Microscopic theory of current-induced skyrmion transport and its application in disordered spin textures

Introduction: Magnetic skyrmions hold great promise for realizing compact and stable memory devices that can be manipulated at very low energy costs via electronic current densities.Methods: In this work, we extend a recently introduced method to describe classical skyrmion textures coupled to dynamical itinerant electrons. In this scheme, the electron dynamics is described via nonequilibrium Green’s function (NEGF) within the generalized Kadanoff–Baym ansatz, and the classical spins are treated via the Landau–Lifshitz–Gilbert equation. Here, the framework is extended to open systems by the introduction of a non-interacting approximation to the collision integral of NEGFs. This, in turn, allows us to perform computations of the real-time response of skyrmions to electronic currents in large quantum systems coupled to electronic reservoirs, which exhibit linear scaling in the number of time steps. We use this approach to investigate how electronic spin currents and dilute spin disorder affect skyrmion transport and the skyrmion Hall drift.Results: Our results show that the skyrmion dynamics is sensitive to a specific form of the spin disorder, such that different disorder configurations lead to qualitatively different skyrmion trajectories for the same applied bias.Discussion: This sensitivity arises from the local spin dynamics around the magnetic impurities, a feature that is expected not to be well-captured by phenomenological or spin-only descriptions. At the same time, our findings illustrate the potential of engineering microscopic impurity patterns to steer skyrmion trajectories

Microscopic theory of current-induced skyrmion transport and its application in disordered spin textures

Magnetic skyrmions hold great promise for realizing compact and stable memory devices that can be manipulated at very low energy costs via electronic current densities. In this work, we extend a recently introduced method to describe classical skyrmion textures coupled to dynamical itinerant electrons. In this scheme, the electron dynamics is described via nonequilibrium Green's functions (NEGF) within the generalized Kadanoff-Baym ansatz, and the classical spins are treated via the Landau-Lifshitz-Gilbert equation. The framework is here extended to open systems, by the introduction of a non-interacting approximation to the collision integral of NEGF. This, in turn, allows us to perform computations of the real-time response of skyrmions to electronic currents in large quantum systems coupled to electronic reservoirs, which exhibit a linear scaling in the number of time steps. We use this approach to investigate how electronic spin currents and dilute spin disorder affects skyrmion transport and the skyrmion Hall drift. Our results show that the skyrmion dynamics is sensitive to the specific form of spin disorder, such that different disorder configurations leads to qualitatively different skyrmion trajectories for the same applied bias. This sensitivity arises from the local spin dynamics around the magnetic impurities, a feature that is expected not to be well captured by phenomenological or spin-only descriptions. At the same time, our findings illustrate the potential of engineering microscopic impurity patterns to steer skyrmion trajectories.Comment: 15 pages and 4 figures. Supplementary: 2 pages and no figure

Second Harmonic Generation from Ultracold Bosons in an Optical Cavity

Within a cavity quantum electrodynamics description, we characterize the fluorescent spectrum from ultracold bosons atoms, in the second harmonic generation (SHG) and resonant cases. Two situations are considered: i) bosons loaded into an optical lattice and ii) in a trapped two-component dilute Bose-Einstein Condensate (BEC), in the regime where the Bogoliubov approximation is often employed. Atom and photon degrees of freedom are treated on equal footing within an exact time-dependent configuration interaction scheme, and cavity leakage is included by including classical oscillator baths. For optical lattices, we consider few bosons in short chains, described via the Bose-Hubbard model with two levels per site, and we find that the spectral response grows on increasing the number of atoms at weak interactions, but diminishes at high interactions (if the number of chain sites does not exceed the number of atoms), and is shifted to lower frequency. In the BEC regime, the spectra display at noticeable extent a scaling behavior with the number of particles and a suitable rescaling of the BEC-cavity and inter-particle interactions, whilst the SHG spectrum redshifts at large atom-atom correlations. Overall, our results provide some general trends for the fluorescence from ultracold bosons in optical cavities, which can be of reference to experimental studies and further theoretical work

Time resolved multi-photon effects in the fluorescence spectra of two-level systems at rest and in motion

We study the time-resolved fluorescence spectrum in two-level systems interacting with an incident coherent field, both in the weak and intermediate coupling regimes. For a single two-level system in the intermediate coupling case, as time flows, the spectrum develops distinct features, that are not captured by a semi-classical treatment of the incident field. Specifically, for a field on resonance with the atomic transition energy, the usual Mollow spectrum is replaced by a four peak structure, and for a frequency that is half of the atomic transition energy, the time-dependent spectrum develops a second harmonic peak with a superimposed Mollow triplet. In the long-time limit, our description recovers results previously found in the literature. After analyzing why a different behavior is observed in the quantum and classical dynamics, the reason for the occurrence of a second harmonic signal in a two-level system is explained via a symmetry analysis of the total (electron and photon) system, and in terms of a three level system operating in limiting regimes. We find an increased second harmonic signal in an array of two-level systems, suggesting a superradiance-like enhancement for multiple two-level systems in cavity setups. Finally, initial explorative results are presented for two-level model atoms entering and exiting a cavity, which hint at an interesting interplay between cavity-photon screening and atomic dynamics effects.Comment: 19 pages, 11 figures. To be published in Phys. Rev.

Zeno-clocking the Auger decay

A tenet of time-resolved spectroscopy is -faster laser pulses for shorter timescales- . Here we suggest turning this paradigm around, and slow down the system dynamics via repeated measurements, to do spectroscopy on longer timescales. This is the principle of the quantum Zeno effect. We exemplify our approach with the Auger process, and find that repeated measurements increase the core-hole lifetime, redistribute the kinetic energy of Auger electrons, and alter entanglement formation. We further provide an explicit experimental protocol for atomic Li, to make our proposal concrete.Comment: 5 pages, 4 figures, supplemental material provide

Light-induced topological magnons in two-dimensional van der Waals magnets

Driving a two-dimensional Mott insulator with circularly polarized light breaks time-reversal and inversion symmetry, which induces an optically-tunable synthetic scalar spin chirality interaction in the effective low-energy spin Hamiltonian. Here, we show that this mechanism can stabilize topological magnon excitations in honeycomb ferromagnets and in optical lattices. We find that the irradiated quantum magnet is described by a Haldane model for magnons that hosts topologically-protected edge modes. We study the evolution of the magnon spectrum in the Floquet regime and via time propagation of the magnon Hamiltonian for a slowly varying pulse envelope. Compared to similar but conceptually distinct driving schemes based on the Aharanov-Casher effect, the dimensionless light-matter coupling parameter $\lambda = eEa/\hbar\omega$ at fixed electric field strength is enhanced by a factor $\sim 10^5$. This increase of the coupling parameter allows to induce a topological gap of the order of $\Delta \approx 2$ meV with realistic laser pulses, bringing an experimental realization of light-induced topological magnon edge states within reach.Comment: 21 pages, 4 figure

Charge separation in donor-C60 complexes with real-time Green's functions: The importance of nonlocal correlations

We use the Nonequilibrium Green's Function (NEGF) method to perform real-time simulations of the ultrafast electron dynamics of photoexcited donor-C60 complexes modeled by a Pariser-Parr-Pople Hamiltonian. The NEGF results are compared to mean-field Hartree-Fock (HF) calculations to disentangle the role of correlations. Initial benchmarking against numerically highly accurate time-dependent Density Matrix Renormalization Group calculations verifies the accuracy of NEGF. We then find that charge-transfer (CT) excitons partially decay into charge separated (CS) states if dynamical non-local correlation corrections are included. This CS process occurs in ~10 fs after photoexcitation. In contrast, the probability of exciton recombination is almost 100% in HF simulations. These results are largely unaffected by nuclear vibrations; the latter become however essential whenever level misalignment hinders the CT process. The robust nature of our findings indicate that ultrafast CS driven by correlation-induced decoherence may occur in many organic nanoscale systems, but it will only be correctly predicted by theoretical treatments that include time-nonlocal correlations.Comment: 9 pages, 6 figures + supplemental information (4 pages)

Observation of a new light-induced skyrmion phase in the Mott insulator Cu2OSeO3

We report the discovery of a novel skyrmion phase in the multiferroic insulator Cu2OSeO3 for magnetic fields below the equilibrium skyrmion pocket. This phase can be accessed by exciting the sample out of equilibrium with near-infrared (NIR) femtosecond laser pulses but can not be reached by any conventional field cooling protocol. From the strong wavelength dependence of the photocreation process and via spin dynamics simulations, we identify the magnetoelastic effect as the most likely photocreation mechanism. This effect results in a transient modification of the magnetic interaction extending the equilibrium skyrmion pocket to lower magnetic fields. Once created, the skyrmions rearrange and remain stable over a long time, reaching minutes. The presented results are relevant for designing high-efficiency non-volatile data storage based on magnetic skyrmions.Comment: 11 pages, 5 figure

Microscopic Theory of Ultrafast Skyrmion Excitation by Light

We propose a microscopic mechanism for ultrafast skyrmion photo-excitation via a two-orbital electronic model. In the strong correlation limit the $d$-electrons are described by an effective spin Hamiltonian, coupled to itinerant $s$-electrons via $s-d$ exchange. Laser-exciting the system by a direct coupling to the electric charge leads to skyrmion nucleation on a 100 fs timescale. The coupling between photo-induced electronic currents and magnetic moments, mediated via Rashba spin-orbit interactions, is identified as the microscopic mechanism behind the ultrafast optical skyrmion excitation.Comment: 11 pages, 3 figure

Photon pumping, photodissociation and dissipation at interplay for the fluorescence of a molecule in a cavity

We introduce a model description of a diatomic molecule in an optical cavity, with pump and fluorescent fields, and electron and nuclear motion are treated on equal footing and exactly. The model accounts for several optical response temporal scenarios: A Mollow spectrum hindered by electron correlations, a competition of harmonic generation and molecular dissociation, a dependence of fluorescence on photon pumping rate and dissipation. It is thus a general and flexible template for insight into experiments where quantum photon confinement, leakage, nuclear motion and electronic correlations are at interplay