Thesis defense of Dag-Björn Hering
0 This thesis investigates two-dimensional quantum antiferromagnets, with a focus on the spin-$\frac{1}{2}$ antiferromagnetic easy-axis XXZ model on a square and a honeycomb lattice. Describing the excitations [...] literature, supporting the validity of CST also for gapped phases. Two-magnon excitations in the $S^z=0$ subspace comprise four bound states, which are tracked across the anisotropy range. Their successive [...] participation ratio. For the honeycomb lattice, the magnon-conserving CST is reliable only up to $\lambda ≲ 0.57$; an adapted CST scheme that decouples only the ground state while preserving couplings between different …
