arXiv subjects

M. C. Payne

Publications and source records attributed to M. C. Payne.

15 recordsLinked to original sources

arXiv2020

Interaction between Surface Waves on Wire Lines

This paper investigates the coupling properties between surface waves on parallel wires. Finite Element Method (FEM) based and analytical models are developed for both single wire Sommerfeld and Goubau lines. Starting with the Sommerfeld type wave, we derive the analytical expression based on the assumption that the two-wire surface wave is a superposition of the two surface waves on the individual wires. Models are validated via measurements and a comparison study conducted between the analytic and the FEM-based computations for coupled Sommerfeld type lines. We then investigate the coupling between two Goubau lines with the FEM model. The measurement and calculations show remarkable agreement. The FEM-based and the analytical models match remarkably well too. The results exhibit new properties of favourable effects on surface waves propagation over multiple conductors. The short range behaviour of the coupled wires and consequently the existence of an optimum separation of coupled wires, is one of the most significant findings of this paper. We comment on the relevance of our results especially in relation to applications of high bandwidth demands and immanent cross-coupling effects.

physics.class-ph↗

arXiv2018

First-principles density-functional calculations using localized spherical-wave basis sets

We present a detailed study of the use of localized spherical-wave basis sets, first introduced in the context of linear-scaling, in first-principles density-functional calculations. Several parameters that control the completeness of this basis set are fully investigated on systems such as molecules and bulk crystalline silicon. We find that the results are in good agreement with those obtained using the extended plane-wave basis set. Since the spherical-wave basis set is accurate, easy to handle, relatively small, and can be systematically improved, we expect it to be of use in other applications.

physics.chem-ph↗

arXiv2011

Accurate ionic forces and geometry optimisation in linear scaling density-functional theory with local orbitals

Linear scaling methods for density-functional theory (DFT) simulations are formulated in terms of localised orbitals in real-space, rather than the delocalised eigenstates of conventional approaches. In local-orbital methods, relative to conventional DFT, desirable properties can be lost to some extent, such as the translational invariance of the total energy of a system with respect to small displacements and the smoothness of the potential energy surface. This has repercussions for calculating accurate ionic forces and geometries. In this work we present results from \textsc{onetep}, our linear scaling method based on localised orbitals in real-space. The use of psinc functions for the underlying basis set and on-the-fly optimisation of the localised orbitals results in smooth potential energy surfaces that are consistent with ionic forces calculated using the Hellmann-Feynman theorem. This enables accurate geometry optimisation to be performed. Results for surface reconstructions in silicon are presented, along with three example systems demonstrating the performance of a quasi-Newton geometry optimisation algorithm: an organic zwitterion, a point defect in an ionic crystal, and a semiconductor nanostructure.

cond-mat.mtrl-sci↗

arXiv2005

All possible bipartite positive-operator-value measurements of two-photon polarization states

Here we propose an implementation of all possible Positive Operator Value Measures (POVMs) of two-photon polarization states. POVMs are the most general class of quantum measurements. Our setup requires linear optics, Bell State measurements and an entangled three-photon ancilla state, which can be prepared separately and in advance (or 'off-line'). As an example we give the detailed settings for a simultaneous measurement of all four Bell States for an arbitrary two-photon polarization state, which is impossible with linear optics alone.

quant-ph↗

arXiv2005

Reply to "Comment on 'A linear optics implementation of weak values in Hardy's paradox'"

The comment by Lundeen et al. contains two criticisms of our proposal. While we agree that the state-preparation procedure could be replaced by a simpler setup as proposed by the authors of the comment, we do not agree with the authors on their second, and more important point regarding two-particle weak measurements. We believe this to be the result of a misunderstanding of our original paper.

quant-ph↗

arXiv2004

General implementation of all possible positive-operator-value measurements of single photon polarization states

Positive Operator Value Measures (POVMs) are the most general class of quantum measurements. We propose a setup in which all possible POVMs of a single photon polarization state (corresponding to all possible sets of two-dimensional Kraus operators) can be implemented easily using linear optics elements. This method makes it possible to experimentally realize any projective orthogonal, projective non-orthogonal or non-projective sets of any number of POVM operators. Furthermore our implementation only requires vacuum ancillas, and is deterministic rather than probabilistic. Thus it realizes every POVM with the correct set of output states. We give the settings required to implement two different well-known non-orthogonal projective POVMs.

quant-ph↗

arXiv2004

Linear optics implementation of weak values in Hardy's paradox

We propose an experimental setup for the implementation of weak measurements in the context of the gedankenexperiment known as Hardy's Paradox. As Aharonov et al. showed, these weak values form a language with which the paradox can be resolved. Our analysis shows that this language is indeed consistent and experimentally testable. It also reveals exactly how a combination of weak values can give rise to an apparently paradoxical result.

quant-ph↗

arXiv2004

Weak Measurement of the Arrival Times of Single Photons and Pairs of Entangled Photons

In this paper we propose a setup for the weak measurement of photon arrival time. It is found that the weak values of this arrival time can lie far away from the expectation value, and in principle also in regions forbidden by special relativity. We discuss in brief the implications of these results as well as their reconciliation with the principle of causality. Furthermore, an analysis of the weak arrival times of a pair of photons in a Bell state shows that these weak arrival times are correlated.

quant-ph↗

arXiv2003

Improving the convergence of defect calculations in supercells - an ab initio study of the neutral silicon vacancy

We present a systematic methodology for the accurate calculation of defect structures in supercells which we illustrate with a study of the neutral vacancy in silicon. This is a prototypical defect which has been studied extensively using ab initio methods, yet remarkably there is still no consensus about the energy or structure of this defect, or even whether the nearest neighbour atoms relax inwards or outwards. In this paper we show that the differences between previous calculations can be attributed to supercell convergence errors, and we demonstrate how to systematically reduce each such source of error. The various sources of scatter in previous theoretical studies are discussed and a new effect, that of supercell symmetry, is identified. It is shown that a consistent treatment of this effect is crucial to understanding the systematic effects of increasing the supercell size. This work therefore also presents the best converged ab initio study of the neutral silicon vacancy to date.

cond-mat.mtrl-sci↗

arXiv2001

Cubic boron nitride: an experimental and theoretical ELNES study

A comparison between experimental and theoretical electron Energy Loss Near Edge Structure (ELNES) of B and N K-edges in cubic boron nitride is presented. The electron energy loss spectra of cubic boron nitride particles were measured using a scanning transmission electron microscope. The theoretical calculation of the ELNES was performed within the framework of density functional theory including single particle core-hole effects. It is found that experimental and calculated ELNES of both the B and N K-edges in cubic boron nitride show excellent agreement.

cond-mat.mtrl-sci↗

arXiv1999

Thermal Contraction and Disordering of the Al(110) Surface

Al(110) has been studied for temperatures up to 900 K via ensemble density-functional molecular dynamics. The strong anharmonicity displayed by this surface results in a negative coefficient of thermal expansion, where the first interlayer distance decreases with increasing temperature. Very shallow channels of oscillation for the second-layer atoms in the direction perpendicular to the surface support this anomalous contraction, and provide a novel mechanism for the formation of adatom-vacancy pairs, preliminary to the disordering and premelting transition. Such characteristic behavior originates in the free-electron-gas bonding at a loosely packed surface.

cond-mat.mtrl-sci↗

arXiv1998

Microscopic mechanism for mechanical polishing of diamond (110) surfaces

Mechanically induced degradation of diamond, as occurs during polishing, is studied using total--energy pseudopotential calculations. The strong asymmetry in the rate of polishing between different directions on the diamond (110) surface is explained in terms of an atomistic mechanism for nano--groove formation. The post--polishing surface morphology and the nature of the polishing residue predicted by this mechanism are consistent with experimental evidence.

cond-mat.mtrl-sci↗

arXiv1997

Supercell technique for total-energy calculations of finite charged and polar systems

We study the behaviour of total-energy supercell calculations for dipolar molecules and charged clusters. Using a cutoff Coulomb interaction within the framework of a plane-wave basis set formalism, with all other aspects of the method (pseudopotentials, basis set, exchange-correlation functional) unchanged, we are able to assess directly the interaction effects present in the supercell technique. We find that the supercell method gives structures and energies in almost total agreement with the results of calculations for finite systems, even for molecules with large dipole moments. We also show that the performance of finite-grid calculations can be improved by allowing a degree of aliasing in the Hartree energy, and by using a reciprocal space definition of the cutoff Coulomb interaction.

cond-mat.mtrl-sci↗

arXiv1997

Ensemble density-functional theory for ab-initio molecular dynamics of metals and finite-temperature insulators

A new method is presented for performing first-principles molecular-dynamics simulations of systems with variable occupancies. We adopt a matrix representation for the one-particle statistical operator Gamma, to introduce a ``projected'' free energy functional G that depends on the Kohn-Sham orbitals only and that is invariant under their unitary transformations. The Liouville equation [ Gamma , H ] = 0 is always satisfied, guaranteeing a very efficient and stable variational minimization algorithm that can be extended to non-conventional entropic formulations or fictitious thermal distributions.

cond-mat.mtrl-sci↗

arXiv1995

Water Chemisorption and Reconstruction of the MgO Surface

The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with Delta E = -90.2kJ/mol. We conclude that protonation stabilizes the otherwise unstable (111) surface and that this, not the bare (001), is the most stable surface of MgO under ambient conditions.

mtrl-th↗