99 research outputs found
Bifurcation and chaos in spin-valve pillars in a periodic applied magnetic field
We study the bifurcation and chaos scenario of the macro-magnetization vector
in a homogeneous nanoscale-ferromagnetic thin film of the type used in
spin-valve pillars. The underlying dynamics is described by a generalized
Landau-Lifshitz-Gilbert (LLG) equation. The LLG equation has an especially
appealing form under a complex stereographic projection, wherein the
qualitative equivalence of an applied field and a spin-current induced torque
is transparent. Recently chaotic behavior of such a spin vector has been
identified by Zhang and Li using a spin polarized current passing through the
pillar of constant polarization direction and periodically varying magnitude,
owing to the spin-transfer torque effect. In this paper we show that the same
dynamical behavior can be achieved using a periodically varying applied
magnetic field, in the presence of a constant DC magnetic field and constant
spin current, which is technically much more feasible, and demonstrate
numerically the chaotic dynamics in the system for an infinitely thin film.
Further, it is noted that in the presence of a nonzero crystal anisotropy field
chaotic dynamics occurs at much lower magnitudes of the spin-current and DC
applied field.Comment: 8 pages, 7 figures. To appear in Chao
Sost and its paralog Sostdc1 coordinate digit number in a Gli3-dependent manner.
WNT signaling is critical in most aspects of skeletal development and homeostasis, and antagonists of WNT signaling are emerging as key regulatory proteins with great promise as therapeutic agents for bone disorders. Here we show that Sost and its paralog Sostdc1 emerged through ancestral genome duplication and their expression patterns have diverged to delineate non-overlapping domains in most organ systems including musculoskeletal, cardiovascular, nervous, digestive, reproductive and respiratory. In the developing limb, Sost and Sostdc1 display dynamic expression patterns with Sost being restricted to the distal ectoderm and Sostdc1 to the proximal ectoderm and the mesenchyme. While Sostdc1(-/-) mice lack any obvious limb or skeletal defects, Sost(-/-) mice recapitulate the hand defects described for Sclerosteosis patients. However, elevated WNT signaling in Sost(-/-); Sostdc1(-/-) mice causes misregulation of SHH signaling, ectopic activation of Sox9 in the digit 1 field and preaxial polydactyly in a Gli1- and Gli3-dependent manner. In addition, we show that the syndactyly documented in Sclerosteosis is present in both Sost(-/-) and Sost(-/-); Sostdc1(-/-) mice, and is driven by misregulation of Fgf8 in the AER, a region lacking Sost and Sostdc1 expression. This study highlights the complexity of WNT signaling in skeletal biology and disease and emphasizes how redundant mechanism and non-cell autonomous effects can synergize to unveil new intricate phenotypes caused by elevated WNT signaling
Genetic evidence that SOST inhibits WNT signaling in the limb
AbstractSOST is a negative regulator of bone formation, and mutations in human SOST are responsible for sclerosteosis. In addition to high bone mass, sclerosteosis patients occasionally display hand defects, suggesting that SOST may function embryonically. Here we report that overexpression of SOST leads to loss of posterior structures of the zeugopod and autopod by perturbing anterior–posterior and proximal–distal signaling centers in the developing limb. Mutant mice that overexpress SOST in combination with Grem1 and Lrp6 mutations display more severe limb defects than single mutants alone, while Sost−/− significantly rescues the Lrp6−/− skeletal phenotype, signifying that SOST gain-of-function impairs limb patterning by inhibiting the WNT signaling through LRP5/6
Power corrections in e+ e- --> pi+ pi-, K+ K- and B --> K pi, pi pi
CLEO-c measurements of the timelike form factors F_pi, F_K at \sqrt{s}=3.671
GeV provide a direct probe of power corrections (PC's) at energies near m_B.
PC's in F_pi, F_K and B \to K pi, pi pi are separated into perturbative and
soft parts. In F_pi, F_K the latter are \ge O(10) larger. A PC fit to the B \to
K pi, pi pi data also yields a \ge O(10) soft-to-perturbative hierarchy for the
QCD penguin PC's. Hence, both can be attributed to dominance of the soft-ovelap
between energetic (approximately) back-to-back collinear partons, and
consistency of the B\to K pi, pi pi fit with the Standard Model appears to be
naturally realized. The CP asymmetries S_{K_s pi^0}, C_{K_s pi^0} are well
determined, providing a clean test for new physics.Comment: 4 pages, 4 figures, version published in Eur. Phys. J. C; elaborated
on the connection between power corrections in e+e- ->M1 M2 and the B-> M1 M2
QCD penguin amplitudes; removed speculation that soft-overlaps are much
larger for PP than for VP and VV final states, this is not supported by a new
analysis in preparatio
Experimental Study on Strength Properties of Triple Blended Self-Compacting Concrete
Self-compacting concrete (SCC) is a flowing concrete mixture that can self-consolidate under its own weight, and it is one of the most significant advancements in concrete technology in the recent decade. SCC's very fluid character makes it ideal for use in challenging situations and in sections with crowded reinforcing. SCC, which was initially created in the late 1980s, has since expanded around the world, with an ever-growing variety of applications. Because of its unique features, SCC has the potential to significantly improve the quality of concrete buildings while also opening up new areas for concrete use. The addition of treated and untreated industrial byproducts, raw materials, and home wastes to SCC is becoming increasingly popular as a way to make it more durable and cost effective. This not only allows waste materials to be reused, but it also results in a eco-friendly environment. The Strength Characteristics of Self-Compacting Concrete were determined by an experimental investigation (SCC). The goal of this research is to see how Fly Ash (FA), Ground Granulated Blast Furnace Slag (GGBS), and Silica Fume (SF) can be used as cement substitutes and what influence they have on the fresh and hardened qualities of concrete. The investigation involves the notion of using a triple blend of Fly ash, GGBS, and Silica Fume to produce better concrete. This triple blend takes use of the favorable properties of Pozzolanic materials
Equatorial and related non-equilibrium states in magnetization dynamics of ferromagnets: Generalization of Suhl's spin-wave instabilities
We investigate the nonlinear dynamics underlying the evolution of a 2-D
nanoscale ferromagnetic film with uniaxial anisotropy in the presence of
perpendicular pumping. Considering the associated Landau-Lifshitz spin
evolution equation with Gilbert damping together with Maxwell equation for the
demagnetization field, we study the dynamics in terms of the stereographic
variable. We identify several new fixed points for suitable choice of external
field in a rotating frame of reference. In particular, we identify explicit
equatorial and related fixed points of the spin vector in the plane transverse
to the anisotropy axis when the pumping frequency coincides with the amplitude
of the static parallel field. We then study the linear stability of these novel
fixed points under homogeneous and spin wave perturbations and obtain a
generalized Suhl's instability criterion, giving the condition for exponential
growth of P-modes under spin wave perturbations. Two parameter phase diagrams
(in terms of amplitudes of static parallel and oscillatory perpendicular
magnetic fields) for stability are obtained, which differ qualitatively from
those for the conventional ferromagnetic resonance near thermal equilibrium and
are amenable to experimental tests.Comment: 23 pages, 5 figures, To appear in Physica
Spin-transfer torque induced reversal in magnetic domains
Using the complex stereographic variable representation for the macrospin,
from a study of the nonlinear dynamics underlying the generalized
Landau-Lifshitz(LL) equation with Gilbert damping, we show that the
spin-transfer torque is effectively equivalent to an applied magnetic field. We
study the macrospin switching on a Stoner particle due to spin-transfer torque
on application of a spin polarized current. We find that the switching due to
spin-transfer torque is a more effective alternative to switching by an applied
external field in the presence of damping. We demonstrate numerically that a
spin-polarized current in the form of a short pulse can be effectively employed
to achieve the desired macro-spin switching.Comment: 16 pages, 6 figure
Nonlinear Dynamics of Moving Curves and Surfaces: Applications to Physical Systems
The subject of moving curves (and surfaces) in three dimensional space (3-D)
is a fascinating topic not only because it represents typical nonlinear
dynamical systems in classical mechanics, but also finds important applications
in a variety of physical problems in different disciplines. Making use of the
underlying geometry, one can very often relate the associated evolution
equations to many interesting nonlinear evolution equations, including soliton
possessing nonlinear dynamical systems. Typical examples include dynamics of
filament vortices in ordinary and superfluids, spin systems, phases in
classical optics, various systems encountered in physics of soft matter, etc.
Such interrelations between geometric evolution and physical systems have
yielded considerable insight into the underlying dynamics. We present a
succinct tutorial analysis of these developments in this article, and indicate
further directions. We also point out how evolution equations for moving
surfaces are often intimately related to soliton equations in higher
dimensions.Comment: Review article, 38 pages, 7 figs. To appear in Int. Jour. of Bif. and
Chao
Sostdc1 deficiency accelerates fracture healing by promoting the expansion of periosteal mesenchymal stem cells
Loss of Sostdc1, a growth factor paralogous to Sost, causes the formation of ectopic incisors, fused molars, abnormal hair follicles, and resistance to kidney disease. Sostdc1 is expressed in the periosteum, a source of osteoblasts, fibroblasts and mesenchymal progenitor cells, which are critically important for fracture repair. Here, we investigated the role of Sostdc1 in bone metabolism and fracture repair. Mice lacking Sostdc1 (Sostdc1−/−) had a low bone mass phenotype associated with loss of trabecular bone in both lumbar vertebrae and in the appendicular skeleton. In contrast, Sostdc1−/− cortical bone measurements revealed larger bones with higher BMD, suggesting that Sostdc1 exerts differential effects on cortical and trabecular bone. Mid-diaphyseal femoral fractures induced in Sostdc1−/− mice showed that the periosteal population normally positive for Sostdc1 rapidly expands during periosteal thickening and these cells migrate into the fracture callus at 3 days post fracture. Quantitative analysis of mesenchymal stem cell (MSC) and osteoblast populations determined that MSCs express Sostdc1, and that Sostdc1−/− 5 day calluses harbor > 2-fold more MSCs than fractured wildtype controls. Histologically a fraction of Sostdc1-positive cells also expressed nestin and α-smooth muscle actin, suggesting that Sostdc1 marks a population of osteochondral progenitor cells that actively participate in callus formation and bone repair. Elevated numbers of MSCs in D5 calluses resulted in a larger, more vascularized cartilage callus at day 7, and a more rapid turnover of cartilage with significantly more remodeled bone and a thicker cortical shell at 21 days post fracture. These data support accelerated or enhanced bone formation/remodeling of the callus in Sostdc1−/− mice, suggesting that Sostdc1 may promote and maintain mesenchymal stem cell quiescence in the periosteum
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