414 research outputs found
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
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
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
Tailoring Josephson coupling through superconductivity-induced nonequilibrium
The distinctive quasiparticle distribution existing under nonequilibrium in a
superconductor-insulator-normal metal-insulator-superconductor (SINIS)
mesoscopic line is proposed as a novel tool to control the supercurrent
intensity in a long Josephson weak link. We present a description of this
system in the framework of the diffusive-limit quasiclassical Green-function
theory and take into account the effects of inelastic scattering with arbitrary
strength. Supercurrent enhancement and suppression, including a marked
transition to a -junction are striking features leading to a fully tunable
structure.Comment: 4 pages, 4 figure
Mesoscopic supercurrent transistor controlled by nonequilibrium cooling
The distinctive quasiparticle distribution existing under nonequilibrium in a
superconductor-insulator-normal metal-insulator-superconductor (SINIS)
mesoscopic line is proposed as a novel tool to control the supercurrent
intensity in a long Josephson weak link. We present a description of this
system in the framework of the diffusive-limit quasiclassical Green-function
theory and take into account the effects of inelastic scattering with arbitrary
strength. Supercurrent enhancement and suppression, including a marked
transition to a -junction are striking features leading to a fully tunable
structure. The role of the degree of nonequilibrium, temperature, and materials
choice as well as features like noise, switching time, and current and power
gain are also addressed.Comment: 8 pages, 9 figures, submitted to Journal of Low Temperature Physic
Coherent transport in Nb/delta-doped-GaAs hybrid microstructures
Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is
presented. The structures fabrication procedure is based on delta-doped layers
grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap
layer to protect the active semiconductor layers during ex situ transfer. The
superconductor is then sputter deposited in situ after thermal desorption of
the protective layer. Two types of structures in particular will be discussed,
i.e., a reference junction and the engineered one that contains an additional
insulating AlGaAs barrier inserted during the growth in the semiconductor. This
latter configuration may give rise to controlled interference effects and
realizes the model introduced by de Gennes and Saint-James in 1963. While both
structures show reflectionless tunneling-dominated transport, only the
engineered junction shows additionally a low-temperature single marked
resonance peaks superimposed to the characteristic Andreev-dominated subgap
conductance. The analysis of coherent magnetotransport in both microstructures
is successfully performed within the random matrix theory of Andreev transport
and ballistic effects are included by directly solving the Bogoliubov-de Gennes
equations. The impact of junction morphology on reflectionless tunneling and
the application of the employed fabrication technique to the realization of
complex semiconductor-superconductor systems are furthermore discussed.Comment: 9 pages, 8 figures, invited review paper, to be published in Mod.
Phys. Lett.
Singlet-triplet transition in a few-electron lateral InGaAs-InAlAs quantum dot
The magnetic-field evolution of Coulomb blockade peaks in lateral
InGaAs/InAlAs quantum dots in the few-electron regime is reported. Quantum dots
are defined by gates evaporated onto a 60 nm-thick hydrogen silsesquioxane
insulating film. A gyromagnetic factor of 4.4 is measured via zero-bias spin
spectroscopy and a transition from singlet to triplet spin configuration is
found at an in-plane magnetic field B = 0.7 T. This observation opens the way
to the manipulation of singlet and triplet states at moderate fields and its
relevance for quantum information applications will be discussed.Comment: 4 pages, 3 figure
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
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
Resonant Transport in Nb/GaAs/AlGaAs/GaAs Microstructures
Resonant transport in a hybrid semiconductor-superconductor microstructure
grown by MBE on GaAs is presented. This structure experimentally realizes the
prototype system originally proposed by de Gennes and Saint-James in 1963 in
\emph{all}-metal structures. A low temperature single peak superimposed to the
characteristic Andreev-dominated subgap conductance represents the mark of such
resonant behavior. Random matrix theory of quantum transport was employed in
order to analyze the observed magnetotransport properties and ballistic effects
were included by directly solving the Bogoliubov-de Gennes equations.Comment: 7 pages REVTeX, 4 figures, to be published by World Scientific in
Proceedings of International Symposium on Mesoscopic Superconductivity and
Spintronics (NTT R&D Center Atsugi, Japan, March 2002
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