275 research outputs found
Spectroscopy and critical temperature of diffusive superconducting/ferromagnetic hybrid structures with spin-active interfaces
The description of the proximity effect in superconducting/ferromagnetic
heterostructures requires to use spin-dependent boundary conditions. Such
boundary conditions must take into account the spin dependence of the phase
shifts acquired by electrons upon scattering on the boundaries of ferromagnets.
The present article shows that this property can strongly affect the critical
temperature and the energy dependence of the density of states of diffusive
heterostructures. These effects should allow a better caracterisation of
diffusive superconductor/ferromagnet interfaces.Comment: 12 pages, 6 figures, to be published in Phys. Rev.
Theory of tunneling spectroscopy of normal metal/ferromagnet/spin-triplet superconductor junctions
We study the tunneling conductance of a ballistic normal metal / ferromagnet
/ spin-triplet superconductor junction using the extended
Blonder-Tinkham-Klapwijk formalism as a model for a -axis oriented Au /
SrRuO / SrRuO junction. We compare chiral -wave (CPW) and
helical -wave (HPW) pair potentials, combined with ferromagnet magnetization
directions parallel and perpendicular to the interface. For fixed ,
where is a direction of magnetization in the ferromagnet measured
from the -axis, the tunneling conductance of CPW and HPW clearly show
different voltage dependencies. It is found that the cases where the -vector
is perpendicular to the magnetization direction (CPW with
and HPW with ) are identical. The obtained results serve as a
guide to determine the pairing symmetry of the spin-triplet superconductor
SrRuO.Comment: 12 pages, 7 figures. There is also a supplementary (not uploaded
Odd-frequency Pairs and Josephson Current through a Strong Ferromagnet
We study Josephson current in superconductor / diffusive ferromagnet
/superconductor junctions by using the recursive Green function method. When
the exchange potential in a ferromagnet is sufficiently large as compared to
the pair potential in a superconductor, an ensemble average of Josephson
current is much smaller than its mesoscopic fluctuations. The Josephson current
vanishes when the exchange potential is extremely large so that a ferromagnet
is half-metallic. Spin-flip scattering at junction interfaces drastically
changes the characteristic behavior of Josephson current. In addition to
spin-singlet Cooper pairs, equal-spin triplet pairs penetrate into a half
metal. Such equal-spin pairs have an unusual symmetry property called
odd-frequency symmetry and carry the Josephson current through a half metal.
The penetration of odd-frequency pairs into a half metal enhances the low
energy quasiparticle density of states, which could be detected experimentally
by scanning tunneling spectroscopy. We will also show that odd-frequency pairs
in a half metal cause a nonmonotonic temperature dependence of the critical
Josephson current.Comment: 12 pages 14 figures embedde
Fiske Steps and Abrikosov Vortices in Josephson Tunnel Junctions
We present a theoretical and experimental study of the Fiske resonances in
the current-voltage characteristics of "small" Josephson junctions with
randomly distributed misaligned Abrikosov vortices. We obtained that in the
presence of Abrikosov vortices the resonant interaction of electromagnetic
waves, excited inside a junction, with the ac Josephson current manifests
itself by Fiske steps in a current-voltage characteristics even in the absence
of external magnetic field. We found that the voltage positions of the Fiske
steps are determined by a junction size, but the Fiske step magnitudes depend
both on the density of trapped Abrikosov vortices and on their misalignment
parameter. We measured the magnetic field dependence of both the amplitude of
the first Fiske step and the Josephson critical current of low-dissipative
small based Josephson tunnel junctions with artificially introduced
Abrikosov vortices. A strong decay of the Josephson critical current and a weak
non-monotonic decrease of the first Fiske step amplitude on the Abrikosov
vortex density were observed. The experimentally observed dependencies are well
described by the developed theory.Comment: 21 pages, 7 figures, submitted to Physical Review
Two-gap superconductivity in MgB: clean or dirty?
A large number of experimental facts and theoretical arguments favor a
two-gap model for superconductivity in MgB. However, this model predicts
strong suppression of the critical temperature by interband impurity scattering
and, presumably, a strong correlation between the critical temperature and the
residual resistivity. No such correlation has been observed. We argue that this
fact can be understood if the band disparity of the electronic structure is
taken into account, not only in the superconducting state, but also in normal
transport
"Chain scenario" for Josephson tunneling with pi-shift in YBa2Cu3O7
We point out that all current Josephson-junction experiments probing directly
the symmetry of the superconducting state in YBa2Cu3O7, can be interpreted in
terms of the bilayer antiferromagnetic spin fluctuation model, which renders
the superconducting state with the order parameters of extended symmetry,
but with the opposite signs in the bonding and antibonding Cu-O plane bands.
The essential part of our interpretation includes the Cu-O chain band which
would have the order parameter of the same sign as antibonding plane band. We
show that in this case net Josephson currents along and perpendicular to the
chains have the phase shift equal to pi.Comment: 4 pages, revtex, 1 figure uuencoded (POSTSCRIPT figure replaced - the
previous file did not print Greek letters correctly
Mesoscopic fluctuations of the supercurrent in diffusive Josephson junctions
We study mesoscopic fluctuations and weak localization correction to the
supercurrent in Josephson junctions with coherent diffusive electron dynamics
in the normal part. Two kinds of junctions are considered: a chaotic dot
coupled to superconductors by tunnel barriers and a diffusive junction with
transparent normal--superconducting interfaces. The amplitude of current
fluctuations and the weak localization correction to the average current are
calculated as functions of the ratio between the superconducting gap and the
electron dwell energy, temperature, and superconducting phase difference across
the junction. Technically, fluctuations on top of the spatially inhomogeneous
proximity effect in the normal region are described by the replicated version
of the \sigma-model. For the case of diffusive junctions with transparent
interfaces, the magnitude of mesoscopic fluctuations of the critical current
appears to be nearly 3 times larger than the prediction of the previous theory
which did not take the proximity effect into account.Comment: 19 pages, 14 figures, 2 table
Andreev reflection in layered structures: implications for high T_c grain boundary Josephson junctions
Andreev reflection is investigated in layered anisotropic normal metal /
superconductor (N/S) systems in the case of an energy gap \Delta in S not
negligible with respect to the Fermi energy E_F, as it probably occurs with
high critical temperature superconductors (HTS). We find that in these limits
retro-reflectivity, which is a fundamental feature of Andreev reflection, is
broken modifying sensitively transport across S/N interfaces. We discuss the
consequences for supercurrents in HTS Josephson junctions and for the midgap
states in S-N contactsComment: 4 pages, 4 figures, to be published in Phys. Rev.
Critical temperature and giant isotope effect in presence of paramagnons
We reconsider the long-standing problem of the effect of spin fluctuations on
the critical temperature and isotope effect in a phonon-mediated
superconductor. Although the general physics of the interplay between phonons
and paramagnons had been rather well understood, the existing approximate
formulas fail to describe the correct behavior of for general phonon
and paramagnon spectra. Using a controllable approximation, we derive an
analytical formula for which agrees well with exact numerical solutions
of the Eliashberg equations for a broad range of parameters. Based on both
numerical and analytical results, we predict a strong enhancement of the
isotope effect when the frequencies of spin fluctuation and phonons are of the
same order. This effect may have important consequences for near-magnetic
superconductors such as MgCNiComment: 5 pages, 2 figure
Optimizing the speed of a Josephson junction
We review the application of dynamical mean-field theory to Josephson
junctions and study how to maximize the characteristic voltage IcRn which
determines the width of a rapid single flux quantum pulse, and thereby the
operating speed in digital electronics. We study a wide class of junctions
ranging from SNS, SCmS (where Cm stands for correlated metal), SINIS (where the
insulating layer is formed from a screened dipole layer), and SNSNS structures.
Our review is focused on a survey of the physical results; the formalism has
been developed elsewhere.Comment: (36 pages, 15 figures, to appear in Int. J. Mod. Phys. B
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