2,166 research outputs found
Interaction of a Nanomagnet with a Weak Superconducting Link
We study electromagnetic interaction of a nanomagnet with a weak
superconducting link. Equations that govern coupled dynamics of the two systems
are derived and investigated numerically. We show that the presence of a small
magnet in the proximity of a weak link may be detected through Shapiro-like
steps caused by the precession of the magnetic moment. Despite very weak
magnetic field generated by the weak link, a time-dependent bias voltage
applied to the link can initiate a non-linear dynamics of the nanomagnet that
leads to the reversal of its magnetic moment. We also consider quantum problem
in which a nanomagnet interacting with a weak link is treated as a two-state
spin system due to quantum tunneling between spin-up and spin-down states.Comment: 7 pages, 4 figure
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
Single-dopant resonance in a single-electron transistor
Single dopants in semiconductor nanostructures have been studied in great
details recently as they are good candidates for quantum bits, provided they
are coupled to a detector. Here we report coupling of a single As donor atom to
a single-electron transistor (SET) in a silicon nanowire field-effect
transistor. Both capacitive and tunnel coupling are achieved, the latter
resulting in a dramatic increase of the conductance through the SET, by up to
one order of magnitude. The experimental results are well explained by the rate
equations theory developed in parallel with the experiment.Comment: 16 pages, 8 figure
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
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
Experimental realization of a measurement conditional unitary operation at single photon level and application to detector characterization
Our last experimental results on the realization of a measurement-conditional
unitary operation at single photon level are presented. This gate operates by
rotating by the polarization of a photon produced by means of Type-II
Parametric Down Conversion conditional to a polarization measurement on the
correlated photon. We then propose a new scheme for measuring the quantum
efficiency of a single photon detection apparatus by using this set-up. We
present experimental results obtained with this scheme compared with {\it
traditional} biphoton calibration. Our results show the interesting
potentiality of the suggested scheme.Comment: to appear in Proc. of SPIE meeting, Denver august 200
Coulomb blockade in quantum dots under AC pumping
We study conductance through a quantum dot under Coulomb blockade conditions
in the presence of an external periodic perturbation. The stationary state is
determined by the balance between the heating of the dot electrons by the
perturbation and cooling. We analyze two cooling mechanisms: electron exchange
with the cold contacts and emission of phonons. Together with the usual linear
Ohmic heating of the dot electrons we consider possible effects of dynamic
localization. The combination of the abovementioned factors may result in a
drastic change of the shape of the Coulomb blockade peak with respect to the
usual equilibrium one.Comment: 12 pages, 8 figure
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
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