860 research outputs found
Cooling electrons from 1 K to 400 mK with V-based nanorefrigerators
The fabrication and operation of V-based superconducting nanorefrigerators is
reported. Specifically, electrons in an Al island are cooled thanks to
hot-quasiparticle extraction provided by tunnel-coupled V electrodes.
Electronic temperature reduction down to 400 mK starting from 1 K is
demonstrated with a cooling power ~20 pW at 1 K for a junction area of 0.3
micron^2. The present architecture extends to higher temperatures refrigeration
based on tunneling between superconductors and paves the way to the
implementation of a multi-stage on-chip cooling scheme operating from above 1 K
down to the mK regime.Comment: 3+ pages, 4 color figure
Huge nonequilibrium magnetoresistance in hybrid superconducting spin valves
A hybrid ferromagnet-superconductor spin valve is proposed. Its operation
relies on the interplay between nonequilibrium transport and proximity-induced
exchange coupling in superconductors. Huge tunnel magnetoresistance values as
large as some 10^6% can be achieved in suitable ferromagnet-superconductor
combinations under proper voltage biasing. The controllable spin-filter nature
of the structure combined with its intrinsic simplicity make this setup
attractive for low-temperature spintronic applications where reduced power
dissipation is an additional requirement.Comment: 4 pages, 4 figure
Ultra-low dissipation Josephson transistor
A superconductor-normal metal-superconductor (SNS) transistor based on
superconducting microcoolers is presented. The proposed 4-terminal device
consists of a long SNS Josephson junction whose N region is in addition
symmetrically connected to superconducting reservoirs through tunnel barriers
(I). Biasing the SINIS line allows to modify the quasiparticle temperature in
the weak link, thus controlling the Josephson current. We show that, in
suitable voltage and temperature regimes, large supercurrent enhancements can
be achieved with respect to equilibrium, due to electron ``cooling'' generated
by the control voltage. The extremely low power dissipation intrinsic to the
structure makes this device relevant for a number of electronic applications.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter
Textbooks of English and Portuguese as a Foreign Language in Europe in the XIX Century
Short Research Pape
Ultra-efficient Cooling in Ferromagnet-Superconductor Microrefrigerators
A promising scheme for electron microrefrigeration based on
ferromagnet-superconductor contacts is presented. In this setup, cooling power
densities up to 600 nW/m can be achieved leading to electronic
temperature reductions largely exceeding those obtained with existing
superconductor-normal metal tunnel contacts. Half-metallic CrO/Al bilayers
are indicated as ideal candidates for the implementation of the device.Comment: 9 pages, 3 figures, submitted to Applied Physics Letter
Interplay between disorder and intersubband collective excitations in the two-dimensional electron gas
Intersubband absorption in modulation-doped quantum wells is usually
appropriately described as a collective excitation of the confined
two-dimensional electron gas. At sufficiently low electron density and low
temperatures, however, the in-plane disorder potential is able to damp the
collective modes by mixing the intersubband charge-density excitation with
single-particle localized modes. Here we show experimental evidence of this
transition. The results are analyzed within the framework of the density
functional theory and highlight the impact of the interplay between disorder
and the collective response of the two-dimensional electron gas in
semiconductor heterostructures.Comment: 5 pages, 4 figures, RevTeX. Accepted for publication in Phys. Rev. B
(Rapid. Comm.
Impact of classical forces and decoherence in multi-terminal Aharonov-Bohm networks
Multi-terminal Aharonov-Bohm (AB) rings are ideal building blocks for quantum
networks (QNs) thanks to their ability to map input states into controlled
coherent superpositions of output states. We report on experiments performed on
three-terminal GaAs/Al_(x)Ga_(1-x)As AB devices and compare our results with a
scattering-matrix model including Lorentz forces and decoherence. Our devices
were studied as a function of external magnetic field (B) and gate voltage at
temperatures down to 350 mK. The total output current from two terminals while
applying a small bias to the third lead was found to be symmetric with respect
to B with AB oscillations showing abrupt phase jumps between 0 and pi at
different values of gate voltage and at low magnetic fields, reminiscent of the
phase-rigidity constraint due to Onsager-Casimir relations. Individual outputs
show quasi-linear dependence of the oscillation phase on the external electric
field. We emphasize that a simple scattering-matrix approach can not model the
observed behavior and propose an improved description that can fully describe
the observed phenomena. Furthermore, we shall show that our model can be
successfully exploited to determine the range of experimental parameters that
guarantee a minimum oscillation visibility, given the geometry and coherence
length of a QN.Comment: 7 pages, 8 figure
Estudo dos efeitos da descontinuidade dielétrica em uma “brush” de polieletrólitos esférica : modelagem via dinâmica molecular
In this work we present a molecular dynamics simulation of a polyelectrolyte spherical brush and counterions in a salt-free medium, in which the dielectric inhomogeneity between materials is taken in consideration. Polyelectrolyte brushes have been studied experimentally broadly, having shown a range of different applications such as for bioseparation and targeted drug/gene delivery. In spite of that, formal simulations and theories explaining its behavior are not as numerous. The theory and the work we present are unfold into more details throughout the thesis in the form of multiple sections, but the results remain contained to the paper annexed1, published in 2017. We start with a brief introduction of the work and then present the paper, later on, the theory is further explored in the methodology appendix, and we finish with the final considerations for the work results and the project conclusion. The project consists of the aforementioned simulations with the main purpose of investigating the effect of the dielectric discontinuity, between the brush core and its surrounding medium, over the dynamics of the system. This is investigated through the use of the method of image charges. Properties of the polyelectrolyte brush are obtained for different parameters, including valence of the counterions, radius of the nanoparticle and the brush total charge. A mean-field theory is presented for the comparison with density profiles obtained for monovalent counterions, and we finish the paper by presenting the osmotic properties of the system.Neste trabalho apresentamos simulações em dinâmica molecular de uma “brush” de polieletrólitos esférica, cercada de contraíons, em um meio livre de sais, onde a heterogeneidade dielétrica entre os materiais é levada em consideração. Estes conjuntos de polieletrólitos tem sido estudados experimentalmente de maneira ampla, tendo mostrado uma gama de diferentes aplicações como o uso para biosseparação e como portadores de drogas/genes para transporte controlado. Entretanto, teorias e simulações formais que expliquem o seu comportamento não são tão numerosas. A teoria e o trabalho presentes são detalhados nesta dissertação na forma de múltiplas seções, mas os resultados permanecem contidos ao artigo anexado1 publicado em 2017. Começamos com uma breve introdução do trabalho e então apresentamos o artigo, posteriormente a teoria é melhor explorada no apêndice da metodologia, finalmente, terminamos com as considerações finais para com os resultados do trabalho e as conclusões do projeto. O projeto consiste das simulações anteriormente mencionadas, as quais tinham o propósito principal de investigar os efeitos da descontinuidade dielétrica, entre o núcleo da “brush” e o meio em que está envolta, sobre a dinâmica do sistema. Isso é investigado através do uso do método de cargas imagem. As propriedades da “brush” de polieletrólitos também são obtidas para diferentes parâmetros, dentre os quais, a valência dos contra íons, o raio da nanopartícula central e a carga total da “brush”. Uma teoria de campo médio é apresentada para comparação com os perfis de densidade obtidos para os contra íons monovalentes, e nós terminamos o artigo apresentando as propriedades osmóticas do sistema
Hybrid InAs nanowire-vanadium proximity SQUID
We report the fabrication and characterization of superconducting quantum
interference devices (SQUIDs) based on InAs nanowires and vanadium
superconducting electrodes. These mesoscopic devices are found to be extremely
robust against thermal cycling and to operate up to temperatures of ~K
with reduced power dissipation. We show that our geometry allows to obtain
nearly-symmetric devices with very large magnetic-field modulation of the
critical current. All these properties make these devices attractive for
on-chip quantum-circuit implementation.Comment: 3 pages, 3 figure
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