Comparative analysis of different preparation methods of chalcogenide glasses: Molecular dynamics structure simulations

Two different preparation methods (liquid-quenching and evaporation) of chalcogenide glasses have been investigated by molecular dynamics simulations. Our particular aim was to determine how the structural changes occur due to the different preparation methods. We applied a classical empirical three-body potential of selenium to describe the interactions between atoms. Our simulation shows that a significant difference can be observed in the homogeneities

Photo-induced volume changes in selenium. Tight-binding molecular dynamics study

Tight-binding molecular dynamics simulations of photo-excitations in small Se clusters (isolated Se$_8$ ring and helical Se chain) and glassy Se networks (containing 162 atoms) were carried out in order to analyse the photo induced instability inside the amorphous selenium. In the cluster systems after taking an electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital a bond breaking occurs. In the glassy networks photoinduced volume expansion was observed and at the same time the number of coordination defects changed significantly due to illumination

Ultrahigh Storage Densities via the Scaling of Patterned Probe Phase-Change Memories

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this record.The scaling potential of patterned probe phase-change memory (PP-PCM) cells is investigated, down to single-nanometer dimensions, using physically realistic simulations that combine electro-thermal modelling with a Gillespie Cellular Automata (GCA) phase-change model. For this study, a trilayer TiN/Ge 2 Sb 2 Te 5 /TiN cell structure (isolated by a SiO 2 insulator) was preferred, due to its good performance and practicability, over previously investigated probe-based structures such as those that used diamond-like carbon capping layers or immersion in an inert liquid to protect the phase-change layer (while still allowing for electrical contact). We found that PP-PCM cells with dimensions as small as 5 nm could be successfully amorphized and re-crystallized (RESET and SET) using moderate voltage pulses. The resistance window between the RESET/SET states decreased with a reduction in cell dimensions, but it was still more than order of magnitude even for the smallest cells, predicting that PP-PCM cells are indeed scalable and operable in the sub-10 nm region. Most importantly, it was found that the storage density could be increased by cell size scaling with storage densities as high as 10 Tb/in 2 being achieved, which is significantly higher than the storage densities previously reported in phase-change probe storage, and other probe-based technologies such as thermomechanical, magnetic and ferroelectric probe storage.Hasan Hayat would like to thank the College of Engineering, Mathematics and Physical Sciences at the University of Exeter for PhD scholarship funding while carrying out this work. C. David Wright would like to thank the EPSRC for funding via grant EP/M015130/1

The Limits of the Primitive Equations of Dynamics for Warm, Slowly Rotating Small Neptunes and Super Earths (article)

This is the author accepted manuscript. The final version is available from American Astronomical Society / IOP Publishing via the DOI in this record.The dataset associated with this article is located in ORE at: https://doi.org/10.24378/exe.1023We present significant differences in the simulated atmospheric flow for warm, tidally-locked small Neptunes and super Earths (based on a nominal GJ 1214b) when solving the simplified, and commonly used, primitive dynamical equations or the full Navier-Stokes equations. The dominant prograde, superrotating zonal jet is markedly different between the simulations which are performed using practically identical numerical setups, within the same model. The differences arise due to the breakdown of the so-called `shallow-fluid' and traditional approximations, which worsens when rotation rates are slowed, and day{night temperature contrasts are increased. The changes in the zonal advection between simulations solving the full and simplified equations, give rise to significant differences in the atmospheric redistribution of heat, altering the position of the hottest part of the atmosphere and temperature contrast between the day and night sides. The implications for the atmospheric chemistry and, therefore, observations need to be studied with a model including a more detailed treatment of the radiative transfer and chemistry. Small Neptunes and super Earths are extremely abundant and important, potentially bridging the structural properties (mass, radius, composition) of terrestrial and gas giant planets. Our results indicate care is required when interpreting the output of models solving the primitive equations of motion for such planets.Leverhulme TrustScience and Technology Facilities CouncilEuropean Research Counci

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

Overcast on Osiris: 3D radiative-hydrodynamical simulations of a cloudy hot Jupiter using the parametrized, phase-equilibrium cloud formation code EDDYSED (article)

This is the final version. Available from OUP via the DOI in this recordThe dataset associated with this article is available in ORE: https://doi.org/10.24378/exe.1483We present results from 3D radiative-hydrodynamical simulations of HD 209458b with a fully coupled treatment of clouds using the EDDYSED code, critically, including cloud radiative feedback via absorption and scattering. We demonstrate that the thermal and optical structure of the simulated atmosphere is markedly different, for the majority of our simulations, when including cloud radiative effects, suggesting this important mechanism cannot be neglected. Additionally, we further demonstrate that the cloud structure is sensitive to not only the cloud sedimentation efficiency (termed fsed in EDDYSED), but also the temperature–pressure profile of the deeper atmosphere. We briefly discuss the large difference between the resolved cloud structures of this work, adopting a phase-equilibrium and parametrized cloud model, and our previous work incorporating a cloud microphysical model, although a fairer comparison where, for example, the same list of constituent condensates is included in both treatments is reserved for a future work. Our results underline the importance of further study into the potential condensate size distributions and vertical structures, as both strongly influence the radiative impact of clouds on the atmosphere. Finally, we present synthetic observations from our simulations reporting an improved match, over our previous cloud-free simulations, to the observed transmission, HST WFC3 emission, and 4.5 μm Spitzer phase curve of HD 209458b. Additionally, we find all our cloudy simulations have an apparent albedo consistent with observations.Leverhulme TrustScience and Technology Facilities Council (STFC

The Impact of Phase Equilibrium Cloud Models on GCM Simulations of GJ~1214b

We investigate the impact of clouds on the atmosphere of GJ~1214b using the radiatively-coupled, phase-equilibrium cloud model {\sc EddySed} coupled to the {\sc Unified Model} general circulation model. We find that, consistent with previous investigations, high metallicity ($100\times$ solar) and clouds with large vertical extents (a sedimentation factor of $f_\mathrm{sed} = 0.1$) are required to best match the observations, although metallicities even higher than those investigated here may be required to improve agreement further. We additionally find that in our case which best matches the observations ($f_\mathrm{sed}=0.1$), the velocity structures change relative to the clear sky case with the formation of a superrotating jet being suppressed, although further investigation is required to understand the cause of the suppression. The increase in cloud extent with $f_\mathrm{sed}$ results in a cooler planet due to a higher albedo, causing the atmosphere to contract. This also results in a reduced day-night contrast seen in the phase curves, although the introduction of cloud still results in a reduction of the phase offset. We additionally investigate the impact the the {\sc Unified Model}'s pseudo-spherical irradiation scheme on the calculation of heating rates, finding that the introduction of nightside shortwave heating results in slower mid-latitude jets compared to the plane parallel irradiation scheme used in previous works. We also consider the impact of a gamma distribution, as opposed to a log-normal distribution, for the distribution of cloud particle radii and find the impact to be relatively minor.Comment: Accepted to MNRA

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

Contact printing of colloidal nanocrystal thin films for hybrid organic/quantum dot optoelectronic devices

Novel thin film optoelectronic devices containing both inorganic colloidal semiconductor quantum dots (QDs) and organic semiconductor thin films have been widely investigated in recent years for a variety of applications. Here, we review one of the most versatile and successful methods developed to integrate these two dissimilar material classes into a functional multilayered device: contact printing of colloidal QD films. Experimental details regarding the contact printing process are outlined, and the key advantages of this QD deposition method over other commonly encountered techniques are discussed. The use of tapping mode atomic force microscopy (AFM) to effectively characterize QD film morphology both on an elastomeric stamp (before contact printing) and as-transferred to the organic semiconductor receiving film (after contact printing) is also described. Finally, we offer suggestions for future efforts directed toward the goal of rapid, continuous QD deposition over larger substrates for the advancement of hybrid optoelectronic thin film devices

Atmospheric convection plays a key role in the climate of tidally-locked terrestrial exoplanets: insights from high-resolution simulations

This is the final version. Available from IOP Piblishing via the DOI in this recordUsing a 3D general circulation model (GCM), we investigate the sensitivity of the climate of tidallylocked Earth-like exoplanets, Trappist-1e and Proxima Centauri b, to the choice of a convection parameterization. Compared to a mass-flux convection parameterization, a simplified convection adjustment parameterization leads to a >60 % decrease of the cloud albedo, increasing the mean day-side temperature by ≈10 K. The representation of convection also affects the atmospheric conditions of the night side, via a change in planetary-scale wave patterns. As a result, using the convection adjustment scheme makes the night-side cold traps warmer by 17–36 K for the planets in our simulations. The day-night thermal contrast is sensitive to the representation of convection in 3D GCM simulations, so caution should be taken when interpreting emission phase curves. The choice of convection treatment, however, does not alter the simulated climate enough to result in a departure from habitable conditions, at least for the atmospheric composition and planetary parameters used in our study. The near-surface conditions both in the Trappist-1e and Proxima b cases remain temperate, allowing for an active water cycle. We further advance our analysis using high-resolution model experiments, in which atmospheric convection is simulated explicitly. Our results suggest that in a hypothetical global convection-permitting simulation the surface temperature contrast would be higher than in the coarse-resolution simulations with parameterized convection. In other words, models with parameterized convection may overestimate the inter-hemispheric heat redistribution efficiency.Leverhulme TrustScience and Technology Facilities Council (STFC