Computer simulations of structural and hopping conduction properties of disordered solids

1. Introduction 2. Molecular dynamics simulation of amorphous carbon structures 3. Atomistic simulation of the bombardment process during the BEN phase of diamond CVD 4. Growth of amorphous silicon 5. One-dimensional hopping in disordered organic solidsComment: PhD thesis, 107 pages +45 figures, LaTeX, some color figures availble on the web (see details in the text

Ozone chemistry on tidally locked M dwarf planets

This is the final version. Available from Oxford University Press via the DOI in this recordWe use the Met Office Unified Model to explore the potential of a tidally locked M dwarf planet, nominally Proxima Centauri b irradiated by a quiescent version of its host star, to sustain an atmospheric ozone layer. We assume a slab ocean surface layer, and an Earth-like atmosphere of nitrogen and oxygen with trace amounts of ozone and water vapour. We describe ozone chemistry using the Chapman mechanism and the hydrogen oxide (HOx, describing the sum of OH and HO2) catalytic cycle. We find that Proxima Centauri radiates with sufficient UV energy to initialize the Chapman mechanism. The result is a thin but stable ozone layer that peaks at 0.75 parts per million at 25 km. The quasi-stationary distribution of atmospheric ozone is determined by photolysis driven by incoming stellar radiation and by atmospheric transport. Ozone mole fractions are smallest in the lowest 15 km of the atmosphere at the sub-stellar point and largest in the nightside gyres. Above 15 km the ozone distribution is dominated by an equatorial jet stream that circumnavigates the planet. The nightside ozone distribution is dominated by two cyclonic Rossby gyres that result in localized ozone hotspots. On the dayside the atmospheric lifetime is determined by the HOx catalytic cycle and deposition to the surface, with nightside lifetimes due to chemistry much longer than timescales associated with atmospheric transport. Surface UV values peak at the substellar point with values of 0.01 W/m2 , shielded by the overlying atmospheric ozone layer but more importantly by water vapour clouds.Leverhulme TrustScience and Technology Facilities Council (STFC

Determination of the anisotropic elastic properties of rocksalt Ge2Sb2Te5 by XRD, residual stress, and DFT

© 2016 American Chemical Society. The chalcogenide material Ge2Sb2Te5 is the prototype phase-change material, with widespread applications for optical media and random access memory. However, the full set of its independent elastic properties has not yet been published. In this study, we determine the elastic constants of the rocksalt Ge2Sb2Te5, experimentally by X-ray diffraction (XRD) and residual stress and computationally by density functional theory (DFT). The stiffnesses (XRD-stress/DFT) in GPa are C11 = 41/58, C12 = 7/8, and C44 = 8/12, and the Zener ratio is 0.46/0.48. These values are important to understand the effect of elastic distortions and nonmelting processes on the performances of increasingly small phase change data bits

Threshold switching via electric field induced crystallization in phase-change memory devices

Copyright © 2012 American Institute of PhysicsPhase-change devices exhibit characteristic threshold switching from the reset (off) to the set (on) state. Mainstream understanding of this electrical switching phenomenon is that it is initiated electronically via the influence of high electric fields on inter-band trap states in the amorphous phase. However, recent work has suggested that field induced (crystal) nucleation could instead be responsible. We compare and contrast these alternative switching “theories” via realistic simulations of device switching both with and without electric field dependent contributions to the system free energy. Results show that although threshold switching can indeed be obtained purely by electric field induced nucleation, the fields required are significantly larger than experimentally measured values

Phase-change technologies: from PCRAM to probe-storage to processors

Phase-change materials based on chalcogenide alloys, for example GeSbTe and AgInSbTe, show remarkable properties such as: the ability to be crystallized by pulses in the (hundreds of) femtoseconds region while at the same time withstanding spontaneous crystallization for many years; the ability to be cycled between phases 1012 times or more; the existence of a huge contrast between the refractive index of the phases; the existence of a huge electrical contrast between phases. These remarkable properties make phase-change materials suitable for a wide range of optical and electrical applications, for optical and electrical memories, for optical routers, for optical and electrical processors. In this paper we describe theoretical and experimental investigations of some of the key application areas, with a view to providing insights into the possible future use of phase-change materials

Simulations of idealised 3D atmospheric flows on terrestrial planets using LFRic-Atmosphere

We demonstrate that LFRic-Atmosphere, a model built using the Met Office's GungHo dynamical core, is able to reproduce idealised large-scale atmospheric circulation patterns specified by several widely-used benchmark recipes. This is motivated by the rapid rate of exoplanet discovery and the ever-growing need for numerical modelling and characterisation of their atmospheres. Here we present LFRic-Atmosphere's results for the idealised tests imitating circulation regimes commonly used in the exoplanet modelling community. The benchmarks include three analytic forcing cases: the standard Held-Suarez test, the Menou-Rauscher Earth-like test, and the Merlis-Schneider Tidally Locked Earth test. Qualitatively, LFRic-Atmosphere agrees well with other numerical models and shows excellent conservation properties in terms of total mass, angular momentum and kinetic energy. We then use LFRic-Atmosphere with a more realistic representation of physical processes (radiation, subgrid-scale mixing, convection, clouds) by configuring it for the four TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) scenarios. This is the first application of LFRic-Atmosphere to a possible climate of a confirmed terrestrial exoplanet. LFRic-Atmosphere reproduces the THAI scenarios within the spread of the existing models across a range of key climatic variables. Our work shows that LFRic-Atmosphere performs well in the seven benchmark tests for terrestrial atmospheres, justifying its use in future exoplanet climate studies.Comment: 34 pages, 9(12) figures; Submitted to Geoscientific Model Development; Comments are welcome (see Discussion tab on the journal's website: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-647

The 3D thermal, dynamical and chemical structure of the atmosphere of HD 189733b: implications of wind-driven chemistry for the emission phase curve (dataset)

UM model output in .pp format (UM custom format) and converted netCDF formatu-au007 - high (spectral) resolution output (relaxation)u-as783 - high (spectral) resolution output (equilibrium)u-ay698 - contribution function (relaxation)u-ay390 - contribution function (equilibrium)u-aw848 - chemical relaxation (timescale *0.1)u-aw847 - chemical relaxation (timescale *10)u-ax477 - chemical relaxation (H2O and CO fixed to equilibrium in heating rate calculation)u-as904 - chemical relaxationu-aq803 - chemical equilibriumThe data contained in this submission is associated with the publication Drummond et al, ApJ, 2018.The article associated with this dataset is located in ORE at: http://hdl.handle.net/10871/34680In this paper we present three-dimensional atmospheric simulations of the hot Jupiter HD~189733b under two different scenarios: local chemical equilibrium and including advection of the chemistry by the resolved wind. Our model consistently couples the treatment of dynamics, radiative transfer and chemistry, completing the feedback cycle between these three important processes. The effect of wind--driven advection on the chemical composition is qualitatively similar to our previous results for the warmer atmosphere of HD~209458b, found using the same model. However, we find more significant alterations to both the thermal and dynamical structure for the cooler atmosphere of HD~189733b, with changes in both the temperature and wind velocities reaching $\sim10\%$. We also present the contribution function, diagnosed from our simulations, and show that wind--driven chemistry has a significant impact on its three--dimensional structure, particularly for regions where methane is an important absorber. Finally, we present emission phase curves from our simulations and show the significant effect of wind--driven chemistry on the thermal emission, particularly within the 3.6 \textmu m Spitzer/IRAC channel.This work is partly supported by the European Research Council under the European Communitys Seventh Framework Programme (FP7/2007-2013 Grant Agreement No. 336792-CREATES and No. 320478 TOFU). N.J.M. and J.G. are partially funded by a Leverhulme Trust Research Project Grant. J.M. acknowledges the support of a Met Office Academic Partnership secondment. This work was performed using the DiRAC Data Intensive service at Leicester, operated by the University of Leicester IT Services, which forms part of the TFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment was funded by BEIS capital funding via STFC capital grants ST/K000373/1 and ST/R002363/1 and STFC DiRAC Operations grant ST/R001014/1. DiRAC is part of the National e-Infrastructure. This work also used the University of Exeter Supercomputer ISCA