Electrical transport properties of nanostructured ferromagnetic perovskite oxides La_0.67Ca_0.33MnO_3 and La_0.5Sr_0.5CoO_3 at low temperatures (5 K > T >0.3 K) and high magnetic field

We report a comprehensive study of the electrical and magneto-transport properties of nanocrystals of La_0.67Ca_0.33MnO_3 (LCMO) (with size down to 15 nm) and La_0.5Sr_0.5CoO_3 (LSCO) (with size down to 35 nm) in the temperature range 0.3 K to 5 K and magnetic fields upto 14 T. The transport, magnetotransport and non-linear conduction (I-V curves) were analysed using the concept of Spin Polarized Tunnelling in the presence of Coulomb blockade. The activation energy of transport, \Delta, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (\chi) and the height of the tunnelling barrier (\Phi_B). The magnetotransport data were used to find out the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the MR contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and the MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to the barrier height persists. The idea of inter-grain tunnelling has been validated by direct measurements of the non-linear I-V data in this temperature range and the I-V data was found to be strongly dependent on magnetic field. We made the important observation that a gap like feature (with magnitude ~ E_C, the Coulomb charging energy) shows up in the conductance g(V) at low bias for the systems with smallest nanocrystal size at lowest temperatures (T < 0.7 K). The gap closes as the magnetic field and the temperature are increased.Comment: 13 figure

Remarks on hard Lefschetz conjectures on Chow groups

We propose two conjectures of Hard Lefschetz type on Chow groups and prove them for some special cases. For abelian varieties, we shall show they are equivalent to well-known conjectures of Beauville and Murre.Comment: to appear in Sciences in China, Ser. A Mathematic

Motion primitives and 3D path planning for fast flight through a forest

This paper presents two families of motion primitives for enabling fast, agile flight through a dense obstacle field. The first family of primitives consists of a time-delay dependent 3D circular path between two points in space and the control inputs required to fly the path. In particular, the control inputs are calculated using algebraic equations which depend on the flight parameters and the location of the waypoint. Moreover, the transition between successive maneuver states, where each state is defined by a unique combination of constant control inputs, is modeled rigorously as an instantaneous switch between the two maneuver states following a time delay which is directly related to the agility of the robotic aircraft. The second family consists of aggressive turn-around (ATA) maneuvers which the robot uses to retreat from impenetrable pockets of obstacles. The ATA maneuver consists of an orchestrated sequence of three sets of constant control inputs. The duration of the first segment is used to optimize the ATA for the spatial constraints imposed by the turning volume. The motion primitives are validated experimentally and implemented in a simulated receding horizon control (RHC)-based motion planner. The paper concludes with inverse-design pointers derived from the primitives

Aerodynamic Sensing for a Fixed Wing UAS Operating at High Angles of Attack

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97104/1/AIAA2012-4416.pd

Phenomenological constraints on Lemaitre-Tolman-Bondi cosmological inhomogeneities from solar system dynamics

We, first, analytically work out the long-term, i.e. averaged over one orbital revolution, perturbations on the orbit of a test particle moving in a local Fermi frame induced therein by the cosmological tidal effects of the inhomogeneous Lemaitre-Tolman-Bondi (LTB) model. The LTB solution has recently attracted attention, among other things, as a possible explanation of the observed cosmic acceleration without resorting to dark energy. Then, we phenomenologically constrain both the parameters K_1 = -\ddot R/R and K_2 = -\ddot R^'/R^' of the LTB metric in the Fermi frame by using different kinds of solar system data. The corrections $\Delta\dot\varpi$ to the standard Newtonian/Einsteinian precessions of the perihelia of the inner planets recently estimated with the EPM ephemerides, compared to our predictions for them, yield K_1 = (4+8) 10^-26 s^-2, K_2 = (3+7) 10^-23 s^-2. The residuals of the Cassini-based Earth-Saturn range, compared with the numerically integrated LTB range signature, allow to obtain K_1/2 = 10^-27 s^-2. The LTB-induced distortions of the orbit of a typical object of the Oort cloud with respect to the commonly accepted Newtonian picture, based on the observations of the comet showers from that remote region of the solar system, point towards K_1/2 <= 10^-30-10^-32 s^-2. Such figures have to be compared with those inferred from cosmological data which are of the order of K1 \approx K2 = -4 10^-36 s^-2.Comment: LaTex2e, 18 pages, 3 tables, 3 figures. Minor changes. Reference added. Accepted by Journal of Cosmology and Astroparticle Physics (JCAP

Closed-Loop Perching and Spatial Guidance Laws for Bio-Inspired Articulated Wing MAV

This paper presents the underlying theoretical developments and successful experimental demonstrations of perching of an aerial robot. The open-loop lateral-directional dynamics of the robot are inherently unstable because it lacks a vertical tail for agility, similar to birds. A unique feature of this robot is that it uses wing articulation for controlling the flight path angle as well as the heading. New guidance algorithms with guaranteed stability are obtained by rewriting the flight dynamic equations in the spatial domain rather than as functions of time, after which dynamic inversion is employed. It is shown that nonlinear dynamic inversion naturally leads to proportional-integral-derivative (PID) controllers, thereby providing an exact method for tuning the gains. The effectiveness of the proposed bio-inspired robot design and its novel closed-loop perching controller has been successfully demonstrated with perched landings on a human hand

Inhibition of Tau Aggregation by Three Aspergillus nidulans Secondary Metabolites: 2,ω-Dihydroxyemodin, Asperthecin, and Asperbenzaldehyde

This is the published version. Copyright 2014 George Theime Verlag. All rights reserved.The aggregation of the microtubule-associated protein tau is a significant event in many neurodegenerative diseases including Alzheimerʼs disease. The inhibition or reversal of tau aggregation is therefore a potential therapeutic strategy for these diseases. Fungal natural products have proven to be a rich source of useful compounds having wide varieties of biological activity. We have screened Aspergillus nidulans secondary metabolites containing aromatic ring structures for their ability to inhibit tau aggregation in vitro using an arachidonic acid polymerization protocol and the previously identified aggregation inhibitor emodin as a positive control. While several compounds showed some activity, 2,ω-dihydroxyemodin, asperthecin, and asperbenzaldehyde were potent aggregation inhibitors as determined by both a filter trap assay and electron microscopy. In this study, these three compounds were stronger inhibitors than emodin, which has been shown in a prior study to inhibit the heparin induction of tau aggregation with an IC50 of 1–5 µM. Additionally, 2,ω-dihydroxyemodin, asperthecin, and asperbenzaldehyde reduced, but did not block, tau stabilization of microtubules. 2,ω-Dihydroxyemodin and asperthecin have similar structures to previously identified tau aggregation inhibitors, while asperbenzaldehyde represents a new class of compounds with tau aggregation inhibitor activity. Asperbenzaldehyde can be readily modified into compounds with strong lipoxygenase inhibitor activity, suggesting that compounds derived from asperbenzaldehyde could have dual activity. Together, our data demonstrates the potential of 2,ω-dihydroxyemodin, asperthecin, and asperbenzaldehyde as lead compounds for further development as therapeutics to inhibit tau aggregation in Alzheimerʼs disease and neurodegenerative tauopathies

An Improved Calculation of the Non-Gaussian Halo Mass Function

The abundance of collapsed objects in the universe, or halo mass function, is an important theoretical tool in studying the effects of primordially generated non-Gaussianities on the large scale structure. The non-Gaussian mass function has been calculated by several authors in different ways, typically by exploiting the smallness of certain parameters which naturally appear in the calculation, to set up a perturbative expansion. We improve upon the existing results for the mass function by combining path integral methods and saddle point techniques (which have been separately applied in previous approaches). Additionally, we carefully account for the various scale dependent combinations of small parameters which appear. Some of these combinations in fact become of order unity for large mass scales and at high redshifts, and must therefore be treated non-perturbatively. Our approach allows us to do this, and to also account for multi-scale density correlations which appear in the calculation. We thus derive an accurate expression for the mass function which is based on approximations that are valid over a larger range of mass scales and redshifts than those of other authors. By tracking the terms ignored in the analysis, we estimate theoretical errors for our result and also for the results of others. We also discuss the complications introduced by the choice of smoothing filter function, which we take to be a top-hat in real space, and which leads to the dominant errors in our expression. Finally, we present a detailed comparison between the various expressions for the mass functions, exploring the accuracy and range of validity of each.Comment: 28 pages, 13 figures; v2: text reorganized and some figured modified for clarity, results unchanged, references added. Matches version published in JCA

Back-reaction and effective acceleration in generic LTB dust models

We provide a thorough examination of the conditions for the existence of back-reaction and an "effective" acceleration (in the context of Buchert's averaging formalism) in regular generic spherically symmetric Lemaitre-Tolman-Bondi (LTB) dust models. By considering arbitrary spherical comoving domains, we verify rigorously the fulfillment of these conditions expressed in terms of suitable scalar variables that are evaluated at the boundary of every domain. Effective deceleration necessarily occurs in all domains in: (a) the asymptotic radial range of models converging to a FLRW background, (b) the asymptotic time range of non-vacuum hyperbolic models, (c) LTB self-similar solutions and (d) near a simultaneous big bang. Accelerating domains are proven to exist in the following scenarios: (i) central vacuum regions, (ii) central (non-vacuum) density voids, (iii) the intermediate radial range of models converging to a FLRW background, (iv) the asymptotic radial range of models converging to a Minkowski vacuum and (v) domains near and/or intersecting a non-simultaneous big bang. All these scenarios occur in hyperbolic models with negative averaged and local spatial curvature, though scenarios (iv) and (v) are also possible in low density regions of a class of elliptic models in which local spatial curvature is negative but its average is positive. Rough numerical estimates between -0.003 and -0.5 were found for the effective deceleration parameter. While the existence of accelerating domains cannot be ruled out in models converging to an Einstein de Sitter background and in domains undergoing gravitational collapse, the conditions for this are very restrictive. The results obtained may provide important theoretical clues on the effects of back-reaction and averaging in more general non-spherical models.Comment: Final version accepted for publication in Classical and Quantum Gravity. 47 pages in IOP LaTeX macros, 12 pdf figure