177 research outputs found
Spin polarized tunneling in ferromagnet/unconventional superconductor junctions
We study tunneling in ferromagnet/unconventional superconductor (F/S)
junctions. We include the effects of spin polarization, interfacial resistance,
and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev
reflection (AR) at the F/S interface, governing tunneling at low bias voltage,
is strongly modified by these parameters. The conductance exhibits a very wide
variety of features as a function of applied voltage.Comment: Revision includes new figures with angular averages and correction of
minor error
Current and Spin-Torque in Double Tunnel Barrier Ferromagnet - Superconductor - Ferromagnet Systems
We calculate the current and the spin-torque in small symmetric double tunnel
barrier ferromagnet - superconductor - ferromagnet (F-S-F) systems.
Spin-accumulation on the superconductor governs the transport properties when
the spin-flip relaxation time is longer than the transport dwell time. In the
elastic transport regime, it is demonstrated that the relative change in the
current (spin-torque) for F-S-F systems equals the relative change in the
current (spin-torque) for F-N-F systems upon changing the relative
magnetization direction of the two ferromagnets. This differs from the results
in the inelastic transport regime where spin-accumulation suppresses the
superconducting gap and dramatically changes the magnetoresistance [S.
Takahashi, H. Imamura, and S. Maekawa, Phys. Rev. Lett. 82, 3911 (1999)]. The
experimental relevance of the elastic and inelastic transport regimes,
respectively, as well as the reasons for the change in the transport properties
are discussed.Comment: 7 page
Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems
We present a theory of Andreev reflection in a
ferromagnet/superconductor/ferromagnet double junction system. The spin
polarized quasiparticles penetrate to the superconductor in the range of
penetration depth from the interface by the Andreev reflection. When the
thickness of the superconductor is comparable to or smaller than the
penetration depth, the spin polarized quasiparticles pass through the
superconductor and therefore the electric current depends on the relative
orientation of magnetizations of the ferromagnets. The dependences of the
magnetoresistance on the thickness of the superconductor, temperature, the
exchange field of the ferromagnets and the height of the interfacial barriers
are analyzed. Our theory explains recent experimental results well.Comment: 8 pages, 9 figures, submitted to Phys. Rev.
Spin Injection and Detection in Magnetic Nanostructures
We study theoretically the spin transport in a nonmagnetic metal connected to
ferromagnetic injector and detector electrodes. We derive a general expression
for the spin accumulation signal which covers from the metallic to the
tunneling regime. This enables us to discuss recent controversy on spin
injection and detection experiments. Extending the result to a superconducting
device, we find that the spin accumulation signal is strongly enhanced by
opening of the superconducting gap since a gapped superconductor is a low
carrier system for spin transport but not for charge. The enhancement is also
expected in semiconductor devices.Comment: 4 pages, 3 figure
Spin-Polarized Transport Across an LaSrMnO/YBaCuO Interface: Role of Andreev Bound States
Transport across an
LaSr_{3}/YBa_{3}_{7}_{3}$/YBCO and Ag/YBCO. In all cases, YBCO is used as bottom layer to
eliminate the channel resistance and to minimize thermal effects. The observed
differential conductance re ects the role of Andreev bound states in a-b
planes, and brings out for the first time the suppression of such states by the
spin-polarized transport across the interface. The theoretical analysis of the
measured data reveals decay of the spin polarization near the LSMO surface with
temperature, consistent with the reported photoemission data.Comment: 5 pages LaTeX, 3 eps figures included, accepted by Physical Review
A Real Space Description of Magnetic Field Induced Melting in the Charge Ordered Manganites: I. The Clean Limit
We study the melting of charge order in the half doped manganites using a
model that incorporates double exchange, antiferromagnetic superexchange, and
Jahn-Teller coupling between electrons and phonons. We primarily use a real
space Monte Carlo technique to study the phase diagram in terms of applied
field and temperature , exploring the melting of charge order with
increasing and its recovery on decreasing . We observe hysteresis in
this response, and discover that the `field melted' high conductance state can
be spatially inhomogeneous even without extrinsic disorder. The hysteretic
response plays out in the background of field driven equilibrium phase
separation. Our results, exploring , , and the electronic parameter
space, are backed up by analysis of simpler limiting cases and a Landau
framework for the field response. This paper focuses on our results in the
`clean' systems, a companion paper studies the effect of cation disorder on the
melting phenomena.Comment: 16 pages, pdflatex, 11 png fig
A Real Space Description of Field Induced Melting in the Charge Ordered Manganites: II. the Disordered Case
We study the effect of A site disorder on magnetic field induced melting of
charge order (CO) in half doped manganites using a Monte-Carlo technique.
Strong A-site disorder destroys CO even without an applied field. At moderate
disorder, the zero field CO state survives but has several intriguing features
in its field response. Our spatially resolved results track the broadening of
the field melting transition due to disorder and explain the unusual dependence
of the melting scales on bandwidth and disorder. In combination with our
companion paper on field melting of charge order in clean systems we provide an
unified understanding of CO melting across all half doped manganites.Comment: 9 pages, pdflatex, 10 embedded png fig
Influence of infiltration temperature on the microstructure and oxidation behavior of SiC-ZrC ceramic coating on C/C composites prepared by reactive melt infiltration
SiC–ZrC ceramic coating on C/C composites was prepared by reactive melt infiltration (RMI) using a powder mixture composed of Zr, Si and C as the infiltrator. The phase composition and microstructure of the ceramic coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The oxidation resistance of the as-prepared composites was tested at 1550 °C in static air. The results indicate that the infiltration temperature has remarkable effects on the phase composition and microstructure of the ceramic coating, as well as on the oxidation resistance of the composites. The SiC–ZrC coated C/C composites prepared at 2000 °C exhibit an excellent oxidation resistance. They gain weight about 5.9 wt% after oxidation at 1550 °C in static air for 5 h, whereas the SiC–ZrC coated C/C composites prepared at 1800 °C lose weight about 3.2 wt%. As a comparison, SiC coated C/C composites prepared at 2000 °C by RMI show an inferior oxidation resistance. After 5 h oxidation, SiC coated C/C composites are severely damaged and their weight loss reaches up to 44.3 wt%. The outstanding oxidation resistance of the SiC–ZrC coated C/C composites prepared at 2000 °C can be attributed to the rapid formation of a continuous glass-like layer composed of ZrO2, ZrSiO4 and SiO2, which covers the surface of the composites and retards the oxygen diffusion and the attack on the underlying C/C substrate. For SiC coated C/C composites, the large SiC particles formed on the surface of the composites are difficult to oxidize rapidly and so a continuous and dense SiO2 layer cannot be formed in time to significantly hinder fast oxygen diffusion leading to the consequent severe oxidation of the C/C substrate
Search for the Chiral Magnetic Effect in Au+Au collisions at GeV with the STAR forward Event Plane Detectors
A decisive experimental test of the Chiral Magnetic Effect (CME) is
considered one of the major scientific goals at the Relativistic Heavy-Ion
Collider (RHIC) towards understanding the nontrivial topological fluctuations
of the Quantum Chromodynamics vacuum. In heavy-ion collisions, the CME is
expected to result in a charge separation phenomenon across the reaction plane,
whose strength could be strongly energy dependent. The previous CME searches
have been focused on top RHIC energy collisions. In this Letter, we present a
low energy search for the CME in Au+Au collisions at
GeV. We measure elliptic flow scaled charge-dependent correlators relative to
the event planes that are defined at both mid-rapidity and at
forward rapidity . We compare the results based on the
directed flow plane () at forward rapidity and the elliptic flow plane
() at both central and forward rapidity. The CME scenario is expected
to result in a larger correlation relative to than to , while
a flow driven background scenario would lead to a consistent result for both
event planes[1,2]. In 10-50\% centrality, results using three different event
planes are found to be consistent within experimental uncertainties, suggesting
a flow driven background scenario dominating the measurement. We obtain an
upper limit on the deviation from a flow driven background scenario at the 95\%
confidence level. This work opens up a possible road map towards future CME
search with the high statistics data from the RHIC Beam Energy Scan Phase-II.Comment: main: 8 pages, 5 figures; supplementary material: 2 pages, 1 figur
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