arXiv subjects

Ruben Burkard

Publications and source records attributed to Ruben Burkard.

4 recordsLinked to original sources

arXiv2026

Anisotropic vacancy-induced magnetization textures in altermagnets

We study magnetic textures induced by vacancies in altermagnets using microscopic simulations and low-energy field theory. We show that a vacancy generically produces a real-space anisotropic distortion of the magnetic order, whose structure encodes the symmetry of the underlying altermagnetic state. This impurity response offers a direct route to detecting altermagnetic order with locally resolved probes. We demonstrate this for both classical altermagnets, where vacancies generate anisotropic magnetization textures in a transverse magnetic field, and quantum models, where fluctuations induce longitudinal power-law decaying magnetic distortions even at zero field.

cond-mat.str-el↗

arXiv2025

High-temperature series expansion of the dynamic Matsubara spin correlator

The high-temperature series expansion for quantum spin models is a well-established tool to compute thermodynamic quantities and equal-time spin correlations, in particular for frustrated interactions. We extend the scope of this expansion to the dynamic Matsubara spin-spin correlator and develop an algorithm that yields exact expansion coefficients in the form of rational numbers. We focus on Heisenberg models with a single coupling constant J and spin lengths S=1/2,1. The expansion coefficients up to 12th order in J/T are precomputed on all possible $\sim 10^6$ graphs embeddable in arbitrary lattices and are provided in a repository. This enables calculation of static momentum-resolved susceptibilities for arbitrary site-pairs or wavevectors. We test our results for the antiferromagnetic S=1/2 chain and triangular lattice model. An important application that we discuss in a companion letter is the calculation of real-frequency dynamic structure factors. This is achieved by identifying the high-frequency expansion coefficients of the Matsubara correlator with frequency moments of the spectral function.

cond-mat.str-el↗

arXiv2025

Dynamic correlations of frustrated quantum spins from high-temperature expansion

For quantum spin systems in equilibrium, the dynamic structure factor (DSF) is among the most feature-packed experimental observables. However, from a theory perspective it is often hard to simulate in an unbiased and accurate way, especially for frustrated and high-dimensional models at intermediate temperature. To address this challenge, we compute the DSF from a dynamic extension of the high-temperature expansion to frequency moments. We focus on nearest-neighbor Heisenberg models with spin-lengths S=1/2 and 1. We provide comprehensive benchmarks and consider a variety of frustrated two- and three-dimensional antiferromagnets as applications. In particular we shed new light on the anomalous intermediate temperature regime of the S=1/2 triangular lattice model and reproduce the DSF measured recently for the S=1 pyrochlore material NaCaNi2F7. An open-source numerical implementation for arbitrary lattice geometries is also provided.

cond-mat.str-el↗

arXiv2024

Dipolar ordering transitions in many-body quantum optics: Analytical diagrammatic approach to equilibrium quantum spins

Quantum spin models with a large number of interaction partners per spin are frequently used to describe modern many-body quantum optical systems like arrays of Rydberg atoms, atom-cavity systems or trapped ion crystals. For theoretical analysis the mean-field (MF) ansatz is routinely applied. However, besides special cases of all-to-all or strong long range interactions, the MF ansatz provides only approximate results. Here we present a systematic correction to MF theory based on diagrammatic perturbation theory for quantum spin correlators in thermal equilibrium. Our analytic results are universally applicable for any lattice geometry and spin-length S. We provide pre-computed and easy-to-use building blocks for Ising, Heisenberg and transverse field Ising models in the symmetry-unbroken regime. We showcase the quality and simplicity of the method by computing magnetic phase boundaries and excitations gaps. We also treat the Dicke-Ising model of ground-state superradiance where we show that corrections to the MF phase boundary vanish.

cond-mat.quant-gas↗