An engineering model for high-speed switching in GeSbTe phase-change memory

Ge2Sb2Te5 is the most successful phase-change alloy in non-volatile memory using the amorphous-crystal phase transition. In deriving further high performance in switching, especially SET speed (from amorphous to crystal transition) should still be modified. In this work, It was examined an ideal Ge2Sb2Te5 alloy based on the Kolobov model using ab-initio molecular dynamics simulations. As a result, it was cleared that a uniaxial exchange between vacancies and Ge atoms is the crucial role in realizing high-speed switching and a large contrast in the resonance bonding state in the alloy. The vacancy engineering enables the alloy switching speed extremely faster.

Static and Dynamic Statistical Correlations in Water: Comparison of Classical Ab Initio Molecular Dynamics at Elevated Temperature With Path Integral Simulations at Ambient Temperature

It is a common practice in ab initio molecular dynamics (AIMD) simulations of water to use an elevated temperature to overcome the over-structuring and slow diffusion predicted by most current density functional theory (DFT) models. The simulation results obtained in this distinct thermodynamic state are then compared with experimental data at ambient temperature based on the rationale that a higher temperature effectively recovers nuclear quantum effects (NQEs) that are missing in the classical AIMD simulations. In this work, we systematically examine the foundation of this assumption for several DFT models as well as for the many-body MB-pol model. We find for the cases studied that a higher temperature does not correctly mimic NQEs at room temperature, which is especially manifest in significantly different three-molecule correlations as well as hydrogen bond dynamics. In many of these cases, the effects of NQEs are the opposite of the effects of carrying out the simulations at an elevated temperature.

Dynamic molecular ordering in multiphasic nanoconfined ionogels detected with time-resolved diffusion NMR

Molecular motion in nanosized channels can be highly complicated. For example, water molecules in hydrophobic nanopores move rapidly and coherently in a chain, following the so-called single file motion. Surprisingly, fast molecular motion is also observed in viscous charged fluids, such as room temperature ionic liquids (RTILs) confined in a nanoporous carbon or silica matrix. The microscopic mechanism of this intriguing effect is still unclear. Here, by combining NMR diffusion experiments in different relaxation windows with ab-initio molecular dynamics simulations, we show that the imidazolium-based RTIL [BMIM]+[TCM]-, entrapped in the MCM-41 silica nanopores, exhibits a complex dynamic molecular ordering (DMO); adsorbed RTIL molecules near the pore walls orient almost vertically to the walls, while at the center of the pores anion-cation pairs diffuse collectively in a single file (SFD). Enlightening this extraordinary effect is of primary importance in designing RTIL-based composite materials with tuned electrochemical properties.

Understanding Proton Transfer in Non-aqueous Biopolymers based on Helical Peptides: A Quantum Mechanical Study

Histidine (an imidazole-based amino acid) is a promising building block for short aromatic peptides containing a proton donor/acceptor moiety. Previous studies have shown that polyalanine helical peptides substituted at regular intervals with histidine residues exhibit both structural stability as well as high proton affinity and high conductivity. Here, we present first-principle calculations of non-aqueous histidine-containing 310-,  and -helices and show that they are able to form hydrogen-bonded networks mimicking proton wires that have the ability to shuttle protons via the Grotthuss shuttling mechanism. The formation of these wires enhances the stability of the helices, and our structural characterizations confirm that the secondary structures are conserved despite distortions of the backbones. In all cases, the helices exhibit high proton affinity and proton transfer barriers on the order of 1~4 kcal/mol. Zero-point energy calculations suggest that for these systems, ground state vibrational energy can provide enough energy to cross the proton transport energy barrier. Additionally, ab initio molecular dynamics results suggests that the protons are transported unidirectionally through the wire at a rate of approximately 2 Å every 20 fs. These results demonstrate that efficient deprotonation-controlled proton wires can be formed using non-aqueous histidine-containing helical peptides.

Using Machine Learning to Greatly Accelerate Path Integral Ab Initio Molecular Dynamics

Ab initio molecular dynamics (AIMD) has become one of the most popular and robust approaches for modeling complicated chemical, liquid, and material systems. However, the formidable computational cost often limits its widespread application in simulations of the largest scale systems. The situation becomes even more severe in cases where the hydrogen nuclei may be better described as quantized particles using a path integral representation. Here, we present a computational approach that combines machine learning with recent advances in path integral contraction schemes, and we achieve a two-orders-of-magnitude acceleration over direct path integral AIMD simulation while at the same time maintaining its accuracy.

Ultrafast Proton Transfer Reaction in Phenol–(Ammonia)n Clusters: An Ab-Initio Molecular Dynamics Investigation.

The ability of phenol to transfer the proton to surrounding ammonia molecules in a phenol-(ammonia)n cluster will depend on the relative orientation of the ammonia molecules and a critical field of about 285 MV cm-1 is essential along the O–H bond for the transfer process. Ab-initio MD simulations reveal that for a spontaneous proton transfer process, the phenol molecule must be embedded in a cluster consisting of at least eight ammonia molecules, even though several local minima with proton transferred can be observed for clusters consisting of 5-7 ammonia molecules. Further, phenol solvated in large clusters of ammonia, the proton transfer is spontaneous with the proton transfer event being instantaneous (about 20-120 fs). These simulations indicate that the rate-determining step for the proton transfer process is the reorganization of the solvent around the OH group and the proton transfer process in phenol-(ammonia)n clusters. The fluctuations in the solvent occur until a particular set of configurations projects the field in excess of critical electric field along the O–H bond which drives the proton transfer process with a respone time of about 70 fs. Further, the proton transfer process follows a curvilinear path which includes the O–H bond elongation and out-of-plane movement of the proton and can be referred to as a “Bend-to-Break” process.

Completely computational model setup for spectroscopic techniques: the ab initio molecular dynamics indirect hard modeling (AIMD-IHM) approach

The spectroscopic quantification of mixture compositions usually requires pure compounds and mixtures of known composition for calibration. Since they are not always available, methods to fill such gaps have evolved, which are, however, not generally applicable. Therefore, calibration can be extremely challenging, especially when multiple instable species, e.g. intermediates, exist in a system. This study presents a new calibration approach that uses ab initio Molecular Dynamics (AIMD)-simulated spectra as to set up and calibrate models for the physics-based spectral analysis method Indirect Hard Modeling (IHM). To demonstrate our approach called AIMD-IHM, we analyze Raman spectra of ternary hydrogen-bonding mixtures of acetone, methanol, and ethanol. The derived AIMD-IHM pure-component models and calibration coefficients are in good agreement with conventionally generated experimental results. The method yields compositions with prediction errors of less than 5% without any experimental calibration input. Our approach can be extended, in principle, to IR and NMR spectroscopy and allows for the analysis of systems that were hitherto inaccessible to quantitative spectroscopic analysis.