137 research outputs found
Kinetic inductance measured in a superconducting wire
Ultrasensitive technique to measure kinetic inductance has test specimen included as part of the inductance of a tank circuit of a tunnel diode oscillator. Frequency counter measures shift in frequency of oscillator, caused by changes in inductance. Frequency shift in tank circuit is proportional to change in kinetic inductanc
Spin accumulation induced resistance in mesoscopic ferromagnet/ superconductor junctions
We present a description of spin-polarized transport in mesoscopic
ferromagnet-superconductor (F/S) systems, where the transport is diffusive, and
the interfaces are transparent. It is shown that the spin reversal associated
with Andreev reflection generates an excess spin density close to the F/S
interface, which leads to a spin contact resistance. Expressions for the
contact resistance are given for two terminal and four terminal geometries. In
the latter the sign depends on the relative magnetization of the ferromagnetic
electrodes.Comment: RevTeX 10 pages, 4 figures, submitted to Phys.Rev. Let
Spin current in ferromagnet/insulator/superconductor junctions
A theory of spin polarized tunneling spectroscopy based on a scattering
theory is given for tunneling junctions between ferromagnets and d-wave
superconductors. The spin filtering effect of an exchange field in the
insulator is also treated. We clarify that the properties of the Andreev
reflection are largely modified due to a presence of an exchange field in the
ferromagnets, and consequently the Andreev reflected quasiparticle shows an
evanescent-wave behavior depending on the injection angle of the quasiparticle.
Conductance formulas for the spin current as well as the charge current are
given as a function of the applied voltage and the spin-polarization in the
ferromagnet for arbitrary barrier heights. It is shown that the surface bound
states do not contribute to the spin current and that the zero-bias conductance
peak expected for a d-wave superconductor splits into two peaks under the
influence of the exchange interaction in the insulator.Comment: 14 pages, 11 figure
Anisotropic Magnetoconductance in Quench-Condensed Ultrathin Beryllium Films
Near the superconductor-insulator (S-I) transition, quench-condensed
ultrathin Be films show a large magnetoconductance which is highly anisotropic
in the direction of the applied field. Film conductance can drop as much as
seven orders of magnitude in a weak perpendicular field (< 1 T), but is
insensitive to a parallel field in the same field range. We believe that this
negative magnetoconductance is due to the field de-phasing of the
superconducting pair wavefunction. This idea enables us to extract the finite
superconducting phase coherence length in nearly superconducting films. Our
data indicate that this local phase coherence persists even in highly
insulating films in the vicinity of the S-I transition.Comment: 4 pages, 4 figure RevTex, Typos Correcte
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 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
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
Self Injection length in La0.7 Ca0.3 Mno3-YBa 2Cu3O7-d ferromagnet- superconductor multi layer thin films
We have carried out extensive studies on the self-injection problem in
barrierless heterojunctions between La0.7Ca0.3MnO3 (LCMO) and YBa2Cu3O7-d
(YBCO). The heterojunctions were grown in situ by sequentially growing LCMO and
YBCO films on LaAlO3 (LAO) substrate using a pulsed laser deposition
(PLD) system. YBCO micro-bridges with 64 microns width were patterned both on
the LAO (control) and LCMO side of the substrate. Critical current, Ic, was
measured at 77K on both the control side as well as the LCMO side for different
YBCO film thickness. It was observed that while the control side showed a Jc of
~2 x 10E6 A/ cm2 the LCMO side showed about half the value for the same
thickness (1800 A). The difference in Jc indicates that a certain thickness of
YBCO has become 'effectively' normal due to self-injection. From the
measurement of Jc at two different thickness' (1800 A and 1500 A) of YBCO both
on the LAO as well as the LCMO side, the value of self-injection length (at
77K) was estimated to be ~900 A self-injection length has been quantified. A
control experiment carried out with LaNiO3 deposited by PLD on YBCO did not
show any evidence of self-injection.Comment: 6 pages, one figure in .ps forma
Tenfold Magnetoconductance in a Non-Magnetic Metal Film
We present magnetoconductance (MC) measurements of homogeneously disordered
Be films whose zero field sheet conductance (G) is described by the
Efros-Shklovskii hopping law . The low field
MC of the films is negative with G decreasing 200% below 1 T. In contrast the
MC above 1 T is strongly positive. At 8 T, G increases 1000% in perpendicular
field and 500% in parallel field. In the simpler parallel case, we observe {\em
field enhanced} variable range hopping characterized by an attenuation of
via the Zeeman interaction.Comment: 9 pages including 5 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
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