37 research outputs found
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.
Thermodynamic properties of ferromagnetic/superconductor/ferromagnetic nanostructures
The theoretical description of the thermodynamic properties of
ferromagnetic/superconductor/ferromagnetic (F/S/F) systems of nanoscopic scale
is proposed. Their superconducting characteristics strongly depend on the
mutual orientation of the ferromagnetic layers. In addition, depending on the
transparency of S/F interfaces, the superconducting critical temperature can
exhibit four different types of dependences on the thickness of the F-layer.
The obtained results permit to give some practical recommendations for the
spin-valve effect experimental observation. In this spin-valve sandwich, we
also expect a spontaneous transition from parallel to anti-parallel
ferromagnetic moment orientation, due to the gain in the superconducting
condensation energy.Comment: 20 pages, 5 figures, submitted to PR
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
Order parameter oscillations in Fe/Ag/Bi2Sr2CaCu2O{8+delta} tunnel junctions
We have performed temperature dependent tunneling conductance spectroscopy on
Fe/Ag/Bi2Sr2CaCu2O8 (BSCCO) planar junctions. The multilayered Fe
counterelectrode was designed to probe the proximity region of the ab-plane of
BSCCO. The spectra manifested a coherent oscillatory behavior with magnitude
and sign dependent on the energy, decaying with increasing distance from the
junction barrier, in conjunction with the theoretical predictions involving
d-wave superconductors coupled with ferromagnets. The conductance oscillates in
antiphase at E = 0 and E = +/-Delta. Spectral features characteristic to a
broken time-reversal pairing symmetry are detected and they do not depend on
the geometrical characteristics of the ferromagnetic film.Comment: 4 pages and 4 figures Submitted to Physical Review Letter
Josephson Current in S-FIF-S Junctions: Nonmonotonic Dependence on Misorientation Angle
Spectra and spin structures of Andreev interface states in S-FIF-S junctions
are investigated with emphasis on finite transparency and misorientation angle
between in-plane magnetizations of ferromagnetic layers in a three-layer
interface. It is demonstrated that the Josephson current in S-FIF-S quantum
point contacts can exhibit a nonmonotonic dependence on the misorientation
angle. The characteristic behavior takes place, if the pi-state is the
equilibrium state of the junction in the particular case of parallel
magnetizations.Comment: 5 pages, 4 figure
Layered ferromagnet-superconductor structures: the state and proximity effects
We investigate clean mutilayered structures of the SFS and SFSFS type, (where
the S layer is intrinsically superconducting and the F layer is ferromagnetic)
through numerical solution of the self-consistent Bogoliubov-de Gennes
equations for these systems. We obtain results for the pair amplitude, the
local density of states, and the local magnetic moment. We find that as a
function of the thickness of the magnetic layers separating adjacent
superconductors, the ground state energy varies periodically between two stable
states. The first state is an ordinary "0-state", in which the order parameter
has a phase difference of zero between consecutive S layers, and the second is
a "-state", where the sign alternates, corresponding to a phase difference
of between adjacent S layers. This behavior can be understood from simple
arguments. The density of states and the local magnetic moment reflect also
this periodicity.Comment: 12 pages, 10 Figure
Periodic alternating -junction structures as realization of -Josephson junctions
We consider the properties of a periodic structure consisting of small
alternating 0- and pi- Josephson junctions. We show that depending on the
relation between the lengths of the individual junctions, this system can be
either in the homogeneous or in the phase-modulated state. The modulated phase
appears via a second order phase transition when the mismatch between the
lengths of the individual junctions exceeds the critical value. The screening
length diverges at the transition point. In the modulated state, the
equilibrium phase difference in the structure can take any value from -pi to pi
(phi-junction). The current-phase relation in this structure has very unusual
shape with two maxima. As a consequence, the field dependence of the critical
current in a small structure is very different from the standard Fraunhofer
dependence. The Josephson vortex in a long structure carries partial magnetic
flux, which is determined by the equilibrium phase.Comment: 4 pages, 3 figues, submitted to Phys. Rev.
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.
Inhomogeneous magnetism induced in a superconductor at superconductor-ferromagnet interface
We study a magnetic proximity effect at superconductor (S) - ferromagnet (F)
interface. It is shown that due to an exchange of electrons between the F and S
metals ferromagnetic correlations extend into the superconductor, being
dependent on interface parameters. We show that ferromagnetic exchange field
pair breaking effect leads to a formation of subgap bands in the S layer local
density of states, that accommodate only one spin-polarized quasiparticles.
Equilibrium magnetization leakage into the S layer as function of SF interface
quality and a value of ferromagnetic interaction have also been calculated. We
show that a damped-oscillatory behavior versus distance from SF interface is a
distinguished feature of the exchange-induced magnetization of the S layer.Comment: 10 pages, 7 Postscript figure
Spontaneous Spin Polarized Currents in Superconductor-Ferromagnetic Metal Heterostructures
We study a simple microscopic model for thin, ferromagnetic, metallic layers
on semi-infinite bulk superconductor. We find that for certain values of the
exchange spliting, on the ferromagnetic side, the ground states of such
structures feature spontaneously induced spin polarized currents. Using a
mean-field theory, which is selfconsistent with respect to the pairing
amplitude , spin polarization and the spontaneous current
, we show that not only there are Andreev bound states in the
ferromagnet but when their energies are near zero they support
spontaneous currents parallel to the ferromagnetic-superconducting interface.
Moreover, we demonstrate that the spin-polarization of these currents depends
sensitively on the band filling.Comment: 4 pages, 5 Postscript figures (included