Wolfram Brenig

A study of the dynamical two-dimensional (2D) nonlinear response of the dimerized spin- 1 / 2 chain to external electric fields is presented. The coupling of the spin system to light is set to arise from the inverse Dzyaloshinskii–Moriya interaction. In the XY-limit, we provide analytical expressions for the second-order nonlinear dynamical response function. Apart from multi spinon continua, this response displays a strong antidiagonal, i.e. galvanoelectric, feature in the 2D frequency plane. This allows to read off scattering rates of the fractional spinon excitations occurring beyond the free XY-limit. For the XXZ-case, we focus on the interaction-driven renormalization of the light–matter coupling by considering vertex corrections which are induced by the zz-exchange. We show this renormalization to modify the spinon joint density of states significantly, potentially allowing for the formation of in-gap bound states. As a result, the vertex corrected light–matter coupling can induce a dramatic spectral weight transfer to lower energies for the dynamical response function within the 2D frequency plane.

We apply the numerical linked cluster expansion (NLCE) to study thermodynamic properties of the proximate Kitaev magnet α -RuCl ₃ on the honeycomb lattice in the presence of a magnetic field. Using the extended spin-1/2 J – K –Γ model and based on documented exchange and magnetoelastic coupling parameters, we present results for the internal energy, the specific heat, and the magnetization. Moreover, the linear magnetostriction coefficient perpendicular to the plane is calculated, which is sensitive to changes of the in-plane spin–spin correlations. We find the magnetostriction to display a dip-like feature, in line with the temperature dependent and field-driven suppression of magnetic order in α -RuCl ₃ . Our results are consistent with previous findings, establishing NLCE also as a tool to study magnetoelastic features of quantum magnets.

We show that the concept of coherent phonon generation by second order response to incident electric laser fields, which is a hallmark of pump-probe spectroscopy on conventional solids, can be expanded to include frustrated quantum magnets. For that purpose, we analyze the Raman force on the shear phonons of a frustrated magnetoelectric bilayer spin system. The bilayer is a stacked triangular magnet, motivated by recently emerging type-II van der Waals multiferroic transition metal dihalides and comprises a spin system which allows for incommensurate spiral order. The magnon excitations are treated by linear spin wave theory. In the spiral state, a finite electric polarization is obtained from the spin-current interaction which induces a coupling of the magnons to the electric field. Scattering of the bilayer shear phonons from the magnons is derived from a magnetoelastic energy. In this scenario, a mixed three-point response function for the Raman force is evaluated. We find it to be strongly anisotropic and very sensitive to the magnon lifetime.

We investigate the effects of hydrostatic pressure on α − RuCl 3 , a prototypical material for the Kitaev spin model on a honeycomb lattice with a possible spin-liquid ground state. Using ultrasound measurements at pressures up to 1.16 GPa, we reveal significant modifications of the acoustic properties and the H − T phase diagram of this material. Hydrostatic pressure suppresses the three-dimensional magnetic order and induces a dimerization transition at higher pressures. At low pressures, the sound attenuation exhibits a linear temperature dependence, while above 0.28 GPa, it becomes nearly temperature independent, suggesting a shift in the phonon scattering regime dominated by Majorana fermions. These findings provide new insights into spin-strain interactions in Kitaev magnets and deliver a detailed characterization of the H − T phase diagram of α − RuCl 3 under hydrostatic pressure.

We study the dynamical response function relevant for two-dimensional coherent nonlinear optical spectroscopy of the antiferromagnetic frustrated J 1 − J 3 Heisenberg model on the square lattice within its long-range-ordered, incommensurate diagonal spiral phase. We argue that in this phase effective dipole coupling to the electric field is important, with the spin-current coupling potentially being the dominant mechanism for spin 1 / 2 . For this setting, we use linear spin wave theory to evaluate the leading nonlinear polarization response which is of second order in the driving field. We show that the response function features a strong antidiagonal, galvanoelectric feature. The width of this feature is set by relaxation rates beyond the noninteracting magnon picture, thereby providing access to single-magnon lifetimes within the multimagnon continuum of the response function. Moreover, the response function is shown to display various structures in the two-dimensional frequency plane related to exceptional regions of the magnon dispersion.

A study of the dynamical two-dimensional (2D) nonlinear response of the dimerized spin-$1{/}2$ chain to external electric fields is presented. The coupling of the spin system to those fields is set to arise from the inverse Dzyaloshinskii-Moriya interaction. In the XY-limit, we provide analytical expressions for the second-order nonlinear dynamical response function. Apart from multi spinon continua, this response displays a strong antidiagonal, i.e. galvanoelectric, feature in the 2D frequency plane. This allows to read off scattering rates of the fractional spinon excitations. For the XXZ-case, we focus on the interaction-driven renormalization of the light-matter coupling by considering vertex corrections which are induced by the zz-exchange. We show this renormalization to modify the spinon joint density of states significantly, potentially allowing for the formation of in-gap bound states. As a result, the vertex corrected light-matter coupling can induce a dramatic spectral weight transfer to lower energies for the dynamical response function within the 2D frequency plane.

We study the dynamical response functions relevant for electric field induced two-dimensional (2D) coherent nonlinear optical spectroscopy in a Kitaev magnet at finite temperature. We show that these response functions are susceptible to both types of fractional quasiparticles of this quantum spin-liquid, i.e. fermions and flux visons. Focusing on the second order response, we find a strong antidiagonal feature in the 2D frequency plane, related to the galvanoelectric effect of the fractional fermions. Perpendicular to the antidiagonal, the width of this feature is set by quasiparticle relaxation rates beyond the bare Kitaev magnet, thereby providing access to single-particle characteristics within a multi-particle continuum. While the 2D spectrum of the response is set by the fermionic quasiparticles and displays Fermi blocking versus temperature, the emergent bond randomness which arises due to thermally populated visons strongly modifies the fermionic spectrum. Therefore also the presence of gauge excitations is manifest in the 2D nonlinear response as the temperature is increased beyond the flux proliferation crossover.

We study electric field induced two-dimensional coherent nonlinear optical spectroscopy (2DCS) in a Kitaev magnet at finite temperature. We show that 2DCS is susceptible to both types of fractional quasiparticles of this quantum spin-liquid, i.e., fermions and flux visons. Focusing on the second order response, we find a strong antidiagonal feature in the two-dimensional frequency plane, related to the galvanoelectric effect of the fractional fermions. Perpendicular to the antidiagonal, the width of this feature is set by quasiparticle relaxation rates beyond the bare Kitaev magnet, thereby providing access to single-particle characteristics within the multi-particle 2DCS continuum. While the structure of the 2DCS susceptibility stems from the fermionic quasiparticles and displays Fermi blocking versus temperature, the emergent bond randomness which arises due to thermally populated visons strongly modifies the fermionic spectrum. Therefore also the presence of gauge excitations is manifest in the 2DCS susceptibility as the temperature is increased beyond the flux proliferation crossover. Our results are consistent with and extend previous findings on second harmonic generation in Kitaev magnets.