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@PHDTHESIS{Neukirchen:1047249,
      author       = {Neukirchen, Alexander},
      title        = {{P}honons in {M}agnetic {S}ystems by means of
                      {D}ensity-{F}unctional {P}erturbation {T}heory},
      volume       = {301},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2025-04180},
      isbn         = {978-3-95806-853-7},
      series       = {Schriften des Forschungszentrums Jülich Reihe
                      Schlüsseltechnologien / Key Technologies},
      pages        = {xviii, 195},
      year         = {2025},
      note         = {Dissertation, RWTH Aachen University, 2025},
      abstract     = {Phonons are quantized vibrational excitations of the
                      crystal lattice. These quasiparticles play a crucial role in
                      understanding many properties of the solid-state system. In
                      this thesis, phonons in terms of dispersion relations and
                      density of states (DOS) are investigated on the basis of the
                      Kohn–Sham density functional theory (DFT), the
                      state-of-the-art ab-initio approach to the electronic
                      structure of specific materials and a proven foundation for
                      the study of lattice vibrations from first-principles. This
                      work relies on the harmonic approximation, in which the
                      properties of phonons are directly related to the second
                      order response of the total energy of the system with
                      respect to the displacement of the atoms in the lattice. Two
                      complementary approaches are used to calculate this
                      response: The first one is the finite displacement (FD)
                      approach, that in which the second order of the energy is
                      approximated as a difference quotient using differences in
                      the forces acting on the atoms. The second one is the
                      density-functional perturbation theory (DFPT), a variational
                      approach that constructs the second order response
                      analytically from the first order response of the wave
                      functions obtained by the self-consistent solution of the
                      Sternheimer equation. In this thesis, I go beyond the
                      conventional application of DFPT to nonmagnetic systems and
                      the conventional realization of DFPT in terms of methods
                      representing the electron wave function in a plane wave (PW)
                      basis and present an implementation in the all-electron
                      full-potential linearized augmented plane-wave (FLAPW)
                      method. This very accurate method, applicable without
                      further ado to all nonmagnetic and magnetic chemical
                      elements of the periodic table, comes with the challenge of
                      an atomic position dependent basis set. I show that the
                      subsequently arising additional matrix elements, so-called
                      correction terms to calculate the response of wave function
                      and energy can be determined accurately. One objective of
                      this thesis is to advance the development of DFPT within the
                      FLAPW method by refining and extending the existing
                      realisation in the community code FLEUR. I present the
                      general theory that leads to the existing implementation
                      and, from this starting point, develop correction terms that
                      improve upon previous results. I extend the framework from
                      the minimum base version towards spin-polarized magnetic
                      systems and systems with more than one atom per unit cell.
                      From the viewpoint of software engineering, I demonstrate
                      efficient integration of DFPT into the existing code,
                      minimizing redundancy and maximizing parallelization
                      options. I benchmark the improved implementation against FD
                      results calculated with FLEUR in conjunction with the
                      phonopy package and obtain an excellent agreement. The
                      validation set spans both materials that were previously
                      established but now show improved results, as well as
                      materials that were previously inaccessible. I calculate
                      both elemental and rare-earth magnets in different magnetic
                      configurations to elucidate, how magnetism and the magnetic
                      order impacts the phonon physics. Finally, I investigate
                      two-dimensional (2D) layered systems and unsupported
                      monolayers. The latter can be efficiently calculated with
                      the thin-film implementation in FLEUR, for which I present
                      an extension to the DFPT plugin.},
      cin          = {PGI-1},
      cid          = {I:(DE-Juel1)PGI-1-20110106},
      pnm          = {5211 - Topological Matter (POF4-521)},
      pid          = {G:(DE-HGF)POF4-5211},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      doi          = {10.34734/FZJ-2025-04180},
      url          = {https://juser.fz-juelich.de/record/1047249},
}