3,940 research outputs found
Spinorial geometry, off-shell Killing spinor identities and higher derivative 5D supergravities
Killing spinor identities relate components of equations of motion to each
other for supersymmetric backgrounds. The only input required is the field
content and the supersymmetry transformations of the fields, as long as an
on-shell supersymmetrization of the action without additional fields exists. If
we consider off-shell supersymmetry it is clear that the same relations will
occur between components of the equations of motion independently of the
specific action considered, in particular the Killing spinor identities can be
derived for arbitrary, including higher derivative, supergravities, with a
specified matter content. We give the Killing spinor identities for
five-dimensional ungauged supergravities coupled to Abelian
vector multiplets, and then using spinorial geometry techniques so that we have
explicit representatives for the spinors, we discuss the particular case of the
time-like class of solutions to theories with perturbative corrections at the
four derivative level. We also discuss the maximally supersymmetric solutions
in the general off-shell case.Comment: 62 pages v2: fewer typos, and a few improvements in the text kindly
suggested by a refere
Entropy theorems in classical mechanics, general relativity, and the gravitational two-body problem
In classical Hamiltonian theories, entropy may be understood either as a
statistical property of canonical systems, or as a mechanical property, that
is, as a monotonic function of the phase space along trajectories. In classical
mechanics, there are theorems which have been proposed for proving the
non-existence of entropy in the latter sense. We explicate, clarify and extend
the proofs of these theorems to some standard matter (scalar and
electromagnetic) field theories in curved spacetime, and then we show why these
proofs fail in general relativity; due to properties of the gravitational
Hamiltonian and phase space measures, the second law of thermodynamics holds.
As a concrete application, we focus on the consequences of these results for
the gravitational two-body problem, and in particular, we prove the
non-compactness of the phase space of perturbed Schwarzschild-Droste
spacetimes. We thus identify the lack of recurring orbits in phase space as a
distinct sign of dissipation and hence entropy production.Comment: 39 pages, 3 figures; v2: version to appear in Phys. Rev. D,
references adde
Direct Observation of Large Amplitude Spin Excitations Localized in a Spin-Transfer Nanocontact
We report the direct observation of large amplitude spin-excitations
localized in a spin-transfer nanocontact using scanning transmission x-ray
microscopy. Experiments were conducted using a nanocontact to an ultrathin
ferromagnetic multilayer with perpendicular magnetic anisotropy. Element
resolved x-ray magnetic circular dichroism images show an abrupt onset of spin
excitations at a threshold current that are localized beneath the nanocontact,
with average spin precession cone angles of 25{\deg} at the contact center. The
results strongly suggest that we have observed a localized magnetic soliton.Comment: 5 pages, 3 figure
Spontaneous heavy cluster emission rates using microscopic potentials
The nuclear cluster radioactivities have been studied theoretically in the
framework of a microscopic superasymmetric fission model (MSAFM). The nuclear
interaction potentials required for binary cold fission processes are
calculated by folding in the density distribution functions of the two
fragments with a realistic effective interaction. The microscopic nuclear
potential thus obtained has been used to calculate the action integral within
the WKB approximation. The calculated half lives of the present MSAFM
calculations are found to be in good agreement over a wide range of observed
experimental data.Comment: 4 pages, 4 figure
Ectopic mineralization in heart valves: New insights from in vivo and in vitro procalcific models and promising perspectives on noncalcifiable bioengineered valves
Ectopic calcification of native and bioprosthetic heart valves represents a major public health
problem causing severe morbidity and mortality worldwide. Valve procalcific degeneration is known to be
caused mainly by calcium salt precipitation onto membranes of suffering non-scavenged cells and dead-cellderived
products acting as major hydroxyapatite nucleators. Although etiopathogenesis of calcification in
native valves is still far from being exhaustively elucidated, it is well known that bioprosthesis mineralization
may be primed by glutaraldehyde-mediated toxicity for xenografts, cryopreservation-related damage for
allografts and graft immune rejection for both. Instead, mechanical valves, which are free from calcification,
are extremely thrombogenic, requiring chronic anticoagulation therapies for transplanted patients. Since
surgical substitution of failed valves is still the leading therapeutic option, progressive improvements in tissue
engineering techniques are crucial to attain readily available valve implants with good biocompatibility,
proper functionality and long-term durability in order to meet the considerable clinical demand for valve
substitutes. Bioengineered valves obtained from acellular non-valvular scaffolds or decellularized native
valves are proving to be a compelling alternative to mechanical and bioprosthetic valve implants, as they
appear to permit repopulation by the host\u2019s own cells with associated tissue remodelling, growth and repair,
besides showing less propensity to calcification and adequate hemodynamic performances. In this review,
insights into valve calcification onset as revealed by in vivo and in vitro procalcific models are updated as well
as advances in the field of valve bioengineering
X-Ray Detection of Transient Magnetic Moments Induced by a Spin Current in Cu
We have used a MHz lock-in x-ray spectro-microscopy technique to directly
detect changes of magnetic moments in Cu due to spin injection from an adjacent
Co layer. The elemental and chemical specificity of x-rays allows us to
distinguish two spin current induced effects. We detect the creation of
transient magnetic moments of on Cu atoms
within the bulk of the 28 nm thick Cu film due to spin-accumulation. The moment
value is compared to predictions by Mott's two current model. We also observe
that the hybridization induced existing magnetic moments on Cu interface atoms
are transiently increased by about 10% or .
This reveals the dominance of spin-torque alignment over Joule heat induced
disorder of the interfacial Cu moments during current flow
Performance of a Y-Ba-Cu-O superconducting filter/GaAs low noise amplifier hybrid circuit
A superconducting 7.3 GHz two-pole microstrip bandpass filter and a GaAs low noise amplifier (LNA) were combined into an active circuit and characterized at liquid nitrogen temperatures. This superconducting/semiconducting circuit's performance was compared to a gold filter/GaAs LNA hybrid circuit. The superconducting filter/GaAs LNA hybrid circuit showed higher gain and lower noise figure than its gold counterpart
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