215 research outputs found
Theory of superconducting and magnetic proximity effect in SF structures with inhomogeneous magnetization textures and spin-active interfaces
We present a study of the proximity effect and the inverse proximity effect
in a superconductorferromagnet bilayer, taking into account several
important factors which mostly have been ignored in the literature so far.
These include spin-dependent interfacial phase shifts (spin-DIPS) and
inhomogeneous textures of the magnetization in the ferromagnetic layer, both of
which are expected to be present in real experimental samples. Our approach is
numerical, allowing us to access the full proximity effect regime. In Part I of
this work, we study the superconducting proximity effect and the resulting
local density of states in an inhomogeneous ferromagnet with a non-trivial
magnetic texture.
Our two main results in Part I are a study of how Bloch and N\'eel domain
walls affect the proximity-induced superconducting correlations and a study of
the superconducting proximity effect in a conical ferromagnet. The latter topic
should be relevant for the ferromagnet Ho, which was recently used in an
experiment to demonstrate the possibility to generate and sustain long-range
triplet superconducting correlations. In Part II of this work, we investigate
the inverse proximity effect with emphasis on the induced magnetization in the
superconducting region as a result of the "leakage" from the ferromagnetic
region. It is shown that the presence of spin-DIPS modify conclusions obtained
previously in the literature with regard to the induced magnetization in the
superconducting region. In particular, we find that the spin-DIPS can trigger
an anti-screening effect of the magnetization, leading to an induced
magnetization in the superconducting region with \textit{the same sign} as in
the proximity ferromagnet.Comment: 16 pages, 18 figures. Accepted for publication in Phys. Rev.
Nonsinusoidal current-phase relations and the transition in diffusive ferromagnetic Josephson junctions
We study the effect of the interfacial transparency on the Josephson current
in a diffusive ferromagnetic contact between two superconductors. In contrast
to the cases of the fully transparent and the low-transparency interfaces, the
current-phase relation is shown to be nonsinusoidal for a finite transparency.
It is demonstrated that even for the nearly fully transparent interfaces the
small corrections due to weak interfacial disorders contribute a small
second-harmonic component in the current-phase relation. For a certain
thicknesses of the ferromagnetic contact and the exchange field this can lead
to a tiny minimum supercurrent at the crossover between 0 and states of
the junction. Our theory has a satisfactory agreement with the recent
experiments in which a finite supercurrent was observed at the transition
temperature. We further explain the possibility for observation of a large
residual supercurrent if the interfaces have an intermediate transparency.Comment: 7 pages, 4 figure
Josephson current in a superconductor-ferromagnet junction with two non-collinear magnetic domains
We study the Josephson effect in a
superconductor--ferromagnet--superconductor (SFS) junction with ferromagnetic
domains of non-collinear magnetization. As a model for our study we consider a
diffusive junction with two ferromagnetic domains along the junction. The
superconductor is assumed to be close to the critical temperature , and
the linearized Usadel equations predict a sinusoidal current-phase relation. We
find analytically the critical current as a function of domain lengths and of
the angle between the orientations of their magnetizations. As a function of
those parameters, the junction may undergo transitions between 0 and
phases. We find that the presence of domains reduces the range of junction
lengths at which the phase is observed. For the junction with two domains
of the same length, the phase totally disappears as soon as the
misorientation angle exceeds . We further comment on possible
implication of our results for experimentally observable 0-- transitions
in SFS junctions.Comment: 9 pages, 4 figures, minor changes, references adde
Triplet proximity effect in FSF trilayers
We study the critical temperature T_c of FSF trilayers (F is a ferromagnet, S
is a singlet superconductor), where the triplet superconducting component is
generated at noncollinear magnetizations of the F layers. An exact numerical
method is employed to calculate T_c as a function of the trilayer parameters,
in particular, mutual orientation of magnetizations. Analytically, we consider
limiting cases. Our results determine conditions which are necessary for
existence of recently investigated odd triplet superconductivity in SF
multilayers.Comment: 5 pages, 4 EPS figures; the style file jetpl.cls is included. Version
2: minor corrections, added reference. Version 3: minor correction
Beyond Moore's technologies: operation principles of a superconductor alternative
The predictions of Moore's law are considered by experts to be valid until
2020 giving rise to "post-Moore's" technologies afterwards. Energy efficiency
is one of the major challenges in high-performance computing that should be
answered. Superconductor digital technology is a promising post-Moore's
alternative for the development of supercomputers. In this paper, we consider
operation principles of an energy-efficient superconductor logic and memory
circuits with a short retrospective review of their evolution. We analyze their
shortcomings in respect to computer circuits design. Possible ways of further
research are outlined.Comment: OPEN ACCES
Identifying the odd-frequency superconducting state by a field-induced Josephson effect
Superconducting order parameters that are odd under exchange of
time-coordinates of the electrons constituting a Cooper-pair, are potentially
of great importance both conceptually and technologically. Recent experiments
report that such an odd-frequency superconducting {\it bulk} state may be
realized in certain heavy-fermion compounds. While the Josephson current
normally only flows between superconductors with the same symmetries with
respect to frequency, we demonstrate that an exchange field may induce a
current between diffusive even- and odd-frequency superconductors. This
suggests a way to identify the possible existence of bulk odd-frequency
superconductors.Comment: 10 pages, 7 figures. To appear in Physical Review
Superconducting decay length in a ferromagnetic metal
The complex decay length xi characterizing penetration of superconducting
correlations into a ferromagnet due to the proximity effect is studied
theoretically in the frame of the linearized Eilenberger equations. The real
part xi_1 and imaginary part xi_2 of the decay length are calculated as
functions of exchange energy and the rates of ordinary, spin flip and spin
orbit electronic scattering in a ferromagnet. The lengths xi_1,2 determine the
spatial scales of, respectively, decay and oscillation of a critical current in
SFS Josephson junctions in the limit of large distance between superconducting
electrodes. The developed theory provides the criteria of applicability of the
expressions for xi_1 and xi_2 in the dirty and the clean limits which are
commonly used in the analysis of SF hybrid structures.Comment: 5 pages, 3 figure
Method for reliable realization of a varphi Josephson junction
We propose a method to realize a Josephson junction by combining
alternating 0 and parts (sub junctions) with an intrinsically
non-sinusoidal current-phase relation (CPR). Conditions for the realization of
the ground state are analyzed. It is shown that taking into account the
non-sinusoidal CPR for a "clean junction with a ferromagnetic (F) barrier, one
can significantly enlarge the domain (regime of suitable F-layer thicknesses)
of the ground state and make the practical realization of
Josephson junctions feasible. Such junctions may also have two different stable
solutions, such as 0 and , 0 and , or and
Nonequilibrium effects in tunnel Josephson junctions
We study nonequilibrium effects in current transport through voltage biased
tunnel junction with long diffusive superconducting leads at low applied
voltage, , and finite temperatures. Due to a small value of the
Josephson frequency, the quasiparticle spectrum adiabatically follows the time
evolution of the superconducting phase difference, which results in the
formation of oscillating bound states in the vicinity of the tunnel junction
(Andreev band). The quasiparticles trapped by the Andreev band generate higher
even harmonics of the Josephson ac current, and also, in the presence of
inelastic scattering, a non-equilibrium dc current, which may considerably
exceed the dc quasiparticle current given by the tunnel model. The distribution
of travelling quasiparticles also deviates from the equilibrium due to the
spectrum oscillations, which results in an additional contribution to the dc
current, proportional to .Comment: 11 pages, 7 figures, to be published in Phys. Rev.
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