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| Home > Publications database > Multidimensional and Multimodal Soft X-ray Methods for Quantum Materials Research |
| Book/Dissertation / PhD Thesis | PUBDB-2025-01721 |
; ;
2025
Verlag Deutsches Elektronen-Synchrotron DESY
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2025-01721
Report No.: DESY-THESIS-2025-009
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.
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