method for the study of the spin
texture of the quantum Hall states
Plateau insulator transition in graphene
The quantum Hall effect in a single-layer graphene sample is studied in
strong magnetic fields up to 28 T. Our measurements reveal the existence of a
metal- insulator transition from filling factor to . The value
of the universal scaling exponent is found to be in graphene and
therefore in a truly two-dimensional system. This value of is in
agreement with the accepted universal value for the plateau-insulator
transitions in standard quasi two-dimensional electron and hole gases.Comment: 10 pages, 5 figure
A study of local approximation for polarization potentials
We discuss the derivation of an equivalent \textit{l}-independent
polarization potential for use in the optical Schr\"{o}dinger equation that
describes the elastic scattering of heavy ions. Three diffferent methods are
used for this purpose. Application of our theory to the low energy scattering
of the halo nucleus Li from a C target is made. It is found that
the notion of \textit{l}-independent polarization potential has some validity
but can not be a good substitute for the \textit{l}-dependent local equivalent
Feshbach polarization potential.Comment: 8 pages, 4 figure
High-cadence spectroscopy of M-dwarfs – II. Searching for stellar pulsations with HARPS
Stellar oscillations appear all across the Hertzsprung–Russell diagram. Recent theoretical studies support their existence also in the atmosphere of M dwarfs. These studies predict for them short periodicities ranging from 20 min to 3 h. Our Cool Tiny Beats (CTB) programme aims at finding these oscillations for the very first time. With this goal, CTB explores the short time domain of M dwarfs using radial velocity data from the High Accuracy Radial velocity Planet Searcher (HARPS)-European Southern Observatory and HARPS-N high-precision spectrographs. Here we present the results for the two most long-term stable targets observed to date with CTB, GJ 588 and GJ 699 (i.e. Barnard's star). In the first part of this work we detail the correction of several instrumental effects. These corrections are especially relevant when searching for subnight signals. Results show no significant signals in the range where M dwarfs pulsations were predicted. However, we estimate that stellar pulsations with amplitudes larger than ∼0.5 m s−1 can be detected with a 90 per cent completeness with our observations. This result, along with the excess of power regions detected in the periodograms, opens the possibility of non-resolved very low amplitude pulsation signals. Next generation more precise instrumentation would be required to detect such oscillations. However, the possibility of detecting pulsating M-dwarf stars with larger amplitudes is feasible due to the short size of the analysed sample. This motivates the need for completeness of the CTB survey
Raising awareness of the accessibility challenges in mathematics MOOCs
MOOCs provide learning environments that make it easier for learners to study from anywhere, at their own pace and with open access to content. This has revolutionised the field of eLearning, but accessibility continues to be a problem, even more so if we include the complexity of the STEM disciplines which have their own specific characteristics. This work presents an analysis of the accessibility of several MOOC platforms which provide courses in mathematics. We attempt to visualise the main web accessibility problems and challenges that disabled learners could face in taking these types of courses, both in general and specifically in the context of the subject of mathematics
Pb/InAs nanowire Josephson junction with high critical current and magnetic flux focusing
We have studied mesoscopic Josephson junctions formed by highly -doped
InAs nanowires and superconducting Ti/Pb source and drain leads. The
current-voltage properties of the system are investigated by varying
temperature and external out-of-plane magnetic field. Superconductivity in the
Pb electrodes persists up to K and with magnetic field values up to
0.4 T. Josephson coupling at zero backgate voltage is observed up to 4.5 K and
the critical current is measured to be as high as 615 nA. The supercurrent
suppression as a function of the magnetic field reveals a diffraction pattern
that is explained by a strong magnetic flux focusing provided by the
superconducting electrodes forming the junction.Comment: 6 pages, 5 color figure
Multi-Channel Inverse Scattering Problem on the Line: Thresholds and Bound States
We consider the multi-channel inverse scattering problem in one-dimension in
the presence of thresholds and bound states for a potential of finite support.
Utilizing the Levin representation, we derive the general Marchenko integral
equation for N-coupled channels and show that, unlike to the case of the radial
inverse scattering problem, the information on the bound state energies and
asymptotic normalization constants can be inferred from the reflection
coefficient matrix alone. Thus, given this matrix, the Marchenko inverse
scattering procedure can provide us with a unique multi-channel potential. The
relationship to supersymmetric partner potentials as well as possible
applications are discussed. The integral equation has been implemented
numerically and applied to several schematic examples showing the
characteristic features of multi-channel systems. A possible application of the
formalism to technological problems is briefly discussed.Comment: 19 pages, 5 figure
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