101 research outputs found
Detection of tightly closed flaws by nondestructive testing (NDT) methods in steel and titanium
X-radiographic, liquid penetrant, ultrasonic, eddy current and magnetic particle testing techniques were optimized and applied to the evaluation of 4340 steel (180 KSI-UTS) and 6Al-4V titanium (STA) alloy specimens. Sixty steel specimens containing a total of 176 fatigue cracks and 60 titanium specimens containing a total of 135 fatigue cracks were evaluated. The cracks ranged in length from .043 cm (0.017 inch) to 1.02 cm (.400 inch) and in depth from .005 cm (.002 inch) to .239 cm (.094 inch) for steel specimens. Lengths ranged from .048 cm (0.019 inch) to 1.03 cm (.407 inch) and depths from 0.010 cm (.004 inch) to .261 cm (0.103 inch) for titanium specimens. Specimen thicknesses were nominally .152 cm (0.060 inch) and 0.635 cm (0.250 inch) and surface finishes were nominally 125 rms. Specimens were evaluated in the "as machined" surface condition, after etch surface and after proof loading in a randomized inspection sequence
Boltzmann Equations for Spin and Charge Relaxations in Superconductors
In a superconductor coupled with a ferromagnetic metal, spin and charge
imbalances can be induced by injecting spin-polarized electron current from the
ferromagnetic metal. We theoretically study a nonequilibrium distribution of
quasiparticles in the presence of spin and charge imbalances. We show that four
distribution functions are needed to characterize such a nonequilibrium
situation, and derive a set of linearized Boltzmann equations for them by
extending the argument by Schmid and Sch\"{o}n based on the quasiclassical
Green's function method. Using the Boltzmann equations, we analyze the spin
imbalance in a thin superconducting wire weakly coupled with a ferromagnetic
electrode. The spin imbalance induces a shift () of
the chemical potential for up-spin (down-spin) quasiparticles. We discuss how
is relaxed by spin-orbit impurity scattering.Comment: 16 pages, 2 figure
Spin-accumulation in small ferromagnetic double barrier junctions
The non-equilibrium spin accumulation in ferromagnetic double barrier
junctions is shown to govern the transport in small structures. Transport
properties of such systems are described by a generalization of the theory of
the Coulomb blockade. The spin accumulation enhances the magnetoresistance. The
transient non-linear transport properties are predicted to provide a unique
experimental evidence of the spin-accumulation in the form of a reversed
current on time scales of the order of the spin-flip relaxation time.Comment: 4 pages, 3 figures, to appear in PR
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
Spin Bottlenecks in the Quantum Hall Regim
We present a theory of time-dependent tunneling between a metal and a
partially spin-polarized two-dimensional electron system (2DES). We find that
the leakage current which flows to screen an electric field between the metal
and the 2DES is the sum of two exponential contributions whose relative weights
depend on spin-dependent tunneling conductances, on quantum corrections to the
electrostatic capacitance of the tunnel junction, and on the rate at which the
2DES spin-polarization approaches equilibrium. For high-mobility and
homogeneous 2DES's at Landau level filling factor , we predict a ratio
of the fast and slow leakage rates equal to where is the number
of reversed spins in the skyrmionic elementary charged excitations.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
On The Mobile Behavior of Solid He at High Temperatures
We report studies of solid helium contained inside a torsional oscillator, at
temperatures between 1.07K and 1.87K. We grew single crystals inside the
oscillator using commercially pure He and He-He mixtures containing
100 ppm He. Crystals were grown at constant temperature and pressure on the
melting curve. At the end of the growth, the crystals were disordered,
following which they partially decoupled from the oscillator. The fraction of
the decoupled He mass was temperature and velocity dependent. Around 1K, the
decoupled mass fraction for crystals grown from the mixture reached a limiting
value of around 35%. In the case of crystals grown using commercially pure
He at temperatures below 1.3K, this fraction was much smaller. This
difference could possibly be associated with the roughening transition at the
solid-liquid interface.Comment: 15 pages, 6 figure
Crossed Andreev reflection at ferromagnetic domain walls
We investigate several factors controlling the physics of hybrid structures
involving ferromagnetic domain walls (DWs) and superconducting (S) metals. We
discuss the role of non collinear magnetizations in S/DW junctions in a spin
Nambu Keldysh formalism. We discuss transport in S/DW/N and
S/DW/S junctions in the presence of inelastic scattering in the domain wall. In
this case transport properties are similar for the S/DW/S and S/DW/N junctions
and are controlled by sequential tunneling of spatially separated Cooper pairs
across the domain wall. In the absence of inelastic scattering we find that a
Josephson current circulates only if the size of the ferromagnetic region is
smaller than the elastic mean free path meaning that the Josephson effect
associated to crossed Andreev reflection cannot be observed under usual
experimental conditions. Nevertheless a finite dc current can circulate across
the S/DW/S junction due to crossed Andreev reflection associated to sequential
tunneling.Comment: 18 pages, 8 figures, references added at the end of the introductio
Spintronic transport and Kondo effect in quantum dots
We investigate the spin-dependent transport properties of quantum-dot based
structures where Kondo correlations dominate the electronic dynamics. The
coupling to ferromagnetic leads with parallel magnetizations is known to give
rise to nontrivial effects in the local density of states of a single quantum
dot. We show that this influence strongly depends on whether charge
fluctuations are present or absent in the dot. This result is confirmed with
numerical renormalization group calculations and perturbation theory in the
on-site interaction. In the Fermi-liquid fixed point, we determine the
correlations of the electric current at zero temperature (shot noise) and
demonstrate that the Fano factor is suppressed below the Poissonian limit for
the symmetric point of the Anderson Hamiltonian even for nonzero lead
magnetizations. We discuss possible avenues of future research in this field:
coupling to the low energy excitations of the ferromagnets (magnons), extension
to double quantum dot systems with interdot antiferromagnetic interaction and
effect of spin-polarized currents on higher symmetry Kondo states such as
SU(4).Comment: 11 pages, 5 figures. Proceedings of the 3rd Intl. Conf. on Physics
and Applications of Spin-Related Phenomena in Semiconductors, Santa Barbara,
200
Electron and hole transmission through superconductor - normal metal interfaces
We have investigated the transmission of electrons and holes through
interfaces between superconducting aluminum (Tc = 1.2 K) and various normal
non-magnetic metals (copper, gold, palladium, platinum, and silver) using
Andreev-reflection spectroscopy at T = 0.1 K. We analyzed the point contacts
with the modified BTK theory that includes Dynes' lifetime as a fitting
parameter G in addition to superconducting energy gap 2D and normal reflection
described by Z. For contact areas from 1 nm^2 to 10000 nm^2 the BTK Z parameter
was 0.5, corresponding to transmission coefficients of about 80 %, independent
of the normal metal. The very small variation of Z indicates that the
interfaces have a negligible dielectric tunneling barrier. Fermi surface
mismatch does not account for the observed transmission coefficient.Comment: 9 pages, 4 figures, submitted to Proceedings of the 19th
International Conference on Magnetism ICM2012 (Busan 2012
Josephson effect in double-barrier superconductor-ferromagnet junctions
We study the Josephson effect in ballistic double-barrier SIFIS planar
junctions, consisting of bulk superconductors (S), a clean metallic ferromagnet
(F), and insulating interfaces (I). We solve the scattering problem based on
the Bogoliubov--de Gennes equations and derive a general expression for the dc
Josephson current, valid for arbitrary interfacial transparency and Fermi wave
vectors mismatch (FWVM). We consider the coherent regime in which quasiparticle
transmission resonances contribute significantly to the Andreev process. The
Josephson current is calculated for various parameters of the junction, and the
influence of both interfacial transparency and FWVM is analyzed. For thin
layers of strong ferromagnet and finite interfacial transparency, we find that
coherent (geometrical) oscillations of the maximum Josephson current are
superimposed on the oscillations related to the crossover between 0 and
states. For the same case we find that the temperature-induced
transition occurs if the junction is very close to the crossovers at zero
temperature.Comment: 13 pages, 6 figure
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