J. L. Fry

Interlayer magnetic coupling in transition‐metal multilayered structures

The magnetic coupling of two layers of Fe through an intervening layer of Cr as a function of the Cr layer thickness has been computed. By including both RKKY and superexchangelike interactions between planes of Fe, and accounting for the surface roughness of the interfaces, the experimentally observed oscillations in the coupling between Fe layers as a function of Cr layer width could be reproduced. The predominance of antiferromagnetic coupling reflects the dominance of superexchange over RKKY exchange and is a consequence of the position of the Fermi level of Cr in a minimum between peaks in the density of states. This condition can be used to explain similar behavior in Co/Ru and to identify other possible transition metal systems for which this behavior may be observed.

Magnetic structure of bcc and fcc manganese

Self‐consistent band‐structure calculations are used to obtain the ferromagnetic moment as a function of lattice constant for bcc and fcc Mn. The ferromagnetic moment of bcc Mn is found to change discontinuously from a small to a large value as the lattice constant increases, while the fcc moment is found to change discontinuously from zero to a large value with increasing atomic volume. In the bcc case, the transition occurs inside a narrow double moment region in which the low spin and high spin states coexist. Information from magnetic susceptibility calculations on bcc and fcc Mn is used to predict whether the ground state is ferromagnetic or antiferromagnetic in certain ranges of lattice constant.

Annihilation of positrons trapped at the (100) and (111) surfaces of Si

Abstract We present results of theoretical studies of positron surface states and positron annihilation characteristics at the clean non reconstructed (100) and (111) surfaces of Si performed within the modified atomistic, superposition method. It is found that in the case of non reconstructed semiconductor surfaces, the positron surface state is localized mainly on the vacuum side of the topmost layer. The computed positron surface state energies E b at the (100) and (111) surfaces of Si are −2.81 and −2.69 eV. In addition, calculations of the positron work functions with respect to the vacuum for bulk Si(100) and Si(111) yielded 2.34 and 2.23 eV, respectively demonstrating the stability of positron surface state on these surfaces. The positron surface state lifetime as well as probabilities for a positron trapped in a surface state to annihilate with relevant core-level electrons are computed for both surfaces, and compared with available experimental data.

Application of the Feynman–Kac path integral method in finding excited states of quantum systems

Abstract Numerical methods of applying the Feynman-Kac path integral approach to quantum mechanics are presented. Themethods are demonstrated on simple quantum mechanical systems, including the hydrogen atom, the simple harmonic oscillator and infinite square wells. New analytic results for the Wiener integrals are obtained and compared with numerical results. A measure of the statistical uncertainty is introduced and rates of convergence are investigated. Implementation of the method on both serial and parallel computers is discussed

Magnetism in expanded 3d transition metals

For sufficiently large expansion of the lattice, transition metals will eventually exhibit ferromagnetism in accordance with Hund’s rule. This problem has been reexamined on the basis of calculations of the paramagneticsusceptibility to determine instabilities of the paramagnetic phase. Comparison of predictions using this method for the critical lattice constant for ferromagnetism are in good agreement with previous total energy calculations. However, this study also yields new predictions of antiferromagnetism for a range of lattice constants less than that for onset of ferromagnetism. The susceptibility is formulated in a multiband generalization of the Stoner approach with many‐body effects incorporated within the local‐density approximation in density functional theory. Slater–Koster band structures are employed which permit lattice constant variation to be realized through a relatively simple scaling scheme. Both many‐body effects and the distribution of primarily single‐electron states associated with Fermi surface nesting combine to produce the antiferromagnetic instability under expansion. Nesting is probably necessary for incommensurate antiferromagnetism, and its decrease in importance as the lattice constant continues to increase contributes to the incipient ferromagnetic instability.

Temperature dependence of transition‐metal magnetism

A first‐principles tight‐binding formulation of the paramagnetic spin susceptibility is developed to study the temperature dependence of the magnetic properties of transition metals. The formulation, which includes many‐body enhancements, has been successful in predicting magnetic ground‐state properties. The present study introduces a temperature‐dependent analytical tetrahedron method to perform Brillouin zone sums, and employs accurate Slater–Koster band structures for convenience. Critical temperatures and the temperature dependence of the spin‐density wave characterizing antiferromagnetism are determined from instabilities of the paramagnetic state toward magnetic order. The Neel temperature of Cr and the Curie temperature of Fe found in this way are in good agreement with measured values. Nonzero critical temperatures are also found for alternate cubic structures of some transition metals, further supporting the prediction of ferromagnetism in bcc Mn, and antiferromagnetism in fcc Mn and expanded bc...

Alternative pathways to antimatter containment

Abstract Antimatter containment is a gateway technology for future advancements in many areas. Immediate applications in propulsion, medicine, and instrumentation have already been envisioned and many others are yet to be considered. Key to this technological advance is identifying one or more pathways to achieve safe reliable containment of antimatter in sufficient quantities to be useful on an engineering and industrial scale. The goal of this paper is to review current approaches and discuss possible alternative pathways to antimatter containment. Specifically, this paper will address the possibility of designing a solid-state containment system that will safely hold antimatter in quantities dense enough to be of any engineering utility. A discussion of the current research, the needed engineering requirements, and a survey of current research is presented.

Positrons as probes of Si(100) surface with adsorbed hydrogen and oxygen

Abstract Positron annihilation induced Auger spectra from the Si(100) surface exposed to hydrogen and oxygen are analyzed by performing calculations of positron surface states and annihilation characteristics of surface trapped positrons. Positron binding energies and work functions are also computed. It is found that the adsorption of hydrogen and oxygen on the Si(100) surface leads to a displacement of the positron surface state wave function away from the substrate Si atoms. As a result of this displacement, the overlap of the positron wave function with Si core electrons and, consequently, the annihilation probability of Si core electrons reduce, in agreement with experimental data.

A model for the PAES Cu M23VV signal versus Cs coverage on the Cu(100) surface

Positron‐annihilation‐induced Auger‐electron spectroscopy (PAES) employs positrons trapped at the surface to create core‐holes and so initiate the Auger process in atoms in the topmost layer of the surface. This technique has already proved itself by providing the surface layer chemical composition of surface alloys and chemisorbed systems. Recent experimental investigations of the attenuation of the Cu M23VV Auger peak with Cs coverage on Cu (100) at 163 K using PAES revealed that the normalized peak intensity for Cu remains nearly constant at the clean surface value until the Cs coverage reaches approximately 0.7 physical monolayer at which point the peak intensity drops precipitously. We present a semi‐phenomenological analysis of this unusual behavior. Our model treats the positron as trapped in a double well potential in the direction perpendicular to the surface: one well is associated with the Cu substrate and the other with the Cs adsorbate. The sharp drop in the PAES intensity which occurs over a...

Studies of Positron‐Surface Interactions in Semiconductors Using Positron Annihilation Induced Auger Electron Spectroscopy

A surface sensitive technique, Positron‐Annihilation‐Induced Auger‐Electron Spectroscopy (PAES), is becoming a powerful tool available for studies of positron surface phenomena and characterization of semiconductor surfaces. In this paper the results of studies of Si(111) and GaAs(100) surfaces using PAES are analyzed by performing quantum mechanical calculations of positron surface states and annihilation characteristics for the reconstructed Si(111)‐(7×7) surface and for both As‐ and Ga‐rich (100) surfaces of GaAs with c(2×8), (2×4), and c(4×4) reconstructions. Estimates of the positron binding energy, work function, and annihilation characteristics reveal their sensitivity to surface reconstruction and chemical composition of the topmost layers of semiconductors. Calculations show that the positron is getting trapped at the corner hole sites of the reconstructed Si(111)‐(7×7) surface. It is shown that comparison of theoretical positron annihilation probabilities computed for different reconstructed GaA...