arXiv subjects

Emil Viñas Boström

Publications and source records attributed to Emil Viñas Boström.

33 records · Page 2Linked to original sources

arXiv2024

Thermal spin-crossover and temperature-dependent zero-field splitting in magnetic nanographene chains

Nanographene-based magnetism at interfaces offers an avenue to designer quantum materials towards novel phases of matter and atomic-scale applications. Key to spintronics applications at the nanoscale is bistable spin-crossover which however remains to be demonstrated in nanographenes. Here we show that antiaromatic 1,4-disubstituted pyrazine-embedded nanographene derivatives, which promote magnetism through oxidation to a non-aromatic radical are prototypical models for the study of carbon-based thermal spin-crossover. Scanning tunneling spectroscopy studies reveal symmetric spin excitation signals which evolve at Tc to a zero-energy peak, and are assigned to the transition of a S = 3/2 high-spin to a S = 1/2 low-spin state by density functional theory. At temperatures below and close to the spin-crossover Tc, the high-spin S= 3/2 excitations evidence pronouncedly different temperature-dependent excitation energies corresponding to a zero-field splitting in the Hubbard-Kanamori Hamiltonian. The discovery of thermal spin crossover and temperature-dependent zero-field splitting in carbon nanomaterials promises to accelerate quantum information, spintronics and thermometry at the atomic scale.

physics.chem-ph↗

arXiv2024

Magnon-mediated topological superconductivity in a quantum wire

Many emergent phases of matter stem from the intertwined dynamics of quasiparticles. Here we show that a topological superconducting phase emerges as the result of interactions between electrons and magnons in a quantum wire and a helical magnet. The magnon-mediated interaction favors triplet superconductivity over a large magnetic phase space region, and stabilizes topological superconductivity over an extended region of chemical potentials. The superconducting gap depends exponentially on the spin-electron coupling, allowing it to be enhanced through material engineering techniques.

cond-mat.supr-con↗

arXiv2023

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.

cond-mat.mes-hall↗

arXiv2023

Phonon-mediated unconventional $s$- and $f$-wave pairing superconductivity in rhombohedral stacked multilayer graphene

Understanding the origin of superconductivity in correlated two-dimensional materials is a key step in leveraging material engineering techniques for next-generation nanoscale devices. The recent demonstration of superconductivity in Bernal bilayer and rhombohedral trilayer graphene, as well as in a large family of graphene-based moiré systems, indicate a common superconducting mechanism across these platforms. Here we combine first principles simulations with effective low-energy theories to investigate the superconducting mechanism and pairing symmetry in rhombohedral stacked graphene multilayers. We find that a phonon-mediated attraction can quantitatively explain the main experimental findings, namely the displacement field and doping dependence of the critical temperature and the presence of two superconducting regions whose pairing symmetries depend on the parent normal state. In particular, we find that intra-valley phonon scattering favors a triplet $f$-wave pairing out of a spin and valley polarized normal state. We also propose a new and so far unexplored superconducting region at higher hole doping densities $n_h \approx 4 \times 10^{12}$ cm$^{-2}$, and demonstrate how this large hole-doped regime can be reached in heterostructures consisting of monolayer $α$-RuCl$_3$ and rhombohedral trilayer graphene.

cond-mat.supr-con↗

arXiv2023

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.

quant-ph↗

arXiv2023

Cavity-renormalized quantum criticality in a honeycomb bilayer antiferromagnet

Strong light-matter interactions as realized in an optical cavity provide a tantalizing opportunity to control the properties of condensed matter systems. Inspired by experimental advances in cavity quantum electrodynamics and the fabrication and control of two-dimensional magnets, we investigate the fate of a quantum critical antiferromagnet coupled to an optical cavity field. Using unbiased quantum Monte Carlo simulations, we compute the scaling behavior of the magnetic structure factor and other observables. While the position and universality class are not changed by a single cavity mode, the critical fluctuations themselves obtain a sizable enhancement, scaling with a fractional exponent that defies expectations based on simple perturbation theory. The scaling exponent can be understood using a generic scaling argument, based on which we predict that the effect may be even stronger in other universality classes. Our microscopic model is based on realistic parameters for two-dimensional magnetic quantum materials and the effect may be within the range of experimental detection.

cond-mat.str-el↗

arXiv2022

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.

cond-mat.str-el↗

arXiv2022

Direct optical probe of magnon topology in two-dimensional quantum magnets

Controlling edge states of topological magnon insulators is a promising route to stable spintronics devices. However, to experimentally ascertain the topology of magnon bands is a challenging task. Here we derive a fundamental relation between the light-matter coupling and the quantum geometry of magnon states. This allows to establish the two-magnon Raman circular dichroism as an optical probe of magnon topology in honeycomb magnets, in particular of the Chern number and the topological gap. Our results pave the way for interfacing light and topological magnons in functional quantum devices.

cond-mat.mes-hall↗

arXiv2021

Magnon frequency renormalization by the electronic geometrical spin torque in itinerant magnets

We investigate non-adiabatic effects on the magnon frequency in an interacting system of localized spins and itinerant electrons. Including the lowest order corrections to the adiabatic dynamics in an analytically solvable model, applicable to simple ferromagnets like Fe, Co and Ni, we find that the magnon frequency is renormalized by a geometrical torque arising from the electronic spin Berry curvature. Comparison to exact numerical simulations reveals that our analytical solution captures essential low-energy features, and provides a mechanism for the magnon frequency hardening observed in recent first principles calculations for Fe, provided the geometrical torque is taken into account.

cond-mat.mtrl-sci↗

arXiv2021

All-Optical Generation of Antiferromagnetic Magnon Currents via the Magnon Circular Photogalvanic Effect

We introduce the magnon circular photogalvanic effect enabled by stimulated Raman scattering. This provides an all-optical pathway to the generation of directed magnon currents with circularly polarized light in honeycomb antiferromagnetic insulators. The effect is the leading order contribution to magnon photocurrent generation via optical fields. Control of the magnon current by the polarization and angle of incidence of the laser is demonstrated. Experimental detection by sizeable inverse spin Hall voltages in platinum contacts is proposed.

cond-mat.mes-hall↗

arXiv2020

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.

cond-mat.mes-hall↗

arXiv2020

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 $λ= eEa/\hbarω$ 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 $Δ\approx 2$ meV with realistic laser pulses, bringing an experimental realization of light-induced topological magnon edge states within reach.

cond-mat.str-el↗

arXiv2020

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.

quant-ph↗

arXiv2018

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.

quant-ph↗

arXiv2018

Steering Magnetic Skyrmions with Nonequilibrium Green's Functions

Magnetic skyrmions, topologically protected vortex-like configurations in spin textures, are of wide conceptual and practical appeal for quantum information technologies, notably in relation to the making of so-called race-track memory devices. Skyrmions can be created, steered and destroyed with magnetic fields and/or (spin) currents. Here we focus on the latter mechanism, analyzed via a microscopic treatment of the skyrmion-current interaction. The system we consider is an isolated skyrmion in a square-lattice cluster, interacting with electrons spins in a current-carrying quantum wire. For the theoretical description, we employ a quantum formulation of spin-dependent currents via nonequilibrium Green's functions (NEGF) within the generalized Kadanoff-Baym ansatz (GKBA). This is combined with a treatment of skyrmions based on classical localized spins, with the skyrmion motion described via Ehrenfest dynamics. With our mixed quantum-classical scheme, we assess how time-dependent currents can affect the skyrmion dynamics, and how this in turn depends on electron-electron and spin-orbit interactions in the wire. Our study shows the usefulness of a quantum-classical treatment of skyrmion steering via currents, as a way for example to validate/extract an effective, classical-only, description of skyrmion dynamics from a microscopic quantum modeling of the skyrmion-current interaction.

cond-mat.mes-hall↗