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This paper reports the first experimental observation of phonons and their softening on single crystalline LaPt$_2$Si$_2$ via inelastic neutron scattering. From the temperature dependence of the phonon frequency in close proximity to the charge-density wave (CDW) $q$-vector, we obtain a CDW transition temperature of T$_{CDW}$ = 230 K and a critical exponent $\beta$ = 0.28 $\pm$ 0.03. This value is suggestive of a non-conventional critical behavior for the CDW phase transition in LaPt$_2$Si$_2$, compatible with a scenario of CDW discommensuration (DC). The DC would be caused by the existence of two CDWs in this material, propagating separately in the non equivalent (Si1-Pt2-Si1) and (Pt1-Si2-Pt1) layers respectively, with transition temperatures T$_{CDW-1}$ = 230 K and T$_{CDW-2}$ = 110 K. A strong $q$-dependence of the electron-phonon coupling has been identified as the driving mechanism for the CDW transition at T$_{CDW-1}$ = 230 K while a CDW with 3-dimensional character, and Fermi surface quasi-nesting as a driving mechanism, is suggested for the transition at T$_{CDW-2}$ = 110 K. Our results clarify some aspects of the CDW transition in LaPt$_2$Si$_2$, which have been so far misinterpreted by both theoretical predictions and experimental observations, and give direct insight into its actual temperature dependence.

X-ray absorption (XAS) and Resonant Inelastic X-ray Scattering (RIXS) spectra of LaPt$_2$Si$_2$ single crystal at the Si L and La N edges are presented. The data are interpreted in terms of density functional theory, showing that the Si spectra can be described in terms of Si $s$ and $d$ local partial density of states (LPDOS), and the La spectra are due to quasi-atomic local $4f$ excitations. Calculations show that Pt $d$-LPDOS dominates the occupied states, and a sharp localized La $f$ state is found in the unoccupied states, in line with the observations.

X-ray absorption and resonant inelastic x-ray scattering spectra of LaPt2Si2 single crystal at the Si 2p and La 4d edges are presented. The data are interpreted in terms of density functional theory, showing that the Si spectra can be described in terms of Si s and d local partial density of states (LPDOS), and the La spectra are due to quasi-atomic local 4f excitations. Calculations show that Pt d-LPDOS dominates the occupied states, and a sharp localized La f state is found in the unoccupied states, in line with the observations.

Since thermal fluctuations become more important as dimensions shrink, it is expected that low-dimensional magnets are more sensitive to lattice distortions and phonons than bulk systems are. Here we present a fully relativistic first-principles study on the spin-lattice coupling, i.e. how the magnetic interactions depend on local lattice distortions, of the prototypical two-dimensional ferromagnet CrI$_3$. We extract an effective measure of the spin-lattice coupling in CrI$_3$ which is up to ten times larger than what is found for bcc Fe. The magnetic exchange interactions, including Heisenberg and relativistic Dzyaloshinskii-Moriya interactions, are sensitive both to the in-plane motion of Cr atoms and out-of-plane motion of ligand atoms. We find that significant magnetic pair interactions change sign from ferromagnetic (FM) to anti-ferromagnetic (AFM) for atomic displacements larger than 0.16 {\AA}. We explain the observed strong spin-lattice coupling by analyzing the orbital decomposition of isotropic exchange interactions, involving different crystal-field-split Cr$-3d$ orbitals. The competition between the AFM t$_{2g}$ - t$_{2g}$ and FM t$_{2g}$ - e$_{g}$ contributions depends on the bond angle formed by Cr and I atoms as well as Cr-Cr distance. In particular, if a Cr atom is displaced, the FM-AFM sign change when the I-Cr-I bond angle approaches 90$^\circ$. The obtained spin-lattice coupling constants, along with the microscopic orbital analysis can act as a guiding principle for further studies of the thermodynamic properties and combined magnon-phonon excitations in two-dimensional magnets.

Using local density approximation plus dynamical mean-field theory (LDA+DMFT), we have computed the valence band photoelectron spectra of highly popular multiferroic BiFeO$_{3}$. Within DMFT, the local impurity problem is tackled by exact diagonalization (ED) solver. For comparison, we also present result from LDA+U approach, which is commonly used to compute physical properties of this compound. Our LDA+DMFT derived spectra match adequately with the experimental hard X-ray photoelectron spectroscopy (HAXPES) and resonant photoelectron spectroscopy (RPES) for Fe 3$d$ states, whereas the other theoretical method that we employed failed to capture the features of the measured spectra. Thus, our investigation shows the importance of accurately incorporating the dynamical aspects of electron-electron interaction among the Fe 3$d$ orbitals in calculations to produce the experimental excitation spectra, which establishes BiFeO$_{3}$ as a strongly correlated electron system. The LDA+DMFT derived density of states (DOSs) exhibit significant amount of Fe 3$d$ states at the energy of Bi lone-pairs, implying that the latter is not as alone as previously thought in the spectral scenario. Our study also demonstrates that the combination of orbital cross-sections for the constituent elements and broadening schemes for the calculated spectral function are pivotal to explain the detailed structures of the experimental spectra.

The magnetization dynamics of a synthetic antiferromagnet subject to a short magnetic field pulse, has been studied by using a combination of first-principles and atomistic spin dynamics simulations. We observe switching phenomena on the time scale of tens of picoseconds, and inertia-like behavior in the magnetization dynamics. We explain the latter in terms of a dynamic redistribution of magnetic energy from the applied field pulse to other possible energy terms, such as the exchange interaction and the magnetic anisotropy, without invoking concepts such as inertia of an antiferromagnetic vector. We also demonstrate that such dynamics can also be observed in a ferromagnetic material where the incident field pulse pumps energy to the magnetic anisotropy.

The topic of this Thesis is magnetization dynamics on atomic length scales. A computational scheme, Atomistic Spin Dynamics, based on density functional theory, the adiabatic approximation and the atomic moment approximation is presented. Simulations are performed for chemically disordered systems, antiferromagnets and ferrimagnets and also systems with reduced dimensionality The autocorrelation function of the archetypical spin glass alloy CuMn is sampled in simulations following a quenching protocol. The aging regime can be clearly identified and the dependence of the relaxation on the damping parameter is investigated. The time evolution of pair correlation and autocorrelation functions has been studied in simulations of the dilute magnetic semiconductor GaMnAs. The dynamics reveal a substantial short ranged magnetic order even at temperatures at or above the ordering temperature. The dynamics for different concentrations of As antisites are discussed. Antiferromagnets offer opportunities for ultrafast switching, this is studied in simulations of an artificial antiferromagnet. For the right conditions, the cooperative effect of applied field torque and and the torque from the other sublattice enables very fast switching. The dynamics of bcc Fe precessing in a strong uniaxial anisotropy are investigated. It is demonstrated that the magnetization can shrink substantially due to a spin wave instability. The dynamics of a two-component model ferrimagnet at finite temperature are investigated. At temperatures where the magnetic system is close to the magnetic and angular momentum compensations points of the ferrimagnet, the relaxation in a uniaxial easy exis anisotropy resembles results in recent experiments on ferrimagnetic resonance. The different cases of uniaxial or colossal magnetic anisotropy in nanowires at different temperatures are compared. The magnon softening in a ferromagnetic monolayer is investigated, giving results that compare well with recent experiments. The effect of lattice relaxation can be treated in first principles calculations. Subsequent simulations captures the softening of magnons caused by reduced dimensionality and temperature. Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 706

This deliverable documents the initial performance analysis results obtained for all 6 TREX flagship applications. The focus was, in particular, the scaling of the application and the ability to exploit parallelism at all the different levels of modern HPC architectures. This ranges from the efficient use of SIMD instructions to the use of highly parallel compute accelerators like Graphics Processing Units (GPUs). For assessing the applications in terms of scalability, it needs to be taken into account that they differ in terms of their principle ability to be highly parallelized. Some of the applications, e.g. QMC=Chem, implement scalable methods with a strong focus on scalability, which could be demonstrated using up to 32,768 CPU cores. Furthermore, very encouraging results have been obtained for TurboRVB from GPU acceleration using Europe’s currently fastest supercomputer, i.e. JUWELS Booster. The performance results collected for this deliverable will help to guide further work and optimisations during the second half of the project.