8 research outputs found
Nonequilibrium Josephson effect in mesoscopic ballistic multiterminal SNS junctions
We present a detailed study of nonequilibrium Josephson currents and
conductance in ballistic multiterminal SNS-devices. Nonequilibrium is created
by means of quasiparticle injection from a normal reservoir connected to the
normal part of the junction. By applying a voltage at the normal reservoir the
Josephson current can be suppressed or the direction of the current can be
reversed. For a junction longer than the thermal length, , the
nonequilibrium current increases linearly with applied voltage, saturating at a
value equal to the equilibrium current of a short junction. The conductance
exhibits a finite bias anomaly around . For symmetric
injection, the conductance oscillates -periodically with the phase
difference between the superconductors, with position of the minimum
( or ) dependent on applied voltage and temperature. For
asymmetric injection, both the nonequilibrium Josephson current and the
conductance becomes -periodic in phase difference. Inclusion of barriers
at the NS-interfaces gives rise to a resonant behavior of the total Josephson
current with respect to junction length with a period . Both
three and four terminal junctions are studied.Comment: 21 pages, 19 figures, submitted to Phys. Rev.
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
Dissipative Electron Transport through Andreev Interferometers
We consider the conductance of an Andreev interferometer, i.e., a hybrid
structure where a dissipative current flows through a mesoscopic normal (N)
sample in contact with two superconducting (S) "mirrors". Giant conductance
oscillations are predicted if the superconducting phase difference is
varied. Conductance maxima appear when is on odd multiple of due
to a bunching at the Fermi energy of quasiparticle energy levels formed by
Andreev reflections at the N-S boundaries. For a ballistic normal sample the
oscillation amplitude is giant and proportional to the number of open
transverse modes. We estimate using both analytical and numerical methods how
scattering and mode mixing --- which tend to lift the level degeneracy at the
Fermi energy --- effect the giant oscillations. These are shown to survive in a
diffusive sample at temperatures much smaller than the Thouless temperature
provided there are potential barriers between the sample and the normal
electron reservoirs. Our results are in good agreement with previous work on
conductance oscillations of diffusive samples, which we propose can be
understood in terms of a Feynman path integral description of quasiparticle
trajectories.Comment: 24 pages, revtex, 12 figures in eps forma
Nucleation of superconductivity and vortex matter in superconductor - ferromagnet hybrids
The theoretical and experimental results concerning the thermodynamical and
low-frequency transport properties of hybrid structures, consisting of
spatially-separated conventional low-temperature superconductor (S) and
ferromagnet (F), is reviewed. Since the superconducting and ferromagnetic parts
are assumed to be electrically insulated, no proximity effect is present and
thus the interaction between both subsystems is through their respective
magnetic stray fields. Depending on the temperature range and the value of the
external field H_{ext}, different behavior of such S/F hybrids is anticipated.
Rather close to the superconducting phase transition line, when the
superconducting state is only weakly developed, the magnetization of the
ferromagnet is solely determined by the magnetic history of the system and it
is not influenced by the field generated by the supercurrents. In contrast to
that, the nonuniform magnetic field pattern, induced by the ferromagnet,
strongly affect the nucleation of superconductivity leading to an exotic
dependence of the critical temperature T_{c} on H_{ext}. Deeper in the
superconducting state the effect of the screening currents cannot be neglected
anymore. In this region of the phase diagram various aspects of the interaction
between vortices and magnetic inhomogeneities are discussed. In the last
section we briefly summarize the physics of S/F hybrids when the magnetization
of the ferromagnet is no longer fixed but can change under the influence of the
superconducting currents. As a consequence, the superconductor and ferromagnet
become truly coupled and the equilibrium configuration of this "soft" S/F
hybrids requires rearrangements of both, superconducting and ferromagnetic
characteristics, as compared with "hard" S/F structures.Comment: Topical review, submitted to Supercond. Sci. Tech., 67 pages, 33
figures, 439 reference
Random-Matrix Theory of Quantum Transport
This is a comprehensive review of the random-matrix approach to the theory of
phase-coherent conduction in mesocopic systems. The theory is applied to a
variety of physical phenomena in quantum dots and disordered wires, including
universal conductance fluctuations, weak localization, Coulomb blockade,
sub-Poissonian shot noise, reflectionless tunneling into a superconductor, and
giant conductance oscillations in a Josephson junction.Comment: 85 pages including 52 figures, to be published in Rev.Mod.Phy