arXiv subjects

R. J. Cole

Publications and source records attributed to R. J. Cole.

3 recordsLinked to original sources

arXiv2014

Probing atomic environments in alloys by electron spectroscopy

In alloys exhibiting substitutional disorder, the variety of atomic environments manifests itself as a `disorder broadening' in their core level binding energy spectra. Disorder broadening can be measured experimentally, and in principle can be used to deduce information about specific atomic environments within a sample. However, progress in this endeavor is hampered by the lack of a model for this phenomenon which can treat complex systems. In this work we describe such a model. The model is used to elucidate the relationship between charge transfer, atomic environment, and disorder broadening in complex systems, with a focus on the problem of characterizing the interface quality of CuNi multilayers. We also validate the model against the results of ab initio electronic structure calculations. Several counterintuitive aspects of the disorder broadening phenomenon are uncovered, an understanding of which is essential for the correct interpretation of experimental results. For instance, it is shown that systems with inhomogeneous concentration profiles can exhibit disorder broadenings significantly larger than random alloys. Furthermore in some systems a `disorder narrowing' is even possible.

cond-mat.mtrl-sci↗

arXiv2013

Lattice charge models and core level shifts in disordered alloys

Differences in core level binding energies between atoms belonging to the same chemical species can be related to differences in their intra- and extra-atomic charge distributions, and differences in how their core holes are screened. With this in mind, we consider the charge-excess functional model (CEFM) for net atomic charges in alloys [E. Bruno et al., Phys. Rev. Lett. 91, 166401 (2003)]. We begin by deriving the CEFM energy function in order to elucidate the approximations which underpin this model. We thereafter consider the particular case of the CEFM in which the strength of the `local interactions' within all atoms are the same. We show that for binary alloys the ground state charges of this model can be expressed in terms of charge transfer between all pairs of unlike atoms analogously to the linear charge model [R. Magri et al., Phys. Rev. B 42, 11388 (1990)]. Hence the model considered is a generalization of the linear charge model for alloys containing more than two chemical species. We then determine the model's unknown `geometric factors' over a wide range of parameter space. These quantities are linked to the nature of charge screening in the model, and we illustrate that the screening becomes increasingly universal as the strength of the local interactions is increased. We then use the model to derive analytical expressions for various physical quantities, including the Madelung energy and the disorder broadening in the core level binding energies. These expressions are applied to ternary random alloys, for which it is shown that the Madelung energy and magnitude of disorder broadening are maximized at the composition at which the two species with the largest `electronegativity difference' are equal, while the remaining species having a vanishing concentration. This result is somewhat counterintuitive with regards to the disorder broadening since it does not...

cond-mat.mtrl-sci↗

arXiv2006

Quantum Logic of Semantic Space: An Exploratory Investigation of Context Effects in Practical Reasoning

This article is an exploratory account of the the non-monotonic behaviour of conceptual associations in the light of context. Computational approximations of conceptual space are furnished by semantic space models which are emerging from the fields of cognition and computational linguistics. Semantic space models not only provide a cognitively motivated basis to underpin human practical reasoning, but from a mathematical perspective, they are real-valued Hilbert spaces. This introduces the highly speculative prospect of formalizing aspects of human practical reasoning via quantum mechanics. This account focuses on how to formalize context effects in relation to concepts as well as keeping an eye on operational issues.

quant-ph↗