arXiv subjects

Kai Leckron

Publications and source records attributed to Kai Leckron.

8 recordsLinked to original sources

arXiv2026

Interplay of exchange and spin-conserving scattering processes in a ferromagnetic two-sublattice system

In magnetic alloys or hybrid systems formed by molecular magnets coupled to a magnetic substrate, the exchange interaction leads to fast spin dynamics after excitation. We investigate the electronic charge and spin dynamics due to the exchange interaction in a two-sublattice system with antiferromagnetic coupling. We employ a simplified model for a ferrimagnetic alloy as a coupled system of itinerant and localized electron states together with an exchange coupling between the two. For the itinerant system we include electron-electron Coulomb scattering and electron-phonon scattering. We study numerically the heat-induced ultrafast magnetization dynamics due to the interplay of exchange scattering and spin-independent scattering processes and discuss different scenarios for the demagnetization and relaxation dynamics of the sublattices. Our results highlight the impact of spin-conserving electron-electron scattering processes on the exchange-driven spin dynamics on ultrashort timescales.

cond-mat.mtrl-sci↗

arXiv2026

Interplay of electron-magnon scattering and spin-orbit induced electronic spin-flip scattering in a two-band Stoner model

This paper presents a theoretical investigation of electron-magnon scattering processes in the ultrafast demagnetization in itinerant ferromagnets. In the framework of a ferromagnetic model system, we compute the spin-dependent dynamics of electrons in itinerant Bloch states by including electron-magnon and electron-electron scattering processes on an equal footing. While the former process flips the electronic spin accompanied by the creation or destruction of a magnon, the latter exchanges electronic angular momentum with the lattice due to the influence of spin-orbit coupling. We show that, for a realistic choice of the electron-magnon interaction and deposited pulse energy, the interplay of these two different scattering mechanisms leads to the creation of magnons and a transfer of angular momentum to the lattice that constitutes an essentially non-equilibrium microscopic scenario for the ultrafast demagnetization process in itinerant ferromagnets.

cond-mat.mtrl-sci↗

arXiv2025

Optical signatures of bulk g-wave altermagnetism in MnTe

For planar d-wave altermagnets, it has been shown that a spin polarization can be induced in a controlled fashion by ultrashort-pulse excitation, even though the material is magnetically compensated. Here, we theoretically analyze the response of the prototypical bulk $g$-wave altermagnet $\alpha$-MnTe to linearly polarized ultrashort pulses. We demonstrate how the electronic spin response in $\alpha$-MnTe exhibits different symmetry characteristics by calculating the excited electron distributions based on ab initio band structure data. These characteristics depend not only on the nodal planes of the bulk g-wave altermagnet, but also on the excitation pulse. We present a simple procedure to analyze the excited-state characteristics via two-dimensional cuts through the three-dimensional Brillouin zone, which can be used as guidance to present-day magneto-optical techniques.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast electron-phonon scattering in antiferromagnetic Dirac semimetals

Topological antiferromagnetic systems, which exhibit anisotropic band structures combined with complex relativistic spin structures in momentum space, have shown strong magnetoresistance effects driven by Dirac fermion characteristics. While these new antiferromagnets have been studied in transport experiments, very little is known about their spin-dependent electronic dynamics on ultrafast timescales and far-from-equilibrium behavior. This paper investigates theoretically the spin-dependent electronic dynamics due to electron-phonon scattering in a model electronic band structure that corresponds to a Dirac semimetal antiferromagnet. Following a spin conserving instantaneous excitation we obtain a change of the antiferromagnetic spin polarization due to the scattering dynamics for the site-resolved spin expectation values. This allows us to identify fingerprints of the anisotropic band structure in the carrier dynamics on ultrashort timescales.

cond-mat.mes-hall↗

arXiv2024

Ultrafast electron dynamics in altermagnetic materials

Altermagnets constitute a new class of magnetic materials that combine properties previously thought to be exclusive to either antiferromagnets or ferromagnets, and have unique properties of their own. In particular, a combination of symmetries connecting magnetic sublattices gives rise to a band spin splitting exhibiting unconventional d, g, or i-wave character. Their unique electronic properties have already led to new spin-dependent transport effects. Here, we consider their spin and charge dynamics on ultrafast timescales. We use a minimal tight binding model that captures the main features of the altermagnetic candidate material KRu$_4$O$_8$. In the framework of this model, we compute the spin-dependent electronic scattering dynamics after ultrashort-pulse excitation and show through these microscopic calculations how electron-electron and electron-phonon scattering processes redistribute optically excited carriers in a 2D slice of the Brillouin zone. We find that the optically excited spin polarization is long lived (~1ps) compared to the electron-electron momentum scattering lifetime of roughly 10fs. This contrasts remarkably with the much shorter spin lifetimes observed in typical ultrafast electronic spin dynamics in conventional ferromagnets and antiferromagnets, making these pulse-driven spin excitation experiments a key probe of altermagnetism.

cond-mat.mtrl-sci↗

arXiv2024

All optical excitation of spin polarization in d-wave altermagnets

The recently discovered altermagnets exhibit collinear magnetic order with zero net magnetization but with unconventional spin-polarized d/g/i-wave band structures, expanding the known paradigms of ferromagnets and antiferromagnets. In addition to novel current-driven electronic transport effects, the unconventional time-reversal symmetry breaking in these systems also makes it possible to obtain a spin response to \emph{linearly polarized} fields in the optical frequency domain. We show through ab-initio calculations of the prototypical d-wave altermagnet RuO$_2$, with a symmetry combining twofold spin rotation with fourfold lattice rotation, $[C_2\|C_{4z}]$, that there is an optical analogue of a spin splitter effect, as the coupling to a linearly polarized exciting laser field makes the d-wave character of the altermagnet directly visible. By magneto-optical measurements on RuO$_2$ films of different thicknesses ranging from $2$ to $8\,$nanometers, we demonstrate the predicted connection of the linear polarization of an ultrashort pump pulse to the sign and magnitude of the optically excited electronic spin polarization in the ultrathin RuO$_2$ films. The possibility of exciting and controlling an electronic spin polarization by linearly polarized optical pulses in a compensated system is a unique consequence of the altermagnetic material properties. Our experimental results therefore establish an optical pump-probe based protocol for detection of altermagnetic characteristics in ultrathin RuO$_2$ films, but our all-optical approach should apply more generally to materials in this altermagnetic symmetry class.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast Demagnetization Dynamics Due to Electron-Electron Scattering and Its Relation to Momentum Relaxation in Ferromagnets

We analyze theoretically the demagnetization dynamics in a ferromagnetic model system due to the interplay of spin-orbit coupling and electron-electron Coulomb scattering. We compute the $k$-resolved electronic reduced spin-density matrix including precessional dynamics around internal spin-orbit and exchange fields as well as the electron-electron Coulomb scattering for densities and spin coherences. Based on a comparison with numerical solutions of the full Boltzmann scattering integrals, we establish that the $k$-resolved reduced spin-density matrix dynamics are well described using a simpler generalized relaxation-time ansatz for the reduced spin-density matrix. This ansatz allows one to relate the complicated scattering dynamics underlying the demagnetization dynamics to a physically meaningful momentum relaxation time $τ$. Our approach reproduces the behaviors of the demagnetization time $τ_{m} \propto 1/τ$ and $τ_{m} \propto τ$ for the limits of short and long $τ$, respectively, and is also valid for the intermediate regime. The ansatz thus provides a tool to include the correct demagnetization behavior in approaches that treat other contributions to the magnetization dynamics such as transport or magnon/phonon dynamics.

cond-mat.mtrl-sci↗

arXiv2017

Ultrafast spin-lattice relaxation in ferromagnets including effective spin-orbit fields

We investigate ultrafast demagnetization due to electron-phonon interaction in a model band-ferromagnet. We show that the microscopic mechanism behind the spin dynamics due to electron-phonon interaction is the interplay of scattering and the precession around momentum-dependent effective internal spin-orbit magnetic fields. The resulting magnetization dynamics can only be mimicked by spin-flip transitions if the spin precession around the internal fields is sufficiently fast (compared to the scattering time) so that it averages out the transverse spin components.

cond-mat.mtrl-sci↗