Spin-fluctuation spectra in magnetic systems: a novel approach based on TDDFT

Magnetism at the micro- and nano-scale level is a well-established research field, by virtue of its relentless technological impact and astounding variety of structures it can shape in condensed-matter systems. The characterization of most of these structures has become possible in the last fifty years thanks to the development and refinement of magnetic spectroscopies, most notably neutron scattering for bulk magnetism, and electron spectroscopies for surfaces and thin films. A fundamental outcome of the most recent experiments is the need to address magnetism in its full non-collinear nature also at the theoretical level, i.e. by treating the magnetization density as a true vector field, allowed to vary its direction at each point in space. This paves the way to the study of chiral topological magnetic structures such as skyrmions, or of the effect of Spin-Orbit Coupling (SOC) on the ground-state con- figuration and on the excited-state dynamics. Handling non-collinearity however, a far-from-trivial task on its own, proves to be particularly demanding in ab-initio calculations, where, at present, it is far from being a standard tool in the study of excited states. In this thesis we shall focus on the development of a method to study the dynamical spin-fluctuations of magnetic systems in a fully non-collinear framework, within Time-Dependent Density Function Theory (TDDFT). The outline of the thesis follows. In Ch. 1 the technological framework and the main experimental findings which have inspired our work are presented; a link between the experiments and the relevant physical quantities, namely the magnetic susceptibility, will also be shown. In Ch. 2 and 3 the theoretical framework in which we move will be introduced, namely Time-Dependent Density Functional Theory (TDDFT) and linear response. In Ch. 4 and Ch. 5 original work is presented: in the former, we devise a computational approach for the study of magnetic excitations via TDDFT, in a fully non-collinear framework. In the latter, we discuss the implementation and compute the spin-wave dispersion for BCC Iron. The final chapter is devoted to the conclusions

Rationalizing doping and electronic correlations in LaFe2_2As2_2

We compute the electronic properties of the normal state of uncollapsed LaFe$_2$As$_2$, taking into account local dynamical correlations by means of slave-spin mean-field+density-functional theory. Assuming the same local interaction strength used to model the whole electron- and hole-doped BaFe$_2$As$_2$ family, our calculations reproduce the experimental Sommerfeld specific heat coefficient, which is twice the value predicted by uncorrelated band theory. We find that LaFe$_2$As$_2$ has a reduced bare bandwidth and this solves the apparent paradox of its sizeable correlations despite its nominal valence d$^{6.5}$, which would imply extreme overdoping and uncorrelated behaviour in BaFe$_2$As$_2$. Our results yield a consistent picture of the whole 122 family and point at the importance of the correlation strength, rather than sheer doping, in the interpretation of the phase diagram of iron-based superconductorsComment: 5 pages, 4 figure

First-principles study of the gap in the spin excitation spectrum of the CrI3_3 honeycomb ferromagnet

The nature of the gap observed at the zone border in the spin-excitation spectrum of CrI$_3$ quasi-2D single crystals is still controversial. We perform first-principles calculations based on time-dependent density-functional perturbation theory, which indicate that the observed gap results from a combination of spin-orbit and inter-layer interaction effects. The former give rise to the anisotropic spin-spin interactions that are responsible for its very existence, while the latter determine both its displacement from the K point of the Brillouin zone due to the in-plane lattice distortions induced by them, and an enhancement of its magnitude, in agreement with experiments and previous theoretical work based on a lattice model

Magnon-phonon interactions enhance the gap at the Dirac point in the spin-wave spectra of CrI3_3 2D magnets

Recent neutron-diffraction experiments in honeycomb CrI$_3$ quasi-2D ferromagnets have evinced the existence of a gap at the Dirac point in their spin-wave spectra. The existence of this gap has been attributed to strong in-plane Dzyaloshinskii-Moriya or Kitaev (DM/K) interactions and suggested to set the stage for topologically protected edge states to sustain non-dissipative spin transport. We perform state-of-the-art simulations of the spin-wave spectra in monolayer CrI$_3$, based on time-dependent density-functional perturbation theory (TDDFpT) and fully accounting for spin-orbit couplings (SOC) from which DM/K interactions ultimately stem. While our results are in qualitative agreement with experiments, the computed TDDFpT magnon gap at the Dirac point is found to be 0.47~meV, roughly 6 times smaller than the most recent experimental estimates, so questioning that intralayer anisotropies alone can explain the observed gap. Lattice-dynamical calculations, performed within density-functional perturbation theory (DFpT), indicate that a substantial degeneracy and a strong coupling between vibrational and magnetic excitations exist in this system, providing a possible additional gap-opening mechanism in the spin-wave spectra. In order to pursue this path, we introduce an interacting magnon-phonon Hamiltonian featuring a linear coupling between lattice and spin fluctuations, enabled by the magnetic anisotropy induced by SOC. Upon determination of the relevant interaction constants by DFpT and supercell calculations, this model allows us to propose magnon-phonon interactions as an important microscopic mechanism responsible for the enhancement of the gap in the range of $\approx 4$~meV around the Dirac point of the CrI$_3$ monolayer

Ab initio study of electron energy loss spectra of bulk bismuth up to 100 eV

The dynamical charge-density response of bulk bismuth has been studied within time-dependent density functional perturbation theory, explicitly accounting for spin-orbit coupling. The use of the Liouville-Lanczos approach allows us to calculate electron energy loss spectra for excitation energies as large as 100 eV. Effects of 5d semicore electronic states, spin-orbit coupling, exchange and correlation, local fields, and anisotropy are thoroughly investigated. The account of the 5d states in the calculation turns out to be crucial to correctly describe the loss spectra above 10 eV and, in particular, the position and shape of the bulk-plasmon peak at 14.0 eV at vanishing transferred momentum. Our calculations reveal the presence of interband transitions at 16.3 eV, which had never been discussed before. The origin of the peak at 5.8 eV is revisited as due to mixed interband and collective excitations. Finally, our study supplements the lack of experiments at finite transferred momenta

Spin dynamics from time-dependent density functional perturbation theory

We present a new method to model spin-wave excitations in magnetic solids, based on the Liouville-Lanczos approach to time-dependent density functional perturbation theory. This method avoids computationally expensive sums over empty states and naturally deals with the coupling between spin and charge fluctuations, without ever explicitly computing charge-density susceptibilities. Spin-wave excitations are obtained with one Lanczos chain per magnon wave-number and polarization, avoiding the solution of the linear-response problem for every individual value of frequency, as other state-of-the-art approaches do. Our method is validated by computing magnon dispersions in bulk Fe and Ni, resulting in agreement with previous theoretical studies in both cases, and with experiment in the case of Fe. The disagreement in the case of Ni is also comparable with that of previous computations

Accurate modeling of FeSe with screened Fock exchange and Hund metal correlations

International audienc

Manipulation of the magnetic state of a porphyrin-based molecule on gold: From Kondo to quantum nanomagnet via the charge fluctuation regime

8 pages, 4 figures second versionInternational audienceBy moving individual Fe-Porphyrin-based molecules with the tip of Scanning Tunneling Microscope in the vicinity of a Br-atom containing elbow of the herringbone-reconstructed Au(111), we reversibly and continuously control their magnetic state. Several regimes are obtained experimentally and explored theoretically: from the integer spin limit, through intermediate magnetic states with renormalized magnetic anisotropy, until the Kondo-screened regime, corresponding to a progressive increase of charge fluctuations and mixed valency due to an increase in the interaction of the molecular Fe states with the substrate Fermi sea. Our results open a route for the realization, tuning and experimental studies of novel quantum magnetic states in molecule-surface hybrids

Epsilon iron as a spin-smectic state

International audienc