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We predict a general local spin-flip transition mechanism caused by magnetic quantum impurities in (partially) polarized phases of quantum magnets in the absence of conservation laws. This transition arises when a magnon bound state crosses zero energy as function of the magnetic field. As application, we study 2D van der Waals magnets described by the Kitaev-Heisenberg honeycomb model which applies to the transition metal trihalides CrI$_3$ and $\alpha$-RuCl$_3$. We consider adatom and substitutional impurity positions, and show how spin-flip transitions can be detected in scanning tunneling spectroscopy., Comment: 13 pages, 7 figures

We formulate a Majorana mean-field theory for the extended $JK\Gamma$ Kitaev model in a magnetic Zeeman field of arbitrary direction, and apply it for studying spatially inhomogeneous states harboring vortices. This mean-field theory is exact in the pure Kitaev limit and captures the essential physics throughout the Kitaev spin liquid phase. We determine the charge profile around vortices and the corresponding quadrupole tensor. The quadrupole-quadrupole interaction between distant vortices is shown to be either repulsive or attractive, depending on parameters. We predict that electrically biased scanning probe tips enable the creation of vortices at preselected positions. Our results open new perspectives for the electric manipulation of Ising anyons in Kitaev spin liquids., Comment: 15 pages, 12 figures

We describe scanning tunneling spectroscopic signatures of Majorana zero modes (MZMs) in Kitaev spin liquids. The tunnel conductance is determined by the dynamical spin correlations of the spin liquid, which we compute exactly, and by spin-anisotropic cotunneling form factors. Near a $\mathbb{Z}_2$ vortex, the tunnel conductance has a staircase voltage dependence, where conductance steps arise from MZMs and (at higher voltages) from additional vortex configurations. By scanning the probe tip position, one can detect the vortex locations. Our analysis suggests that topological magnon bound states near defects or magnetic impurities generate spectroscopic signatures that are qualitatively different from those of MZMs., Comment: 14 pages, 7 figures

We show that line defects in a non-Abelian Kitaev spin liquid harbor gapless one-dimensional Majorana modes if the interaction across the defect falls below a critical value. Treating the weak interaction at the line defect within a mean-field approximation, we determine the critical interaction strength as a function of the external magnetic field. In the gapless regime, we use the low-energy effective field theory to calculate the spin-lattice relaxation rate for a nuclear spin near the defect and find a cubic temperature dependence that agrees with experiments in the Kitaev material $\alpha\text{-RuCl}_3$., Comment: 11 pages, 4 figures