826 research outputs found
Interplay of spin-discriminated Andreev bound states forming the 0- transition in Superconductor-Ferromagnet-Superconductor Junctions
The Josephson current in S-F-S junctions is described by taking into account
different reflection (transmission) amplitudes for quasiparticles with spin up
and down. We show that the 0- transition in the junctions can take place
at some temperature only for sufficiently strong spin-activity of the
interface. In particular, Andreev interface bound state energies in one spin
channel have to be all negative, while in the other one positive. Only one spin
channel contributes then to the zero-temperature Josephson current. At the
temperature of the 0- transition two spin channels substantially
compensate each other and can result in a pronounced minimum in the critical
current in tunnel junctions. The minimal critical current is quadratic in small
transparency and contains first and second harmonics of one and the same order.Comment: 5 pages, revtex, 2 ps-figure
Broadening of Andreev-Bound States in d_{x^2-y^2} superconductors
We investigate the broadening of the bound states at an interface of an
unconventional superconductor by bulk impurity scattering. We use the
quasiclassical theory and include impurity scattering in the Born and in the
unitarity limit. The broadening of bound states due to unitary scatterers is
shown to be substantially weaker than in the Born limit. We study various model
geometries and calculate the temperature dependence of the Josephson critical
current in the presence of these impurity-broadened bound states.Comment: 6 page including 7 figures, submitted to Phys. Rev.
Repulsive Casimir Force: Sufficient Conditions
In this paper the Casimir energy of two parallel plates made by materials of
different penetration depth and no medium in between is derived. We study the
Casimir force density and derive analytical constraints on the two penetration
depths which are sufficient conditions to ensure repulsion. Compared to other
methods our approach needs no specific model for dielectric or magnetic
material properties and constitutes a complementary analysis.Comment: 11 pages. 3 figures. Misprints corrected in Eq. (4
Superconducting Junctions with Ferromagnetic, Antiferromagnetic or Charge-Density-Wave Interlayers
Spectra and spin structures of Andreev interface states and the Josephson
current are investigated theoretically in junctions between clean
superconductors (SC) with ordered interlayers. The Josephson current through
the ferromagnet-insulator-ferromagnet interlayer 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. We find a novel channel of quasiparticle reflection
(Q reflection) from the simplest two-sublattice antiferromagnet (AF) on a
bipartite lattice. As a combined effect of Andreev and Q reflections, Andreev
states arise at the AF/SC interface. When the Q reflection dominates the
specular one, Andreev bound states have almost zero energy on AF/ s-wave SC
interfaces, whereas they lie near the edge of the continuous spectrum for
AF/d-wave SC boundaries. For an s-wave SC/AF/s-wave SC junction, the bound
states are found to split and carry the supercurrent. Our analytical results
are based on a novel quasiclassical approach, which applies to interfaces
involving itinerant antiferromagnets. Similar effects can take place on
interfaces of superconductors with charge density wave materials (CDW),
including the possible d-density wave state (DDW) of the cuprates.Comment: LT24 conference proceeding, 2 pages, 1 figur
Quasiparticle Interface States in Junctions Involving d-Wave Superconductors
Influence of surface pair breaking, barrier transmission and phase difference
on quasiparticle bound states in junctions with d-wave superconductors is
examined. Based on the quasiclassical theory of superconductivity, an approach
is developed to handle interface bound states. It is shown in SIS' junctions
that low energy bound states get their energies reduced by surface pair
breaking, which can be taken into account by introducing an effective order
parameter for each superconductor at the junction barrier. More interestingly,
for the interface bound states near the continuous spectrum the effect of
surface pair breaking may result in a splitting of the bound states. In the
tunneling limit this can lead to a square root dependence of a nonequilibrium
Josephson current on the barrier transmision, which means an enhancement as
compared to the conventional critical current linear in the transmission.
Reduced broadening of bound states in NIS junctions due to surface pair
breaking is found.Comment: 27 pages, Latex fil
Magnetic interference patterns in superconducting junctions: Effects of anharmonic current-phase relations
A microscopic theory of the magnetic-field modulation of critical currents is
developed for plane Josephson junctions with anharmonic current-phase
relations. The results obtained allow examining temperature-dependent
deviations of the modulation from the conventional interference pattern. For
tunneling through localized states in symmetric short junctions with a
pronounced anharmonic behavior, the deviations are obtained and shown to depend
on distribution of channel transparencies. For constant transparency the
deviations vanish not only near Tc, but also at T=0. If Dorokhov bimodal
distribution for transparency eigenvalues holds, the averaged deviation
increases with decreasing temperature and takes its maximum at T=0.Comment: 6 pages, 6 figure
Subharmonic Gap Structure in Superconductor/Ferromagnet/Superconductor Junctions
The behavior of dc subgap current in magnetic quantum point contact is
discussed for the case of low-transparency junction with different tunnel
probabilities for spin-up () and spin-down ()
electrons. Due to the presence of Andreev bound states in the
system the positions of subgap electric current steps are split at temperature with respect to the
nonmagnetic result . It is found that under the condition
the spin current also manifests subgap
structure, but only for odd values of . The split steps corresponding to
in subgap electric and spin currents are analytically calculated and
the following steps are described qualitatively.Comment: 4 pages, 1 figure, minor stylistic changes, journal-ref adde
Dispersion Interactions between Optically Anisotropic Cylinders at all Separations: Retardation Effects for Insulating and Semiconducting Single Wall Carbon Nanotubes
We derive the complete form of the van der Waals dispersion interaction
between two infinitely long anisotropic semiconducting/insulating thin
cylinders at all separations. The derivation is based on the general theory of
dispersion interactions between anisotropic media as formulated in [J. N.
Munday, D. Iannuzzi, Yu. S. Barash and F. Capasso, {\sl Phys. Rev. A} {\bf 71},
042102 (2005)]. This formulation is then used to calculate the dispersion
interactions between a pair of single walled carbon nanotubes at all
separations and all angles. Non-retarded and retarded forms of the interactions
are developed separately. The possibility of repulsive dispersion interactions
and non-monotonic dispersion interactions is discussed within the framework of
the new formulation
0-pi transitions in Josephson junctions with antiferromagnetic interlayers
We show that the dc Josephson current through
superconductor-antiferromagnet-superconductor (S/AF/S) junctions manifests a
remarkable atomic scale dependence on the interlayer thickness. At low
temperatures the junction is either a 0- or pi-junction depending on whether
the AF interlayer consists of an even or odd number of atomic layers. This is
associated with different symmetries of the AF interlayers in the two cases. In
the junction with odd AF interlayers an additional pi-0 transition can take
place as a function of temperature. This originates from the interplay of
spin-split Andreev bound states. Experimental implications of these theoretical
findings are discussed.Comment: 4 pages, 2 figure
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