arXiv subjects

Dag-Björn Hering

Publications and source records attributed to Dag-Björn Hering.

8 recordsLinked to original sources

arXiv2026

Quantum decay of magnons in the unfrustrated honeycomb Heisenberg model

We investigate the physical properties of elementary magnon excitations of the ordered antiferromagnetic Heisenberg model on the honeycomb lattice using quantum Monte Carlo (QMC) simulations, series expansions (SE), and continuous similarity transformations (CST). The stochastic analytic continuation method is used to determine the dynamic structure factor from correlation functions in imaginary time obtained by QMC. In contrast to the "roton minimum" of the square lattice Heisenberg antiferromagnet, we find that magnons on the honeycomb lattice completely decay in the corner of the Brillouin zone ($K$-point); the entire weight is shifted into the continuum. These findings are fully supported by SE and CST in momentum space. The extrapolated one-magnon dispersion obtained from SE about the Ising limit quantitatively agrees with the extracted QMC excitation energies except around the $K$-point, where large uncertainties in the extrapolation indicate the magnon decay. This quantum decay is further confirmed and understood by the CST, which yields a divergent flow when enforcing a magnon quasi-particle picture. The divergence originates from strong attractive magnon-magnon interactions leading to a bound state and thereby to a three-magnon continuum overlapping with the one-magnon state. This has the magnon quasi-particle picture break down at high energies on the honeycomb lattice.

cond-mat.str-el↗

arXiv2026

Control of the Néel vector in the quantum antiferromagnetic honeycomb lattice

The switching of antiferromagnetic order and its efficient control promise to enable ultrafast manipulation of data and large storage capacity. Recently, the time-dependent Schwinger boson mean-field theory has been successfully developed to study the Néel vector switching in hypercubic antiferromagnetic lattices. In the present article, we aim at demonstrating that the approach is a well-justified framework to capture the essentials of the switching process, even in low-symmetry quantum antiferromagnets. To this end, we show the possibility of the sublattice magnetization reorientation in the quantum antiferromagnetic honeycomb lattice. First, equilibrium properties of the honeycomb lattice are analyzed using the Schwinger boson mean-field theory and compared to the continuous similarity transformation method to justify the applicability of the approach. Then, the Schwinger boson mean-field theory is employed for switching process. We provide a comprehensive answer to the question what the threshold switching fields are when the coordination number of the lattice is varied. Indeed, the results of the study reveal a correspondence between lattice structures and the threshold fields by comparing them for the square and the simple cubic lattices and the honeycomb lattice. The findings of the present article extend the foundation for future theoretical and computational advancements in the field of antiferromagnetic switching. These advancements are of particular relevance for the development of ultrafast spintronic or magnonic devices.

cond-mat.str-el↗

arXiv2025

Quantum effects in the magnon spectrum of 2D altermagnets via continuous similarity transformations

We investigate quantum effects on magnon excitations in a minimal spin-1/2 Heisenberg model for 2D altermagnets on the square lattice. A continuous similarity transformation is applied in momentum space to derive an effective Hamiltonian that conserves the number of magnon excitations. This allows us to quantitatively calculate the one-magnon dispersion, the effects of magnon-magnon interactions, and the dynamic structure factor in a certain range of parameters. In particular, we focus on the altermagnetic spin splitting of the magnon bands and the size of the roton minimum. We further map out divergencies of the continuous similarity transformation for different types of generators, which signal either the breakdown of the Néel-ordered phase or the presence of significant magnon decay.

cond-mat.str-el↗

arXiv2024

Quantum melting of long-range ordered quantum antiferromagnets investigated by momentum-space continuous similarity transformations

We apply continuous similarity transformations (CSTs) to study the zero-temperature breakdown of long-range ordered quantum antiferromagnets. The CST flow equations are truncated in momentum space by the scaling dimension so that all operators with scaling dimension up to two are taken into account. We determine the quantum phase transition out of the Néel-ordered phase in the unfrustrated square lattice Heisenberg bilayer as well as the quantum melting of the Néel-ordered and columnar phase in the highly frustrated $J_1$-$J_2$ model on the square lattice. In all cases the CST is set up to isolate the ground state so that the stability of the flow equations, the ground-state energy, and the sublattice magnetization are used to explore the long-range ordered phases. We extract quantum-critical points which agree well with values in the literature. Further, we estimate the associated critical exponents $α$ and $β$ which turns out to be a challenging task for the CST approach.

cond-mat.str-el↗

arXiv2024

Quantitative description of long-range order in the anisotropic spin-1/2 Heisenberg antiferromagnet on the square lattice

The quantitative description of long-range order remains a challenge in quantum many-body physics. We provide zero-temperature results from two complementary methods for the ground-state energy per site, the sublattice magnetization, the spin gap, and the transverse spin correlation length for the spin-1/2 anisotropic quantum Heisenberg antiferromagnet on the square lattice. On the one hand, we use exact, large-scale quantum Monte Carlo (QMC) simulations. On the other hand, we use the semi-analytic approach based on continuous similarity transformations in terms of elementary magnon excitations. Our findings confirm the applicability and quantitative validity of both approaches along the full parameter axis from the Ising point to the symmetry-restoring phase transition at the Heisenberg point and further provide quantitative reference results in the thermodynamic limit. In addition, we analytically derive the relation between the dispersion and the correlation length at zero temperature in arbitrary dimension, and discuss improved second-moment QMC estimators.

cond-mat.str-el↗

arXiv2022

Continuous similarity transformation for critical phenomena: easy-axis antiferromagnetic XXZ model

We apply continuous similarity transformations (CSTs) to the easy-axis antiferromagnetic XXZ-model on the square lattice. The CST flow equations are truncated in momentum space by the scaling dimension $d$ so that all contributions with $d\le 2$ are taken into account. The resulting quartic magnon-conserving effective Hamiltonian is analyzed in the zero-, one-, and two-magnon sector. In this way, a quantitative description of the ground-state energy, the one-magnon dispersion and its gap as well as of two-magnon bound states is gained for anisotropies ranging from the gapped Ising model to the gapless Heisenberg model. We discuss the critical properties of the gap closing as well as the evolution of the one-magnon roton mininum. The excitation energies of two-magnon bound states are calculated and their decay into the two-magnon continuum is determined via the inverse participation ratio.

cond-mat.str-el↗

arXiv2022

Dynamic mean-field theory for dense spin systems at infinite temperature

A dynamic mean-field theory for spin ensembles (spinDMFT) at infinite temperatures on arbitrary lattices is established. The approach is introduced for an isotropic Heisenberg model with $S = \tfrac12$ and external field. For large coordination numbers, it is shown that the effect of the environment of each spin is captured by a classical time-dependent random mean-field which is normally distributed. Expectation values are calculated by averaging over these mean-fields, i.e., by a path integral over the normal distributions. A self-consistency condition is derived by linking the moments defining the normal distributions to spin autocorrelations. In this framework, we explicitly show how the rotating wave approximation becomes a valid description for increasing magnetic field. We also demonstrate that the approach can easily be extended. Exemplarily, we employ it to reach a quantitative understanding of a dense ensemble of spins with dipolar interaction which are distributed randomly on a plane including static Gaussian noise as well.

cond-mat.stat-mech↗

arXiv2021

Charge dynamics in magnetically disordered Mott insulators

With the aid of both a semi-analytical and a numerically exact method we investigate the charge dynamics in the vicinity of half-filling in the one- and two-dimensional $t$-$J$ model derived from a Fermi-Hubbard model in the limit of large interaction $U$ and hence small exchange coupling $J$. The spin degrees of freedom are taken to be disordered. So we consider the limit $0 < J \ll T \ll W$ where $W$ is the band width. We focus on evaluating the spectral density of a single hole excitation and the charge gap which separates the upper and the lower Hubbard band. One of the key findings is the evidence for the absence of sharp edges of the Hubbard band, instead Gaussian tails appear.

cond-mat.str-el↗