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Opponent: Professor Maiko Kofu, University of Tokyo, Institute of Solid State Physics, Tokyo, Japan
Supervisor: Associate professor Martin Månsson, Ljus och materiens fysik
This thesis presents a series of investigations focusing on emergent phenomena in low-dimensional materials, with particular emphasis on superconductivity, charge density waves (CDWs), and magnetism. The primary experimental techniques employed are muon spin rotation/relaxation/resonance ($\mu^+$SR), neutron scattering, and X-ray scattering. The relevant theoretical background is introduced in Chapter 2. The fundamentals of the experimental techniques are presented in Chapter 3. Chapter 4 outlines the theoretical techniques used in this thesis. Chapter 5 provides a summary of the work performed on the square lattice Heisenberg antiferromagnet CuF$_2$(D$_2$O)$_2$(pyz), focusing on the field effects on the magnetic excitations. Chapter 6 summarizes the appended papers, both the background of the material involved as well as the key results and findings. In the family of transition metal dichalcogenides, materials such as 1T-TiSe$_2$ and 2H-TaS$_2$ are studied to investigate their superconducting ground states. In TiSe$_2$, a multi-gap superconducting state is identified, with evidence pointing to a Lifshitz transition under pressure. In 2H-TaS$_2$, conventional superconductivity is confirmed alongside unconventional CDW behavior, including the first observed Kohn anomaly in the system. These findings underscore the delicate interplay between structural polymorphism, dimensionality, and external tuning parameters. In the field of low-dimensional magnetism, CuF$_2$(D$_2$O)$_2$(pyz) is examined as a realization of the square lattice Heisenberg antiferromagnet. Here, unexpected high-energy excitations are observed under moderate fields, supported by matrix product state simulations. Another key material, (C$_5$H$_9$NH$_3$)$_2$CuBr$_4$, serves as a platform for exploring the quantum spin ladder model under pressure. The application of hydrostatic pressure induces a transition from antiferromagnetic disorder to long-range ferromagnetic order. Chapter 7 concludes the thesis by summarizing the key findings and proposing future avenues of research, including further spectroscopic studies to elucidate the superconducting and magnetic mechanisms at play. While these studies are grounded in fundamental science, their implications may extend toward future materials development for energy-efficient technologies. The work presented here highlights the richness of low-dimensional systems as fertile ground for novel quantum phenomena and deepens our understanding of how interactions and symmetry shape the physical world.