378 research outputs found
Local density of states in superconductor-strong ferromagnet structures
We study the dependence of the local density of states (LDOS) on coordinates
for a superconductor-ferromagnet (S/F) bilayer and a S/F/S structure assuming
that the exchange energy h in the ferromagnet is sufficiently large: where is the elastic relaxation time. This limit cannot be
described by the Usadel equation and we solve the more general Eilenberger
equation. We demonstrate that, in the main approximation in the parameter , the proximity effect does not lead to a modification of the LDOS
in the S/F system and a non-trivial dependence on coordinates shows up in next
orders in In the S/F/S sandwich the correction to the LDOS is
nonzero in the main approximation and depends on the phase difference between
the superconductors. We also calculate the superconducting critical temperature
for the bilayered system and show that it does not depend on the
exchange energy of the ferromagnet in the limit of large h and a thick F layer.Comment: 9 pages, 5 figure
Spin-polarized Josephson and quasiparticle currents in superconducting spin-filter tunnel junctions
We present a theoretical study of the effect of spin-filtering on the
Josephson and dissipative quasiparticle currents in a superconducting tunnel
junction. By combining the quasiclassical Green's functions and the tunneling
Hamiltonian method we describe the transport properties of a generic junction
consisting of two superconducting leads with an effective exchange field h
separated by a spin-filter insulating barrier. We show that besides the
tunneling of Cooper pairs with total spin-projection Sz = 0 there is another
contribution to the Josephson current due to equal-spin Cooper pairs. The
latter is finite and not affected by the spin-filter effect provided that the
fields h and the magnetization of the barrier are non-collinear . We also
determine the quasiparticle current for a symmetric junction and show that the
differential conductance may exhibit peaks at different values of the voltage
depending on the polarization of the spin-filter, and the relative angle
between the exchange fields and the magnetization of the barrier. Our findings
provide a plausible explanation for existing experiments on Josephson junctions
with magnetic barriers, predict new effects and show how spin-polarized
supercurrents in hybrid structures can be created.Comment: 5 pages; 3 figure
Interplay between Josephson effect and magnetic interactions in double quantum dots
We analyze the magnetic and transport properties of a double quantum dot
coupled to superconducting leads. In addition to the possible phase transition
to a state, already present in the single dot case, this system exhibits
a richer magnetic behavior due to the competition between Kondo and inter-dot
antiferromagnetic coupling. We obtain results for the Josephson current which
may help to understand recent experiments on superconductor-metallofullerene
dimer junctions. We show that in such a system the Josephson effect can be used
to control its magnetic configuration.Comment: 5 pages, 4 figure
Feasibility of study magnetic proximity effects in bilayer "superconductor/ferromagnet" using waveguide-enhanced Polarized Neutron Reflectometry
A resonant enhancement of the neutron standing waves is proposed to use in
order to increase the magnetic neutron scattering from a
"superconductor/ferromagnet"(S/F) bilayer. The model calculations show that
usage of this effect allows to increase the magnetic scattering intensity by
factor of hundreds. Aspects related to the growth procedure (order of
deposition, roughness of the layers etc) as well as experimental conditions
(resolution, polarization of the neutron beam, background etc) are also
discussed.
Collected experimental data for the S/F heterostructure
Cu(32nm)/V(40nm)/Fe(1nm)/MgO confirmed the presence of a resonant 60-fold
amplification of the magnetic scattering.Comment: The manuscript of the article submitted to Crysstalography Reports.
23 pages, 5 figure
Manifestation of triplet superconductivity in superconductor-ferromagnet structures
We study proximity effects in a multilayered superconductor/ferromagnet (S/F)
structure with arbitrary relative directions of the magnetization . If
the magnetizations of different layers are collinear the superconducting
condensate function induced in the F layers has only a singlet component and a
triplet one with a zero projection of the total magnetic moment of the Cooper
pairs on the direction. In this case the condensate penetrates the F
layers over a short length determined by the exchange energy . If
the magnetizations are not collinear the triplet component has, in
addition to the zero projection, the projections . The latter component
is even in the momentum, odd in the Matsubara frequency and penetrates the F
layers over a long distance that increases with decreasing temperature and does
not depend on (spin-orbit interaction limits this length). If the thickness
of the F layers is much larger than , the Josephson coupling between
neighboring S layers is provided only by the triplet component, so that a new
type of superconductivity arises in the transverse direction of the structure.
The Josephson critical current is positive (negative) for the case of a
positive (negative) chirality of the vector . We demonstrate that this
type of the triplet condensate can be detected also by measuring the density of
states in F/S/F structures.Comment: 14 pages; 9 figures. Final version, to be published in Phys. Rev.
Full Scale Proton Beam Impact Testing of new CERN Collimators and Validation of a Numerical Approach for Future Operation
New collimators are being produced at CERN in the framework of a large
particle accelerator upgrade project to protect beam lines against stray
particles. Their movable jaws hold low density absorbers with tight geometric
requirements, while being able to withstand direct proton beam impacts. Such
events induce considerable thermo-mechanical loads, leading to complex
structural responses, which make the numerical analysis challenging. Hence, an
experiment has been developed to validate the jaw design under representative
conditions and to acquire online results to enhance the numerical models. Two
jaws have been impacted by high-intensity proton beams in a dedicated facility
at CERN and have recreated the worst possible scenario in future operation. The
analysis of online results coupled to post-irradiation examinations have
demonstrated that the jaw response remains in the elastic domain. However, they
have also highlighted how sensitive the jaw geometry is to its mounting support
inside the collimator. Proton beam impacts, as well as handling activities, may
alter the jaw flatness tolerance value by 70 m, whereas the
flatness tolerance requirement is 200 m. In spite of having validated
the jaw design for this application, the study points out numerical limitations
caused by the difficulties in describing complex geometries and boundary
conditions with such unprecedented requirements.Comment: 22 pages, 17 figures, Prepared for submission to JINS
Unanticipated proximity behavior in ferromagnet-superconductor heterostructures with controlled magnetic noncollinearity
Magnetization noncollinearity in ferromagnet-superconductor (F/S)
heterostructures is expected to enhance the superconducting transition
temperature (Tc) according to the domain-wall superconductivity theory, or to
suppress Tc when spin-triplet Cooper pairs are explicitly considered. We study
the proximity effect in F/S structures where the F layer is a Sm-Co/Py
exchange-spring bilayer and the S layer is Nb. The exchange-spring contains a
single, controllable and quantifiable domain wall in the Py layer. We observe
an enhancement of superconductivity that is nonmonotonic as the Py domain wall
is increasingly twisted via rotating a magnetic field, different from
theoretical predictions. We have excluded magnetic fields and vortex motion as
the source of the nonmonotonic behavior. This unanticipated proximity behavior
suggests that new physics is yet to be captured in the theoretical treatments
of F/S systems containing noncollinear magnetization.Comment: 17 pages, 4 figures. Physical Review Letters in pres
Theoretical description of the ferromagnetic -junctions near the critical temperature
The theory of ferromagnetic Pi-junction near the critical temperature is
presented. It is demonstrated that in the dirty limit the modified Usadel
equation adequately describes the proximity effect in ferromagnets. To provide
the description of an experimentally relevant situation, oscillations of the
Josephson critical current are calculated as a function of ferromagnetic layer
thickness for different transparencies of the superconductor-ferromagnet
interfaces.Comment: 12 pages, 4 figures, submitted to Phys. Rev.
Resistance of a domain wall in the quasiclassical approach
Starting from a simple microscopic model, we have derived a kinetic equation
for the matrix distribution function. We employed this equation to calculate
the conductance in a mesoscopic F'/F/F' structure with a domain wall (DW).
In the limit of a small exchange energy and an abrupt DW, the conductance
of the structure is equal to . Assuming that the scattering times
for electrons with up and down spins are close to each other we show that the
account for a finite width of the DW leads to an increase in this conductance.
We have also calculated the spatial distribution of the electric field in the F
wire. In the opposite limit of large (adiabatic variation of the
magnetization in the DW) the conductance coincides in the main approximation
with the conductance of a single domain structure . The account for rotation of
the magnetization in the DW leads to a negative correction to this conductance.
Our results differ from the results in papers published earlier.Comment: 11 pages; replaced with revised versio
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