4,504 research outputs found
A Microwave Josephson Refrigerator
We present a microwave quantum refrigeration principle based on the Josephson
effect. When a superconducting quantum interference device (SQUID) is pierced
by a time-dependent magnetic flux, it induces changes in the macroscopic
quantum phase and an effective finite bias voltage appears across the SQUID.
This voltage can be used to actively cool well below the lattice temperature
one of the superconducting electrodes forming the interferometer. The
achievable cooling performance combined with the simplicity and scalability
intrinsic to the structure pave the way to a number of applications in quantum
technology.Comment: 6 pages, 3 figure
Radiation comb generation with extended Josephson junctions
We propose the implementation of a Josephson radiation comb generator (JRCG)
based on an extended Josephson junction subject to a time dependent magnetic
field. The junction critical current shows known diffraction patterns and
determines the position of the critical nodes when it vanishes. When the
magnetic flux passes through one of such critical nodes, the superconducting
phase must undergo a -jump to minimize the Josephson energy.
Correspondingly a voltage pulse is generated at the extremes of the junction.
Under periodic driving this allows us to produce a comb-like voltage pulses
sequence. In the frequency domain it is possible to generate up to hundreds of
harmonics of the fundamental driving frequency, thus mimicking the frequency
comb used in optics and metrology. We discuss several implementations through a
rectangular, cylindrical and annular junction geometries, allowing us to
generate different radiation spectra and to produce an output power up to
~pW at ~GHz for a driving frequency of ~MHz.Comment: 4+ pages, 4 color figure
Parasitic effects in SQUID-based radiation comb generators
We study several parasitic effects on the implementation of a Josephson
radiation comb generator (JRCG) based on a dc superconducting quantum
interference device (SQUID) driven by an external magnetic field. This system
can be used as a radiation generator similarly to what is done in optics and
metrology, and allows one to generate up to several hundreds of harmonics of
the driving frequency. First we take into account how assuming a finite loop
geometrical inductance and junction capacitance in each SQUID may alter the
operation of this device. Then, we estimate the effect of imperfections in the
fabrication of an array of SQUIDs, which is an unavoidable source of errors in
practical situations. We show that the role of the junction capacitance is in
general negligible, whereas the geometrical inductance has a beneficial effect
on the performance of the device. The errors on the areas and junction
resistance asymmetries may deteriorate the performance, but their effect can be
limited up to a large extent with a suitable choice of fabrication parameters.Comment: 9 pages, 9 figure
Gate-modulated thermopower in disordered nanowires: I. Low temperature coherent regime
Using a one-dimensional tight-binding Anderson model, we study a disordered
nanowire in the presence of an external gate which can be used for depleting
its carrier density (field effect transistor device configuration). In this
first paper, we consider the low temperature coherent regime where the electron
transmission through the nanowire remains elastic. In the limit where the
nanowire length exceeds the electron localization length, we derive three
analytical expressions for the typical value of the thermopower as a function
of the gate potential, in the cases where the electron transport takes place
(i) inside the impurity band of the nanowire, (ii) around its band edges and
eventually (iii) outside its band. We obtain a very large enhancement of the
typical thermopower at the band edges, while the sample to sample fluctuations
around the typical value exhibit a sharp crossover from a Lorentzian
distribution inside the impurity band towards a Gaussian distribution as the
band edges are approached.Comment: 13 pages, 8 figures, final version as publishe
Photonic heat conduction in Josephson-coupled Bardeen-Cooper-Schrieffer superconductors
We investigate the photon-mediated heat flow between two Josephson-coupled
Bardeen-Cooper-Schrieffer (BCS) superconductors. We demonstrate that in
standard low temperature experiments involving temperature-biased
superconducting quantum interference devices (SQUIDs), this radiative
contribution is negligible if compared to the direct galvanic one, but it
largely exceeds the heat exchanged between electrons and the lattice phonons.
The corresponding thermal conductance is found to be several orders of
magnitude smaller, for real experiments setup parameters, than the universal
quantum of thermal conductance, kappa_0(T)=pi k_B^2T/6hbar.Comment: 8 pages, 6 figure
Absorbing/Emitting Phonons with one dimensional MOSFETs
We consider nanowires in the field effect transistor device configuration.
Modeling each nanowire as a one dimensional lattice with random site
potentials, we study the heat exchanges between the nanowire electrons and the
substrate phonons, when electron transport is due to phonon-assisted hops
between localized states. Shifting the nanowire conduction band with a metallic
gate induces different behaviors. When the Fermi potential is located near the
band center, a bias voltage gives rise to small local heat exchanges which
fluctuate randomly along the nanowire. When it is located near one of the band
edges, the bias voltage yields heat currents which flow mainly from the
substrate towards the nanowire near one boundary of the nanowire, and in the
opposite direction near the other boundary. This opens interesting perspectives
for heat management at submicron scales: Arrays of parallel gated nanowires
could be used for a field control of phonon emission/absorption.Comment: 9 pages, 11 figure
Considérations didactiques autour du répertoire (pluri) linguistique de l’apprenant : du CARAP à la classe de langue
La prise en compte des caractéristiques d'une société de plus en plus multilingue et, par conséquent, du patrimoine (pluri)linguistique et pluriculturel des individus est désormais indispensable, surtout dans le système éducatif et notamment dans la formation de tous les enseignants, ceux qui enseignent les langues et ceux qui enseignent en langue (maternelle, seconde ou étrangère). Cet article propose quelques réflexions pour la mise en place d'une didactique à visées plurilingue et interculturelle à partir des recommandations nationales et européennes les plus récentes et tout spécialement des indications du Cadre de référence pour les approches plurielles (CARAP)
vocoids and their prosodic distribution, with special reference to Italian and Arabic
This study attempts to characterize vocoids, i.e. vowels and semivowels, as a unified class of segments. In order to do so, it investigates the main phenomena concerning the quantitative distribution of these sounds, namely syllabic alternation, length alternations, deletion and insertion. Such phenomena are best analyzed by making reference to prosodic structure, and syllable structure in particular. Therefore, both frameworks adopted in this thesis take into consideration this type of representation. The main approach, which I refer to generally as Derivational Theory (DT), is based on the notion that surface phonetic forms are derived from underlying forms through a series of structural changes taking place at different levels of representation. This model is contrasted with the recently introduced (Prince and Smolensky 1993) Optimality Theory (OT), an output-oriented paradigm based on the parallel evaluation of candidate forms by means of universal but violable constraints. This thesis shows that OT offers some valuable insights into the phenomena under analysis, although there are areas in which it requires integration with derivational tools. This study also makes specific reference to two languages: Ammani Arabic and Standard Italian. These diverge in their treatment of vocoids, but clear general trends may be detected which have also been found in other languages
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