Global Optimization and the Energy Landscapes of Dzugutov Clusters

The global minima of clusters bound by a Dzugutov potential form non-compact polytetrahedral clusters mainly composed of interpenetrating and face-sharing 13-atom icosahedra. As the size increases, these icosahedral units first form linear arrays, then two-dimensional rings, then three-dimensional networks. Characterization of the energy landscapes of these clusters shows that they are particularly rough and generally exhibit a multiple-funnel topography. These results provide new insights into the structure and dynamics of bulk supercooled Dzugutov liquids and the form of the bulk phase diagram.Comment: 9 pages, 4 figure

Extended point defects in crystalline materials: Ge and Si

B diffusion measurements are used to probe the basic nature of self-interstitial 'point' defects in Ge. We find two distinct self-interstitial forms - a simple one with low entropy and a complex one with entropy ~30 k at the migration saddle point. The latter dominates diffusion at high temperature. We propose that its structure is similar to that of an amorphous pocket - we name it a 'morph'. Computational modelling suggests that morphs exist in both self-interstitial and vacancy-like forms, and are crucial for diffusion and defect dynamics in Ge, Si and probably many other crystalline solids

Decoupling of diffusion from structural relaxation and spatial heterogeneity in a supercooled simple liquid

We report a molecular dynamics simulation of a supercooled simple monatomic glass-forming liquid. It is found that the onset of the supercooled regime results in formation of distinct domains of slow diffusion which are confined to the long-lived icosahedrally structured clusters associated with deeper minima in the energy landscape. As these domains, possessing a low-dimensional geometry, grow with cooling and percolate below $T_c$, the critical temperature of the mode coupling theory, a sharp slowing down of the structural relaxation relative to diffusion is observed. It is concluded that this latter anomaly cannot be accounted for by the spatial variation in atomic mobility; instead, we explain it as a direct result of the configuration-space constraints imposed by the transient structural correlations. We also conjecture that the observed tendency for low-dimensional clustering may be regarded as a possible mechanism of fragility.Comment: To be published in PR

Single event burnout sensitivity of SiC and Si

Exposure to ionizing radiation has the potential to catastrophically modify the operation, and destroy, electronic components in microseconds. The electrification of aircraft necessitates the need to use the most power dense and lowest loss semiconductor devices available, and the increasing supply voltages results in extremely high electric fields within the devices. These conditions create the worst case environment for the Single Event Effect (SEE), the instantaneous alteration in device response after high energy particle interaction, with a destructive form of SEE, the Single Event Burnout (SEB), resulting in total failure of the device with potentially explosive consequences. To enable circuits to operate with these high supply voltages, SiC is rapidly becoming the semiconductor of choice. However, the radiation response of SiC power devices during operation is unknown. Here we show that SiC offers a 60% reduction in cosmic ray sensitivity in comparison to Si devices with an equivalent voltage rating. The data show that Si fails when subjected to a heavy ion impact with Linear Energy Transfer (LET) equivalent to 0.2% of the silver ions commonly used for Single Event Effect testing. In total contrast, we show that SiC does not exhibit failure during exposure to any heavy ion LET up to values three times greater than those commonly used in testing at any bias up to 99% of the breakdown voltage. The data show that SiC is a robust material and therefore has the potential to replace Si as the material of choice for high reliability avionic applications, as it far exceeds the performance of Si in cosmic ray environments, facilitating significant advances in the electrification of aircraft to be made in the near future