Marshall Stoneham

The strange magnetism of oxides and carbons

Magnetism is not usually expected in simple sp oxides like MgO or in carbons like graphite. Yet basic intrinsic defects in these systems can be magnetic in ways that seem to be shared by more complex oxides. A second puzzle comes from reports of possible room temperature ferromagnetism in simple oxides, where experiments are not always in agreement. This paper discusses what determines whether point defects like cation vacancies in oxides have magnetic or non-magnetic ground states. It also discusses the possible connections between point defect ground states and oxide ferromagnetism. The connectivity issue raises questions about possible diffuse states in nanocrystalline oxides, several possibilities being outlined. These ideas raise the further possibility that the magnetism might be written in these oxides at the nanoscale, perhaps using atomic force microscopy.

Electrostatic energy calculation for the interpretation of scanning probe microscopy experiments

We discuss the correct expression for the classical electrostatic energy used while analysing scanning probe microscopy (SPM) experiments if either a conducting tip or a substrate or both are used in the experiment. For this purpose a general system consisting of an arbitrary arrangement of finite metallic conductors at fixed potentials (maintained by external sources) and a distribution of point charges in free space are considered using classical electrostatics. We stress the crucial importance of incorporating into the energy the contribution coming from the external sources (the `battery). Using the Green function of the Laplace equation, we show in a very general case that the potential energy of point charges which are far away from metals is equally shared by their direct interaction and the polarization interaction due to charge induced in metals by the remote charges (the image interaction). When the charges are located close to the metals, there is an additional negative term in the energy entirely due to image interaction. The exact Hamiltonian of a quantum system interacting classically with polarized metal conductors is derived and its application in the Hartree-Fock and the density functional theories is briefly discussed. As an illustration of the theory, we consider an interaction of several point charges with a metal plane and a spherical tip, based on the set-up of a real SPM experiment. We show the significance of the image interaction for the force imposed on the tip.

Materials science. Salting the surface.

When one type of material is grown on a very different one, the electrostatic interactions between them become important. These interactions offer new ways to control structure on the nanoscale.

Where are the Gaps

Reading a Handbook like this gives a vivid picture of the enormous vigour and power of materials modelling. One is tempted to believe that we can answer all the questions materials technology might pose. Even if that were partly true, we should be identifying just what we do not know how to do. Some gaps will be depend on new hardware and software, especially when modelling quantum systems. Some gaps will be recognised only after some social or technological change has brought them into focus. Among the developments likely to stimulate innovation could be novel nanoelectronics, or the fields where physics meets biology. Still further gaps exist because we have been slaves to fashion, and have been drawn away from unpopular (roughly translating as “too difficult”) fields; examples might include excited state spectroscopy, or electrical breakdown.

Materials and the Environment: Eco-informed material choice

This book is the first devoted exclusively to the environmental aspects of materials, a core subject area for undergraduate students in several engineering disciplines

Was Schön ever right

A few years ago, Jan Hendrik Schon impressed the world of science with exciting ideas and amazing experiments. Those experiments are believed no more. But were we wrong to admire the ideas at the time, or have we been too ready to turn our backs on novel ideas because they are associated with misconduct?

Modeling of the human enamel laser ablation process at the mesoscopic scale

A mesoscopic simulation of the process of human enamel laser ablation by Er:YAG and CO2 lasers is being developed using the finite element method, taking into account the complex structure and chemical composition of this material. A geometric model that allows studying in detail the temperature, stress and displacement distribution within a few enamel rods is presented. The heat generation that takes place inside the enamel at the centre of the laser spot, caused by a non-ablative laser pulse emitted by CO2 and Er:YAG lasers, was simulated. The sensitivity of our model to the estimated material parameters was studied. Temperature, displacement and stress distribution maps obtained for both lasers are presented. These preliminary results suggest that the temperature distribution across the enamel rods is different in the two situations considered; thermally induced stresses in the material are higher in the regions that are richer in hydroxyapatite (HA), and the higher displacements are observed in the regions that are rich in water. The rod tails inside enamel present higher stresses in the direction perpendicular to the surface of enamel than the ones that are created at the surface of our simulated structure. We conclude that the mesostructure plays a crucial role in the accurate modelling of dental laser ablation.

Making sense of scent

A new angle on our sense of smell, probably the most overlooked of the senses, shows how subtle quantum phenomena can be.

Materials for a life story

Robert Cahns memoirs recount the experiences of a young scientist during the emergence of materials science after World War II, and extol the value of scientific friendships and their international nature

Mesoscopic study of the electronic properties of thin polymer films

Conjugated polymers are very interesting for light emitting diodes. Further improvement in device performance requires a better understanding on the correlation between the polymer structure and device characteristics. A mesoscopic study using a generalised Monte Carlo method of bipolar charge transport in thin poly( p -phenylene-vinylene) (PPV) films is presented in this paper. We show that energy and spatial molecular disorder have a serious influence on migration of charge carriers within the polymer layer. The transfer of carriers between two polymer chains is made dependent on the chemical potential difference as well as on other features such as the distance between both molecules involved and the direction of the electric field. The purpose of the present work is to clarify the effects of local energy and polymer structural disorder on current flow, trapping and recombination on polymer based devices.