Tuning skyrmions in B20 compounds by 4d and 5d doping

Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the 3d sublattice of the B20 structure by substitution of heavier 4d and 5d elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal a helical ground state and field-induced skyrmions for all these systems. Especially small skyrmions $\sim$50 nm are predicted for Co$_{0.75}$Os$_{0.25}$Si, compared to $\sim$148 nm for Fe$_{0.75}$Co$_{0.25}$Si. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.Comment: 18 pages, 21 figures, 9 table

Modeling Molecular Interactions in Water: From Pairwise to Many-Body Potential Energy Functions.

Almost 50 years have passed from the first computer simulations of water, and a large number of molecular models have been proposed since then to elucidate the unique behavior of water across different phases. In this article, we review the recent progress in the development of analytical potential energy functions that aim at correctly representing many-body effects. Starting from the many-body expansion of the interaction energy, specific focus is on different classes of potential energy functions built upon a hierarchy of approximations and on their ability to accurately reproduce reference data obtained from state-of-the-art electronic structure calculations and experimental measurements. We show that most recent potential energy functions, which include explicit short-range representations of two-body and three-body effects along with a physically correct description of many-body effects at all distances, predict the properties of water from the gas to the condensed phase with unprecedented accuracy, thus opening the door to the long-sought "universal model" capable of describing the behavior of water under different conditions and in different environments

Structure, Dynamics and Thermodynamics of Liquid Water : Insights from Molecular Simulations

Water is a complex liquid with many unusual properties. Our understanding of its physical, chemical and biological properties is greatly advanced after a century of dedicated research but there are still many unresolved questions. If answered, they could have important long-term consequences for practical applications ranging from drug design to water purification. This thesis presents results on the structure, dynamics and thermodynamics of liquid water. The focus is on theoretical simulations applied to interpret experimental data from mainly x-ray and neutron scattering and spectroscopy techniques. The structural sensitivity of x-ray and neutron diffraction is investigated using reverse Monte Carlo simulations and information on the pair-correlation functions of water is derived. A new method for structure modeling of computationally demanding data sets is presented and used to resolve an inconsistency between experimental extended x-ray absorption fine-structure and diffraction data regarding oxygen-oxygen pair-correlations. Small-angle x-ray scattering data are modeled using large-scale classical molecular dynamics simulations, and the observed enhanced scattering at supercooled temperatures is connected to the presence of a Widom line emanating from a liquid-liquid critical point in the deeply supercooled high pressure regime. An investigation of inherent structures reveals an underlying structural bimodality in the simulations connected to disordered high-density and ordered low-density molecules, providing a clearer interpretation of experimental small-angle scattering data. Dynamical anomalies in supercooled water observed in inelastic neutron scattering experiments, manifested by low-frequency collective excitations resembling a boson peak, are investigated and found to be connected to the thermodynamically defined Widom line. Finally, x-ray absorption spectra are calculated for simulated water structures using density functional theory. An approximation of intra-molecular zero-point vibrational effects is found to significantly improve the relative spectral intensities but a structural investigation indicates that the classical simulations underestimate the amount of broken hydrogen bonds.Vatten är en komplex vätska med flera ovanliga egenskaper. Vår förståelse av dess fysiska, kemiska och biologiska egenskaper har utvecklats mycket sedan systematiska vetenskapliga studier började genomföras för mer än ett sekel sedan, men många viktiga frågor är fortfarande obesvarade. En ökad förståelse skulle på sikt kunna leda till framsteg inom viktiga områden så som medicinutveckling och vattenrening. Denna avhandling presenterar resultat kring vattnets struktur, dynamik och termodynamik. Fokusen ligger på teoretiska simuleringar som använts för att tolka experimentella data från huvudsakligen röntgen- och neutronspridning samt spektroskopier. Den strukturella känsligheten i röntgen- och neutrondiffraktionsdata undersöks via reverse Monte Carlo metoden och information om de partiella parkorrelationsfunktionerna erhålls. En ny metod för strukturmodellering av beräkningsintensiva data presenteras och används för att lösa en motsägelse mellan experimentell diffraktion och EXAFS angående syre- syre parkorrelationsfunktionen. Data från röntgensmåvinkelspridning modelleras med storskaliga klassiska molekyldynamiksimuleringar, och den observerade förhöjda småvinkelspridningen vid underkylda temperaturer kopplas till existensen av en Widomlinje härrörande från en vätske- vätske kritisk punkt i det djupt underkylda området vid höga tryck. En undersökning av inherenta strukturer i simuleringarna påvisar en underliggande strukturell bimodalitet mellan molekyler i oordnade högdensitetsregioner respektive ordnade lågdensitetsregioner, vilket ger en tydligare tolkning av den experimentella småvinkelspridningen. Dynamiska anomalier i underkylt vatten som har observerats i inelastisk neutronspridning, speciellt förekomsten av lågfrekventa excitationer som liknar en bosontopp, undersöks och kopplas till den termodynamiskt definierade Widomlinjen. Slutligen presenteras densitetsfunktionalberäkningar av röntgenabsorptionsspektra för simulerade vattenstrukturer. En approximation av intramolekylära nollpunktsvibrationseffekter förbättrar relativa intensiteteri spektrumen avsevärt, men en strukturanalys visar att klassiska simuleringar av vatten underskattar andelen brutna vätebindningar.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 6: Submitted. Paper 7: Submitted. Paper 8: Manuscript. Paper 9: Submitted

coderefinery/post-workshop-survey: Version 7 May 2019

CodeRefinery post-workshop survey

The Boson peak in supercooled water

We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores, and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line T-W. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.AuthorCount:5;</p

Tuning skyrmions in B20 compounds by 4d and 5d doping

Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the 3d sublattice of the B20 structure by substitution of heavier 4d and 5d elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal the spin-spiral nature of the magnetic ground state and field-induced skyrmions for all these systems. Especially small skyrmions similar to 50 nm are predicted for Co0.75Os0.25Si, compared to similar to 148 nm for Fe0.75Co0.25Si. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms