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TY - THES AU - Caci, Nils TI - Quantum Monte Carlo studies of spin-nematic phase transitions and frustrated quantum magnets PB - RWTH Aachen University VL - Dissertation CY - Aachen M1 - RWTH-2023-10470 SP - 1 Online-Ressource : Illustrationen, Diagramme PY - 2023 N1 - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University N1 - Dissertation, RWTH Aachen University, 2023 AB - In this thesis, we study different quantum Heisenberg antiferromagnets (AFMs) by means of unbiased large-scale quantum Monte Carlo (QMC) simulations. First, we consider the spin-1 Heisenberg AFM on the honeycomb lattice in the presence of an easy-plane single-ion anisotropy. Here, an easy-plane anisotropy stabilizes correlations in the spin-XY plane and leads to a thermal Berezinskii-Kosterlitz-Thouless (BKT) transition. Motivated by recent experiments on the spin-1 compound BaNi<sub>2</sub>V<sub>2</sub>O<sub>8</sub> we particularly investigate whether reminiscent BKT scaling in the magnetic correlation length prevails when an additional weak easy-axis single-ion anisotropy is considered. Further, we provide a systematic characterization of critical phenomena arising in such planar spin-1 systems. Next, we investigate the spin-1/2 Heisenberg AFM on the diamond-decorated square lattice which is a highly frustrated system of coupled orthogonal dimers. This system is known to feature various interesting ground-state phases in the absence of a magnetic field that consist of a ferrimagnetic phase, a phase where localized spin-singlets form on dimers as well as larger four site tetramer clusters and a phase where the ground state is a product state of decoupled localized spin-singlets and spin-monomers. In the presence of a magnetic field, there is a discontinuous phase transition line separating the ferrimagnetic and monomer-dimer phase, which we find to extend to finite temperature and terminate in a line of critical points belonging to the 2D Ising universality class. The emergence of Ising critical points can be shown analytically in a related Ising-Heisenberg model which we address as well. Further, we examine the thermal properties of the spin-1/2 kagome Heisenberg AFM with a breathing distortion, where the coupling between upwards and downwards facing triangles is different. The spin-1/2 Heisenberg AFM on the kagome lattice is a promising candidate for a quantum spin liquid (QSL) state and was subject to intense research efforts. This QSL state is predicted to be stable even under strong breathing anisotropies. Employing unbiased QMC studies within a combined few-site cluster basis we compare the performance of different computational bases and report the thermodynamic properties for strong breathing anisotropies. Finally, we consider the bilinear-biquadratic spin-1 Heisenberg model on the cubic lattice, which is well known to feature a spin-nematic phase transition. Previous QMC studies report a continuous phase transition for the thermal melting of the spin-nematic state. By large scale QMC studies for system sizes beyond those previously accessible, we show that a weakly first-order phase transition is found instead. Further, we compare the properties of the spin-nematic state to exact analytical predictions based on Poisson-Dirichlet distributions in a loop model formulation. Our results establish weakly first-order phase transitions in basic SU(2) symmetric spin-1 systems where quantitative insights are available. LB - PUB:(DE-HGF)11 DO - DOI:10.18154/RWTH-2023-10470 UR - https://publications.rwth-aachen.de/record/972923 ER -