1,753 research outputs found
Transport and magnetization dynamics in a superconductor/single-molecule magnet/superconductor junction
We study dc-transport and magnetization dynamics in a junction of arbitrary
transparency consisting of two spin-singlet superconducting leads connected via
a single classical spin precessing at the frequency . The presence of
the spin in the junction provides different transmission amplitudes for spin-up
and spin-down quasiparticles as well as a time-dependent spin-flip transmission
term. For a phase biased junction, we show that a steady-state superconducting
charge current flows through the junction and that an out-of-equilibrium
circularly polarized spin current, of frequency , is emitted in the
leads. Detailed understanding of the charge and spin currents is obtained in
the entire parameter range. In the adiabatic regime,
where is the superconducting gap, and for high transparencies of the
junction, a strong suppression of the current takes place around \vp \approx
0 due to an abrupt change in the occupation of the Andreev bound-states. At
higher values of the phase and/or precession frequency, extended
(quasi-particle like) states compete with the bound-states in order to carry
the current. Well below the superconducting transition, these results are shown
to be weakly affected by the back-action of the spin current on the dynamics of
the precessing spin. Indeed, we show that the Gilbert damping due to the
quasi-particle spin current is strongly suppressed at low-temperatures, which
goes along with a shift of the precession frequency due to the condensate. The
results obtained may be of interest for on-going experiments in the field of
molecular spintronics.Comment: 19 pages, 13 figures (v3) Minor modifications per referee's comments.
No change in results. (v2) 2 authors added, 1 reference added (Ref. 25), no
change in the text and result
Meta-analytic Findings on Grouping Programs
Meta-analytic reviews have focused on five distinct instructional programs that separate students by ability: multilevel dasses, cross-grade programs, within-class grouping, enriched classes for the gifted and talented, and accelerated classes. The reviews show that effects are a function of program type. Multilevel classes, which entail only minor adjustment of course content for ability groups, usually have little or no effect on student achievement. Programs that entail more substantial adjustment of curriculum to ability, such as cross-grade and within-class programs, produce clear positive effects. Programs of enrichment and acceleration, which usually involve the greatest amount of curricular adjustment, have the largest effects on student learning. These results doe not support recent claims that no one benefits from grouping or that students in the lower groups are harmed academically and emotionally by grouping.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67315/2/10.1177_001698629203600204.pd
Josephson current in strongly correlated double quantum dots
We study the transport properties of a serial double quantum dot (DQD)
coupled to two superconducting leads, focusing on the Josephson current through
the DQD and the associated 0- transitions which result from the subtle
interplay between the superconductivity, the Kondo physics, and the inter-dot
superexchange interaction. We examine the competition between the
superconductivity and the Kondo physics by tuning the relative strength
of the superconducting gap and the Kondo temperature
, for different strengths of the superexchange coupling determined by the
interdot tunneling relative to the dot level broadening . We find
strong renormalization of , a significant role of the superexchange coupling
, and a rich phase diagram of the 0 and -junction regimes. In
particular, when both the superconductivity and the exchange interaction are in
close competion with the Kondo physics (), there appears
an island of -phase at large values of the superconducting phase
difference.Comment: 4 pages, 4 figure
Rate equations for Coulomb blockade with ferromagnetic leads
We present a density-matrix rate-equation approach to sequential tunneling
through a metal particle weakly coupled to ferromagnetic leads. The
density-matrix description is able to deal with correlations between degenerate
many-electron states that the standard rate equation formalism in terms of
occupation probabilities cannot describe. Our formalism is valid for an
arbitrary number of electrons on the dot, for an arbitrary angle between the
polarization directions of the leads, and with or without spin-orbit scattering
on the metal particle. Interestingly, we find that the density-matrix
description may be necessary even for metal particles with unpolarized leads if
three or more single-electron levels contribute to the transport current and
electron-electron interactions in the metal particle are described by the
`universal interaction Hamiltonian'.Comment: 10 pages, 4 figures, REVTeX
Non-equilibrium effects in a Josephson junction coupled to a precessing spin
We present a theoretical study of a Josephson junction consisting of two
s-wave superconducting leads coupled over a classical spin. When an external
magnetic field is applied, the classical spin will precess with the Larmor
frequency. This magnetically active interface results in a time-dependent
boundary condition with different tunneling amplitudes for spin-up and
spin-down quasiparticles and where the precession produces spin-flip scattering
processes. We show that as a result, the Andreev states develop sidebands and a
non-equilibrium population which depend on the precession frequency and the
angle between the classical spin and the external magnetic field. The Andreev
states lead to a steady-state Josephson current whose current-phase relation
could be used for characterizing the precessing spin. In addition to the charge
transport, a magnetization current is also generated.This spin current is
time-dependent and its polarization axis rotates with the same precession
frequency as the classical spin.Comment: 20 pages, 26 figure
Proximity DC squids in the long junction limit
We report the design and measurement of
Superconducting/normal/superconducting (SNS) proximity DC squids in the long
junction limit, i.e. superconducting loops interrupted by two normal metal
wires roughly a micrometer long. Thanks to the clean interface between the
metals, at low temperature a large supercurrent flows through the device. The
dc squid-like geometry leads to an almost complete periodic modulation of the
critical current through the device by a magnetic flux, with a flux periodicity
of a flux quantum h/2e through the SNS loop. In addition, we examine the entire
field dependence, notably the low and high field dependence of the maximum
switching current. In contrast with the well-known Fraunhoffer-type
oscillations typical of short wide junctions, we find a monotonous gaussian
extinction of the critical current at high field. As shown in [15], this
monotonous dependence is typical of long and narrow diffusive junctions. We
also find in some cases a puzzling reentrance at low field. In contrast, the
temperature dependence of the critical current is well described by the
proximity effect theory, as found by Dubos {\it et al.} [16] on SNS wires in
the long junction limit. The switching current distributions and hysteretic IV
curves also suggest interesting dynamics of long SNS junctions with an
important role played by the diffusion time across the junction.Comment: 12 pages, 16 figure
Aharonov-Bohm differential conductance modulation in defective metallic single-wall carbon nanotubes
Using a perturbative approach, the effects of the energy gap induced by the
Aharonov-Bohm (AB) flux on the transport properties of defective metallic
single-walled carbon nanotubes (MSWCNTs) are investigated. The electronic waves
scattered back and forth by a pair of impurities give rise to Fabry-Perot
oscillations which constitutes a coherent backscattering interference pattern
(CBSIP). It is shown that, the CBSIP is aperiodically modulated by applying a
magnetic field parallel to the nanotube axis. In fact, the AB-flux brings this
CBSIP under control by an additional phase shift. As a consequence, the extrema
as well as zeros of the CBSIP are located at the irrational fractions of the
quantity , where is the flux piercing the
nanotube cross section and is the magnetic quantum flux. Indeed,
the spacing between two adjacent extrema in the magneto-differential
conductance (MDC) profile is decreased with increasing the magnetic field. The
faster and higher and slower and shorter variations is then obtained by
metallic zigzag and armchair nanotubes, respectively. Such results propose that
defective metallic nanotubes could be used as magneto-conductance switching
devices based on the AB effect.Comment: 11 pages, 4 figure
Current-flux characteristics in mesoscopic nonsuperconducting rings
We propose four different mechanisms responsible for paramagnetic or
diamagnetic persistent currents in normal metal rings and determine the
circumstances for change of the current from paramagnetic to diamagnetic ones
and {\it vice versa}. It might qualitatively reproduce the experimental results
of Bluhm et al. (Phys. Rev. Lett. 102, 136802 (2009)).Comment: 8 pages, 1 figur
Spontaneous supercurrent induced by ferromagnetic pi-junctions
We present magnetization measurements of mesoscopic superconducting niobium
loops containing a ferromagnetic (PdNi) pi-junction. The loops are prepared on
top of the active area of a micro Hall-sensor based on high mobility
GaAs/AlGaAs heterostructures. We observe asymmetric switching of the loop
between different magnetization states when reversing the sweep direction of
the magnetic field. This provides evidence for a spontaneous current induced by
the intrinsic phase shift of the pi-junction. In addition, the presence of the
spontaneous current near zero applied field is directly revealed by an increase
of the magnetic moment with decreasing temperature, which results in half
integer flux quantization in the loop at low temperatures.Comment: 4 pages, 4 figure
Josephson current through a Kondo molecule
We investigate transport of Cooper pairs through a double quantum dot (DQD)
in the Kondo regime and coupled to superconducting leads. Within the
non-perturbative slave boson mean-field theory we evaluate the Josephson
current for two different configurations, the DQD coupled in parallel and in
series to the leads. We find striking differences between these configurations
in the supercurrent as a function of the ratio t/\Gamma, where t is the
interdot coupling and \Gamma is the coupling to the leads: the critical current
I_c decreases monotonously with t/\Gamma for the parallel configuration whereas
I_c exhibits a maximum at t/\Gamma=1 in the serial case. These results
demonstrate that a variation of the ratio t/\Gamma enables to control the flow
of supercurrent through the Kondo resonance of the DQD.Comment: 5 pages, 4 figure
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