The Jahn-Teller instability with accidental degeneracy

The Jahn-Teller theorem predicts that orbitally degenerate complexes in solids can lower their energy by a distortion which lowers their symmetry. Exact degeneracy is not necessary for this instability; usually it suffices if the actual separation of the energy levels involved is less than the energy reduction which would result if they were exactly degenerate. We discuss various cases of accidental degeneracy in cubic and tetrahedral systems. With accidental degeneracy it is possible to get mixed distortions which involve both trigonal and tetragonal distortions. These have probably been observed for the negative vacancy in silicon, and possibly for the neutral vacancy in diamond. The theory is applied to both these cases, and it is compared with the earlier qualitative arguments of Watkins

The optical absorption of the neutral vacancy in diamond

Optical absorption by the neutral vacancy in diamond has been predicted to occur between the ground 1E state and the 1T2 excited state. Both these states are orbitally degenerate and should show Jahn Teller distortions. We have calculated the Huang-Rhys Factor for the transition (the fraction of the intensity in the zero phonon line) and the first and second moments of the absorption band. Also we discuss the response of the zero phonon line to externally applied stresses. Our model assumes that the vacancy interacts with just six modes, corresponding to the normal modes of the neighbours of the vacancy. The coupling of the electronic and nuclear motion is estimated from an LCAO model. The results are compared with observations of the GR1 band, which has been attributed to the vacancy. The predicted Huang-Rhys factor S ~ 3.7 and the first and second moments of the band support this identification. Further our model predicts that luminescence from this band should be unpolarised, as observed. However the predicted stress splitting of the zero phonon line differs from that observed. Possible explanations of this are discussed

Book reviews

Teaching/Communication/Extension/Profession,

ELECTRONIC STRUCTURE OF GE IN SIO2

It is argued that one-electron theory is insufficient to account for the origin of the observed spectra of Ge in SiO2 ( alpha -quartz) crystals. A simple model is employed to show that impurity states responsible for ESR spectra of SiO2:Ge are stabilised by many-electron polarisation effects associated with the Ge atom itself and its immediate oxygen neighbour

Band-edge problem in the theoretical determination of defect energy levels: the O vacancy in ZnO as a benchmark case

Calculations of formation energies and charge transition levels of defects routinely rely on density functional theory (DFT) for describing the electronic structure. Since bulk band gaps of semiconductors and insulators are not well described in semilocal approximations to DFT, band-gap correction schemes or advanced theoretical models which properly describe band gaps need to be employed. However, it has become apparent that different methods that reproduce the experimental band gap can yield substantially different results regarding charge transition levels of point defects. We investigate this problem in the case of the (+2/0) charge transition level of the O vacancy in ZnO, which has attracted considerable attention as a benchmark case. For this purpose, we first perform calculations based on non-screened hybrid density functionals, and then compare our results with those of other methods. While our results agree very well with those obtained with screened hybrid functionals, they are strikingly different compared to those obtained with other band-gap corrected schemes. Nevertheless, we show that all the different methods agree well with each other and with our calculations when a suitable alignment procedure is adopted. The proposed procedure consists in aligning the electron band structure through an external potential, such as the vacuum level. When the electron densities are well reproduced, this procedure is equivalent to an alignment through the average electrostatic potential in a calculation subject to periodic boundary conditions. We stress that, in order to give accurate defect levels, a theoretical scheme is required to yield not only band gaps in agreement with experiment, but also band edges correctly positioned with respect to such a reference potential

The quantum in your materials world

New ideas lead to new technologies, and new technologies demand new materials. Quantized matter - atoms - underpinned the 19th century chemical industry and quantized charge - the electron - is the basis of microelectronics. Atoms and electrons were once as exotic as the word ‘quantum’ is today, and just as hard to relate to the very real world of materials. Yet silicon technology spawned a whole series of materials innovations. There were the functional materials: ultrapure Si, strained Si/SiO2, low k materials, Cu and Al for interconnects, etc. There were passive device materials, such as heat sinks, packaging, and diffusion barriers. New materials were needed for fabrication: lithography optics, photoresists, and metalorganics. New fabrication methods, such as ion implantation, transformed dopant control. The public saw the fruits of this research: solid-state lasers for compact disc players, colorful casings and straps for watches, light-emitting diode and liquid crystal displays, etc. Quantum information processing (QIP) computing will offer a new list of materials

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

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