Ferromagnetic materials in the zinc-blende structure

New materials are currently sought for use in spintronics applications. Ferromagnetic materials with half metallic properties are valuable in this respect. Here we present the electronic structure and magnetic properties of binary compounds consisting of 3d transition metals and group V elements viz. P, Sb and As in the zinc-blende structure. We demonstrate that compounds of V, Cr and Mn show half metallic behavior for appropriate lattice constants. By comparing the total energies in the ferromagnetic and antiferromagnetic structures, we have ascertained that the ferromagnetic phase is stable over the antiferromagnetic one. Of the different compounds studied, the Cr based systems exhibit the strongest interatomic exchange interactions, and are hence predicted to have the highest critical temperatures. Also, we predict that VAs under certain growth conditions should be a semiconducting ferromagnet. Moreover, critical temperatures of selected half metallic compounds have been estimated from mean field theory and Monte Carlo simulations using parameters obtained from a {\it ab-initio} non-collinear, tight binding linearized muffin-tin orbital method. From a simple model, we calculate the reflectance from an ideal MnAs/InAs interface considering the band structures of MnAs and InAs. Finally we present results on the relative stabilities of MnAs and CrSb compounds in the NiAs and zinc-blende structures, and suggest a parameter space in substrate lattice spacings for when the zinc-blende structure is expected to be stable.Comment: 7 pages, 6 figure

Two dimensional Sen connections and quasi-local energy-momentum

The recently constructed two dimensional Sen connection is applied in the problem of quasi-local energy-momentum in general relativity. First it is shown that, because of one of the two 2 dimensional Sen--Witten identities, Penrose's quasi-local charge integral can be expressed as a Nester--Witten integral.Then, to find the appropriate spinor propagation laws to the Nester--Witten integral, all the possible first order linear differential operators that can be constructed only from the irreducible chiral parts of the Sen operator alone are determined and examined. It is only the holomorphy or anti-holomorphy operator that can define acceptable propagation laws. The 2 dimensional Sen connection thus naturally defines a quasi-local energy-momentum, which is precisely that of Dougan and Mason. Then provided the dominant energy condition holds and the 2-sphere S is convex we show that the next statements are equivalent: i. the quasi-local mass (energy-momentum) associated with S is zero; ii.the Cauchy development $D(\Sigma)$ is a pp-wave geometry with pure radiation ($D(\Sigma)$ is flat), where $\Sigma$ is a spacelike hypersurface whose boundary is S; iii. there exist a Sen--constant spinor field (two spinor fields) on S. Thus the pp-wave Cauchy developments can be characterized by the geometry of a two rather than a three dimensional submanifold.Comment: 20 pages, Plain Tex, I

Quasi-local mass in the covariant Newtonian space-time

In general relativity, quasi-local energy-momentum expressions have been constructed from various formulae. However, Newtonian theory of gravity gives a well known and an unique quasi-local mass expression (surface integration). Since geometrical formulation of Newtonian gravity has been established in the covariant Newtonian space-time, it provides a covariant approximation from relativistic to Newtonian theories. By using this approximation, we calculate Komar integral, Brown-York quasi-local energy and Dougan-Mason quasi-local mass in the covariant Newtonian space-time. It turns out that Komar integral naturally gives the Newtonian quasi-local mass expression, however, further conditions (spherical symmetry) need to be made for Brown-York and Dougan-Mason expressions.Comment: Submit to Class. Quantum Gra

Reduction of requirement for leg vascular surgery during long-term treatment of claudicant patients with ticlopidine: Results from the swedish ticlopidine multicentre study (STIMS)

Objective:To study the effect of long-term treatment of the platelet inhibitor ticlopidine as secondary prevention against the need of vascular surgery in patients with intermittent claudication.Design:The Swedish Ticlopidine Multicentre Study (STIMS), was conducted in six medical and surgical clinics of university hospitals in Sweden.Methods:687 claudicants were randomised to ticlopidine 250mg bd or placebo and vascular surgery events were recorded prospectively over a 7-year period. Cox proportional hazards models of risk for leg vascular surgery were constructed using drug treatment and 11 putative risk factors for vascular disease as covariates. Surgical event-free survivals were compared by Kaplan-Meier analysis.Results:The overall rate of first operations was 2.4% per annum. More than half of these operations were in the aortoiliac region. One-quarter of patients operated during the period required further operations but amputation was rare. Ticlopidine treatment reduced the need for vascular reconstructive surgery by about half, both in intention-to-treat and on-treatment analyses (unadjusted relative risks 0.486, 95% CI 0.317–0.745: p < 0.001; 0.493, 95% CI 0.290–0.841: p < 0.01, respectively). In Cox model analysis only male sex was confirmed as a risk factor for surgery. Previous peripheral arterial surgery was the strongest predictor of the need for surgery. None of the risk factors examined interacted statistically with the effect of treatment with ticlopidine.Conclusion:In patients with intermittent claudication it seems possible to prevent the need for future vascular surgery by the use of platelet inhibition with ticlopidine

Local Haemodynamic Changes During Carotid Endarterectomy—The Influence on Cerebral Oxygenation

AbstractObjectives. To characterize carotid bifurcation haemodynamics and cerebral oxygenation during clamping and at reperfusion after carotid endarterectomy (CEA).Materials and methods. Sixty-two patients with a symptomatic high-grade stenosis of the internal carotid artery (ICA), who underwent CEA under general anaesthesia, were studied prospectively. Measurements of stump-pressure, volume flow (transit time flowmetry) and changes in cerebral oxygenation (near-infrared spectroscopy (NIRS)) were performed. Selective shunting was based on stump pressure only.Results. Stump pressure correlated with both ICA flow before clamping (r=0.45; p=0.03) and changes in cerebral oxygenation (rSO2) during clamping (r=0.61; p=0.002), the latter was reversed by shunt placement. ICA flow before clamping also correlated with changes in rSO2 during clamping (r=0.41; p=0.01).Conclusion. Measurements with transit time flowmetry and cerebral oximetry are technically easy and help to determine the need for selective shunting during CEA. High ICA flow before clamping in combination with a low stump pressure usually indicates the need for a shunt. Volume flow measurements may also be useful in the quality assessment of the CEA

Intermodulation electrostatic force microscopy for imaging surface photo-voltage

We demonstrate an alternative to Kelvin Probe Force Microscopy for imaging surface potential. The open-loop, single-pass technique applies a low-frequency AC voltage to the atomic force microscopy tip while driving the cantilever near its resonance frequency. Frequency mixing due to the nonlinear capacitance gives intermodulation products of the two drive frequencies near the cantilever resonance, where they are measured with high signal to noise ratio. Analysis of this intermodulation response allows for quantitative reconstruction of the contact potential difference. We derive the theory of the method, validate it with numerical simulation and a control experiment, and we demonstrate its utility for fast imaging of the surface photo-voltage on an organic photo-voltaic material.Comment: 4 pages, 3 figures, peer-reviewed, preprin

Two dimensional Sen connections in general relativity

The two dimensional version of the Sen connection for spinors and tensors on spacelike 2-surfaces is constructed. A complex metric $\gamma_{AB}$ on the spin spaces is found which characterizes both the algebraic and extrinsic geometrical properties of the 2-surface $\$$. The curvature of the two dimensional Sen operator $\Delta_e$ is the pull back to $\$$ of the anti-self-dual part of the spacetime curvature while its `torsion' is a boost gauge invariant expression of the extrinsic curvatures of $\$$. The difference of the 2 dimensional Sen and the induced spin connections is the anti-self-dual part of the `torsion'. The irreducible parts of $\Delta_e$ are shown to be the familiar 2-surface twistor and the Weyl--Sen--Witten operators. Two Sen--Witten type identities are derived, the first is an identity between the 2 dimensional twistor and the Weyl--Sen--Witten operators and the integrand of Penrose's charge integral, while the second contains the `torsion' as well. For spinor fields satisfying the 2-surface twistor equation the first reduces to Tod's formula for the kinematical twistor.Comment: 14 pages, Plain Tex, no report numbe

The Chevreton Tensor and Einstein-Maxwell Spacetimes Conformal to Einstein Spaces

In this paper we characterize the source-free Einstein-Maxwell spacetimes which have a trace-free Chevreton tensor. We show that this is equivalent to the Chevreton tensor being of pure-radiation type and that it restricts the spacetimes to Petrov types \textbf{N} or \textbf{O}. We prove that the trace of the Chevreton tensor is related to the Bach tensor and use this to find all Einstein-Maxwell spacetimes with a zero cosmological constant that have a vanishing Bach tensor. Among these spacetimes we then look for those which are conformal to Einstein spaces. We find that the electromagnetic field and the Weyl tensor must be aligned, and in the case that the electromagnetic field is null, the spacetime must be conformally Ricci-flat and all such solutions are known. In the non-null case, since the general solution is not known on closed form, we settle with giving the integrability conditions in the general case, but we do give new explicit examples of Einstein-Maxwell spacetimes that are conformal to Einstein spaces, and we also find examples where the vanishing of the Bach tensor does not imply that the spacetime is conformal to a $C$-space. The non-aligned Einstein-Maxwell spacetimes with vanishing Bach tensor are conformally $C$-spaces, but none of them are conformal to Einstein spaces.Comment: 22 pages. Corrected equation (12

Dynamical laws of superenergy in General Relativity

The Bel and Bel-Robinson tensors were introduced nearly fifty years ago in an attempt to generalize to gravitation the energy-momentum tensor of electromagnetism. This generalization was successful from the mathematical point of view because these tensors share mathematical properties which are remarkably similar to those of the energy-momentum tensor of electromagnetism. However, the physical role of these tensors in General Relativity has remained obscure and no interpretation has achieved wide acceptance. In principle, they cannot represent {\em energy} and the term {\em superenergy} has been coined for the hypothetical physical magnitude lying behind them. In this work we try to shed light on the true physical meaning of {\em superenergy} by following the same procedure which enables us to give an interpretation of the electromagnetic energy. This procedure consists in performing an orthogonal splitting of the Bel and Bel-Robinson tensors and analysing the different parts resulting from the splitting. In the electromagnetic case such splitting gives rise to the electromagnetic {\em energy density}, the Poynting vector and the electromagnetic stress tensor, each of them having a precise physical interpretation which is deduced from the {\em dynamical laws} of electromagnetism (Poynting theorem). The full orthogonal splitting of the Bel and Bel-Robinson tensors is more complex but, as expected, similarities with electromagnetism are present. Also the covariant divergence of the Bel tensor is analogous to the covariant divergence of the electromagnetic energy-momentum tensor and the orthogonal splitting of the former is found. The ensuing {\em equations} are to the superenergy what the Poynting theorem is to electromagnetism. See paper for full abstract.Comment: 27 pages, no figures. Typos corrected, section 9 suppressed and more acknowledgments added. To appear in Classical and Quantum Gravit