27,210 research outputs found
Spin superconductor in ferromagnetic graphene
We show a spin superconductor (SSC) in ferromagnetic graphene as the
counterpart to the charge superconductor, in which a spin-polarized
electron-hole pair plays the role of the spin `Cooper pair' with
a neutral charge. We present a BCS-type theory for the SSC. With the
`London-type equations' of the super-spin-current density, we show the
existence of an electric `Meissner effect' against a spatial varying electric
field. We further study a SSC/normal conductor/SSC junction and predict a
spin-current Josephson effect.Comment: 6 pages, 4 figure
Mgb2 Nonlinear Properties Investigated under Localized High RF Magnetic Field Excitation
In order to increase the accelerating gradient of Superconducting Radio
Frequency (SRF) cavities, Magnesium Diboride (MgB2) opens up hope because of
its high transition temperature and potential for low surface resistance in the
high RF field regime. However, due to the presence of the small superconducting
gap in the {\pi} band, the nonlinear response of MgB2 is potentially quite
large compared to a single gap s-wave superconductor (SC) such as Nb.
Understanding the mechanisms of nonlinearity coming from the two-band structure
of MgB2, as well as extrinsic sources, is an urgent requirement. A localized
and strong RF magnetic field, created by a magnetic write head, is integrated
into our nonlinear-Meissner-effect scanning microwave microscope [1]. MgB2
films with thickness 50 nm, fabricated by a hybrid physical-chemical vapor
deposition technique on dielectric substrates, are measured at a fixed location
and show a strongly temperature-dependent third harmonic response. We propose
that at least two mechanisms are responsible for this nonlinear response, one
of which involves vortex nucleation and penetration into the film. [1] T. M.
Tai, X. X. Xi, C. G. Zhuang, D. I. Mircea, S. M. Anlage, "Nonlinear Near-Field
Microwave Microscope for RF Defect Localization in Superconductors", IEEE
Trans. Appl. Supercond. 21, 2615 (2011).Comment: 6 pages, 6 figure
A new dromaeosaurid (Dinosauria: Theropoda) from the Upper Cretaceous Wulansuhai Formation of Inner Mongolia, China
We describe a new dromaeosaurid theropod from the Upper Cretaceous Wulansuhai Formation of Bayan Mandahu, Inner
Mongolia. The new taxon, Linheraptor exquisitus gen. et sp. nov., is based on an exceptionally well-preserved, nearly
complete skeleton. This specimen represents the fifth dromaeosaurid taxon recovered from the Upper Cretaceous
Djadokhta Formation and its laterally equivalent strata, which include the Wulansuhai Formation, and adds to the known
diversity of Late Cretaceous dromaeosaurids. Linheraptor exquisitus closely resembles the recently reported Tsaagan
mangas. Uniquely among dromaeosaurids, the two taxa share a large, anteriorly located maxillary fenestra and a contact
between the jugal and the squamosal that excludes the postorbital from the infratemporal fenestra. These features suggest
a sister-taxon relationship between L. exquisitus and T. mangas, which indicates the presence of a unique dromaeosaurid
lineage in the Late Cretaceous of Asia. A number of cranial and dental features seen in L. exquisitus and T. mangas, and
particularly some postcranial features of L. exquisitus, suggest that these two taxa are probably intermediate in
systematic position between known basal and derived dromaeosaurids. The discovery of Linheraptor exquisitus is thus
important for understanding the evolution of some salient features seen in the derived dromaeosaurids
Roles of intrinsic anisotropy and pi-band pairbreaking effects on critical currents in tilted c-axis MgB2 films probed by magneto-optical and transport measurements
Investigations of MgB2 and Fe-based superconductors in recent years have
revealed many unusual effects of multiband superconductivity but manifestations
of anisotropic multiband effects in the critical current density Jc have not
been addressed experimentally, mostly because of the difficulties to measure Jc
along the c-axis. To investigate the effect of very different intrinsic
anisotropies of sigma and pi electron bands in MgB2 on current transport, we
grew epitaxial films with tilted c-axis (THETA ~ 19.5{\deg}), which enabled us
to measure the components of Jc both along the ab-plane and the c-axis using
magneto-optical and transport techniques. These measurements were combined with
scanning and transmission electron microscopy, which revealed terraced steps on
the surface of the c-axis tilted films. The measured field and temperature
dependencies of the anisotropic Jc(H) show that Jc,L parallel to the terraced
steps is higher than Jc,T perpendicular to the terraced steps, and Jc of
thinner films (50 nm) obtained from transport experiments at 0.1 T reaches ~10%
of the depairing current density Jd in the ab plane, while magneto-optical
imaging revealed much higher Jc at lower fields. To analyze the experimental
data we developed a model of anisotropic vortex pinning which accounts for the
observed behavior of Jc in the c-axis tilted films and suggests that the
apparent anisotropy of Jc is affected by current pairbreaking effects in the
weaker {\pi} band. Our results indicate that the out-of-plane current transport
mediated by the {\pi} band could set the ultimate limit of Jc in MgB2
polycrystals.Comment: 21 pges, 13 figure
KINETICS OF GRAIN-GROWTH OF YTTRIUM ALUMINUM GARNET FIBERS PREPARED BY SOL-GEL METHOD
The yttrium aluminum garnet (YAG) long fibers were prepared by the sol-gel method using aluminum chloride, aluminum powder, yttrium oxide and acetic acid as raw materials. The grain growth law is given by Dn – D0n = Kt (D0 = initial grain size, D = average grain size at time t, n = grain growth exponent and K = reaction constant). The grain growth exponent and activation energy of YAG fibers are ≈ 3 and 200 kJ/mol, respectively. The grain-growth behaviors of YAG were influenced by experimental conditions such as raw materials, initial particle size, initial particle distribution, etc
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