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@PHDTHESIS{Schunck:628891,
      author       = {Schunck, Jan},
      othercontributors = {Beye, Martin and Schroer, Christian},
      title        = {{M}ultidimensional and {M}ultimodal {S}oft {X}-ray
                      {M}ethods for {Q}uantum {M}aterials {R}esearch},
      school       = {Universität Hamburg},
      type         = {Dissertation},
      address      = {Hamburg},
      publisher    = {Verlag Deutsches Elektronen-Synchrotron DESY},
      reportid     = {PUBDB-2025-01721, DESY-THESIS-2025-009},
      series       = {DESY-THESIS},
      pages        = {171},
      year         = {2025},
      note         = {Dissertation, Universität Hamburg, 2025},
      abstract     = {Quantum materials are governed by a complex interplay of
                      spin, orbit, charge and lattice degrees of freedom,
                      resulting in emergent phenomena like high-temperature
                      superconductivity, charge and orbital ordering and
                      insulator-to-metal transitions (IMTs). Often, the
                      interaction of these subsystems results in an energy
                      landscape with multiple local minima favouring different
                      phases. In many cases, two or more distinct phases coexist
                      and the macroscopic property of the material is shaped by
                      the properties of the individual phases as well as their
                      interaction. To understand the complexity that shapes
                      quantum materials, their properties need to be studied in
                      multiple dimensions of space, energy and time.X-rays are
                      indispensable tools for the study of quantum materials as
                      they enable probing on atomic length scales as well as
                      excitation of electrons bound in specific core levels.
                      Synchrotron radiation sources provide the coherence,
                      spectral brightness, flexible focusing capabilities and
                      tunability of the photon energy to adapt the X-ray beam
                      properties to the requirements of a specific measurement
                      scheme and sample. The photon energy can be tuned to
                      electronic resonances of one element to disentangle its role
                      for macroscopic functionality. Free-electron lasers (FELs)
                      extend this capability in the time domain down to pico- and
                      femtoseconds, the time scales of atomic and electronic
                      motion.This thesis presents the development of
                      multidimensional and multimodal soft X-ray methods that can
                      be tailored to address specific scientific challenges posed
                      by quantum materials. Multidimensional studies of incident
                      and emitted photon energies and spatial and temporal
                      dependencies as well as the dependence on fluence of a pump
                      laser that drives e.g. an IMT are discussed. Multimodal
                      studies allow observing quantum materials from the point of
                      view of different experimental techniques, like X-ray
                      imaging, X-ray absorption spectroscopy, X-ray emission
                      spectroscopy, (resonant) X-ray diffraction, resonant
                      inelastic X-ray scattering (RIXS) and angle-resolved
                      photoemission spectroscopy (ARPES).First, the RIXS imaging
                      method, which utilizes a transmission Fresnel zone plate to
                      combine soft X-ray absorption spectroscopy with microscopy
                      with a resolution of 1.8 µm, is presented. This method is
                      applied in a study of the IMT of VO$_2$ microsquares
                      measuring 30 µm $\times$ 30 µm. Imaging X-ray absorption
                      spectroscopy (XAS) shows that the phase transition
                      temperature at the edges of the squares is lower in
                      comparison to the centres by 1.2 K. This implies that bulk
                      properties of quantum materials may change upon structuring
                      on the microscale.Second, this method is transferred to
                      imaging X-ray diffraction (XRD) to investigate the doped
                      titanate Y$_{1-x}$Ca$_{x}$TiO$_3$ with $x=0.37$, revealing
                      insulating and metallic phases which coexist in curved,
                      striped domains across unusually large temperature regions.
                      This observation is related to a varying chemical
                      inhomogeneity of about $x\pm{0.01}$, likely arising during
                      crystal growth.Next, excitation of the electronic subsystem
                      in quantum materials with femtosecond infra-red laser pulses
                      also drives insulator-to-metal transitions. For the study of
                      ultrafast dynamics of magnetite (Fe$_3$O$_4$) at an FEL,
                      zone plates can also be used for time-to-space mapping,
                      recording a delay range of several picoseconds as well as an
                      extended fluence range simultaneously. This time-to-space
                      mapping setup combines temporal, spatial and pump fluence
                      information and may be developed to record single-shot
                      experiments in the future.Lastly, a method, termed
                      photoelectron spectrometry for the analysis of X-rays (PAX),
                      which converts RIXS photons to photoelectrons via the
                      photoelectric effect, is developed towards high energy
                      resolution to investigate a sample from the family of
                      high-temperature superconducting cuprates. PAX enables
                      simultaneous recording of a range of photon-sample momentum
                      transfer, corresponding to a significant part of the first
                      Brillouin zone in the investigated system. In comparison to
                      grating-based RIXS spectrometers, a PAX instrument is much
                      more compact, saving money and experimental space. The
                      success of the PAX method resulted in the development of a
                      dedicated ultra-high vacuum chamber, soon to be
                      commissioned, which promises a significant improvement in
                      photon count rate and energy resolution, as well as the
                      combination with ARPES.In summary, this thesis presents
                      experimental developments that enable the study of quantum
                      materials through the utilisation of diverse soft X-ray
                      methods in conjunction with a spatial resolution on the
                      micrometer level, temporal resolution on the level of 100 fs
                      and energy resolution on the level of 100 meV. Furthermore,
                      it outlines concepts to improve this energy and spatial
                      resolution by approximately one order of magnitude. The
                      advancement of the experimental tools described in this
                      thesis will facilitate a deeper comprehension of the
                      complexity of quantum materials and enable us as a society
                      to harness phenomena occurring in quantum materials.},
      cin          = {FS-FLASH},
      cid          = {I:(DE-H253)FS-FLASH-20140814},
      pnm          = {632 - Materials – Quantum, Complex and Functional
                      Materials (POF4-632) / 6G2 - FLASH (DESY) (POF4-6G2) / 6G3 -
                      PETRA III (DESY) (POF4-6G3)},
      pid          = {G:(DE-HGF)POF4-632 / G:(DE-HGF)POF4-6G2 /
                      G:(DE-HGF)POF4-6G3},
      experiment   = {EXP:(DE-H253)F-BL2-20150101 / EXP:(DE-H253)P-P04-20150101 /
                      EXP:(DE-H253)Nanolab-02-20150101},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      doi          = {10.3204/PUBDB-2025-01721},
      url          = {https://bib-pubdb1.desy.de/record/628891},
}