arXiv subjects

Branislav K. Nikolic

Publications and source records attributed to Branislav K. Nikolic.

At least 19 recordsLinked to original sources

arXiv2026

Computational quantum transport: a scattering approach perspective

This review is devoted to the different techniques that have been developed to compute the phase-coherent transport properties of quantum nanoelectronic systems connected to electrodes. Beside a review of the different algorithms proposed in the literature, we provide a comprehensive and pedagogical derivation of the two formalisms on which these techniques are based: the scattering approach and the (nonequilibrium) Green's function approach. We show that the scattering problem can be formulated as a system of linear equations and that different existing algorithms for solving this scattering problem amount to different sequences of Gaussian elimination. We explicitly prove the equivalence of the two formalisms. We discuss the stability and numerical complexity of the existing methods. The review ends with a selection of a few applications where numerical calculations were instrumental in shaping our understanding of the physics.

cond-mat.mes-hall↗

arXiv2026

Long-range and steady-state entanglement of driven-dissipative nitrogen vacancy centers using microwaves as a drive and synthetic antiferromagnet as a dissipator

The search for optimal schemes and dissipative environments for mediating long-range entanglement between two distant nitrogen-vacancy centers (NVCs) in diamond is the subject of ongoing vigorous efforts due to potential applications of such microscopic solid-state qubits in quantum sensing and quantum computing. However, stabilizing entanglement of NVCs into steady-state poses a significant challenge, typically requiring tuning the environment into a {\em nonequilibrium} state. Here we microscopically derive a Lindblad quantum master equation for a system of two driven-dissipative NVCs, where the drive is microwave radiation and dissipation is provided by a single magnetic bath that is kept in {\em equilibrium}. This equation allows us to predict precise conditions for long-range and steady-state entanglement of NVCs, while it also suggests synthetic antiferromagnet as an optimal choice for a dissipative environment. By using realistic parameters from available experiments, we estimate steady-state concurrence reaching $\mathcal{C}\simeq 0.28$ for two NVCs separated by $\sim 100 \: \mathrm{nm}$.

cond-mat.mes-hall↗

arXiv2026

Engineering a driven-dissipative bath of altermagnetic quantum magnons for controlling classical dynamics of spins hosting spin waves, domain walls, or skyrmions

Using Schwinger-Keldysh field theory (SKFT), we engineer a dissipative and driven (i.e., out of equilibrium) bosonic bath acting on classical localized spins within a ferromagnetic insulator (FI) layer whose dynamics is governed by the Landau-Lifshitz-Gilbert equation, as is usually assumed in spintronics and magnonics. The bosonic bath is comprised of quantum magnons within a layer of altermagnetic insulator (AMI) that is attached to a conventional FI layer, often one of the key ingredients within spintronic and magnonic multilayers, so that interaction between slow classical (in the FI layer) and fast quantum (in the AMI layer) localized spins ensues. Such a bath, including its driving to produce a nonequilibrium distribution of altermagnetic magnons, generates a rich structure of the SKFT-derived extended LLG equation for classical spins within the FI layer. Our LLG equation contains two damping terms, both of which are spatially nonlocal and anisotropic, while one of them is also intrinsically non-Markovian, i.e., nonlocal in time. We demonstrate how to exploit these terms for tuning spintronic and magnonic effects within the FI layer of AMI/FI bilayers that involve spin wave or domain wall propagation, as well as skyrmion annihilation.

cond-mat.mes-hall↗

arXiv2026

Fate of entanglement in open quantum spin liquid: Time evolution of its genuine multipartite negativity upon sudden coupling to a dissipative bosonic environment

Many-body entanglement properties of quantum spin liquids (QSLs), persisting at arbitrarily long distances, have been intensely explored over the past two decades, but mostly for QSLs viewed as {\em closed} quantum systems. However, in experiments and potential quantum computing applications, candidate materials for this exotic phase of quantum matter will always interact with a dissipative environment, such as the one generated by bosonic quasiparticles in solids at finite temperature. Here we investigate both the {\em stability} and {\em spatial distribution} of entanglement for the Kitaev model of QSL, which is made {\em open} by its sudden coupling to an infinite bosonic bath of Caldeira-Leggett type and then time-evolved in both Markovian and non-Markovian regimes. From the time-dependent density matrix of QSL subregions, we extract genuine multipartite negativity (GMN), quantum Fisher information, spin-spin correlators, and the expectation value (EV) of the Wilson loop operator. In particular, time dependence of GMN offers the most penetrating insights: ({\em i}) in the Markovian regime, it remains nonzero only in hexagonal loopy subregions of QSL (as also discovered very recently for closed QSLs), eventually vanishing on the same timescale on which the EV of the Wilson loop operator vanishes; ({\em ii}) in the non-Markovian regime with pronounced memory effects, surprisingly, GMN remains nonzero up to much higher temperatures while also remaining zero in non-loopy subregions. In addition, the non-Markovian dynamics generates emergent interactions between spins, thereby opening avenues for tailoring properties of QSL via engineering of dissipation.

cond-mat.str-el↗

arXiv2026

Ultrafast optical excitation of magnons in 2D antiferromagnetic semiconductors via spin torque mediated by unbound electron-hole pairs and excitons: Signatures in magnonic charge pumping

Recent experiments observing how femtosecond laser pulse (fsLP) excites magnons in two-dimensional (2D) antiferromagnetic (AF) semiconductors -- such as CrSBr, NiPS$_3$, and MnPS$_3$, or their van der Waals heterostructures -- suggest an important role played by excitons. However, microscopic details of such an effect remain obscure, as resonant coupling of magnons, living in the sub-meV energy range, to excitons, living in the \mbox{$\sim 1$ eV} range, can hardly be operative. Here, we develop a quantum transport theory of this effect, in which time-dependent nonequilibrium Green's function (TDNEGF) for electrons driven by fsLP is coupled self-consistently to the Landau-Lifshitz-Gilbert (LLG) equation describing classical dynamics of localized magnetic moments (LMMs) residing on magnetic atoms of 2D AF semiconductors. This theory explains how fsLP, of central frequency {\em above} the semiconductor gap, generates a photocurrent that becomes spin-polarized due to the background of LMMs, which, in turn, exerts spin-transfer torque (STT) onto LMMs as a genuinely nonequilibrium spintronic mechanism. The collective motion of LMMs analyzed by windowed Fast Fourier transform (FFT) decodes frequencies of excited magnons, as well as their lifetime governed by {\em nonlocal} damping with the LLG equation due to electronic bath. Finally, our theory also predicts that excited magnons will {\em pump} time-dependent charge currents into the attached electrodes, or locally within 2D AF semiconductor, thereby emitting electromagnetic radiation. The windowed FFT of these two signals contains imprints of excited magnons, as well as possible presence of excitons, which could be exploited as a novel probe in future experiments.

cond-mat.mes-hall↗

arXiv2025

Optically induced magnetic inertia and magnons from non-Markovian extension of the Landau-Lifshitz-Gilbert equation

The Landau-Lisfhitz-Gilbert (LLG) equation has been the cornerstone of modeling the dynamics of localized spins, viewed as classical vectors of fixed length, within nonequilibrium magnets. When light is employed as the nonequilibrium drive, the LLG equation must be supplemented with additional terms that are usually conjectured using phenomenological arguments for direct opto-magnetic coupling between localized spins and (real or effective) magnetic field of light. However, direct coupling of magnetic field to spins is 1/c smaller than coupling of light and electrons; or both magnetic and electric fields are too fast for slow classical spins to be able to follow them. Here, we displace the need for phenomenological arguments by rigorously deriving an extended LLG equation via Schwinger-Keldysh field theory (SKFT). Within such a theory, light interacts with itinerant electrons, and then spin current carried by them exerts spin-transfer torque onto localized spins, so that when photoexcited electrons are integrated out we arrive at a spin-only equation. Unlike the standard phenomenological LLG equation with local-in-time Gilbert damping, our extended one contains a non-Markovian memory kernel whose plot within the plane of its two times variables exhibits fractal properties. By applying SKFT-derived extended LLG equation, as our central result, to a light-driven ferromagnet as an example, we predict an optically induced magnetic inertia term. Its magnitude is governed by spatially nonlocal and time-dependent prefactor, leading to excitation of coherent magnons at sharp frequencies in and outside of the band of incoherent (or thermal) magnons.

cond-mat.mes-hall↗

arXiv2025

Schwinger-Keldysh non-perturbative field theory of open quantum systems beyond the Markovian regime: Application to spin-boson and spin-chain-boson models

We develop a unified framework for open quantum systems composed of many mutually interacting quantum spins, or any isomorphic systems like qubits and qudits, surrounded by one or more independent bosonic baths. Our framework, based on Schwinger-Keldysh field theory (SKFT), can handle arbitrary spin value S, dimensionality of space, and geometry, while being applicable to a large parameter space for system and bath or their coupling. It can probe regimes in which non-Markovian dynamics and nonperturbative effects pose formidable challenges for other state-of-the-art theoretical methods. This is achieved by working with the two-particle irreducible (2PI) effective action, which resums classes of Feynman diagrams of SKFT to an infinite order. Furthermore, such diagrams are generated via an expansion in 1/N, where N is the number of Schwinger bosons we employ to map spin operators onto canonically commuting ones, rather than via conventional expansion in system-bath coupling constant. We carefully benchmark our SKFT+2PI-computed results vs. numerically (quasi)exact ones from tensor network calculations applied to the archetypical spin-boson model where both methodologies are applicable. Additionally, we demonstrate the capability of SKFT+2PI to handle a much more complex spin-chain-boson model with multiple baths interacting with each spin where no benchmark from other methods is available at present. The favorable numerical cost of solving integro-differential equations produced by the SKFT+2PI framework with an increasing number of spins and time steps makes it a promising route for simulating driven-dissipative systems in quantum computing, quantum magnonics, and quantum spintronics.

quant-ph↗

arXiv2025

Deciphering the origin of spin current in spintronic terahertz emitters and its imprint on their electromagnetic radiation via time-dependent density functional theory

Spin current flowing between femtosecond laser pulse (fsLP)-driven ferromagnetic metal and adjacent normal metal (NM) hosting strong spin-orbit coupling is invariably invoked to explain terahertz (THz) radiation believed to be emitted solely by NM layer. Despite being such a central concept, the microscopic origin of interlayer spin current remains vague. Here, we employ recently developed [A. Kefayati {\em et al.}, Phys. Rev. Lett. {\bf 133}, 136704 (2024)] time-dependent density functional theory plus Jefimenko equations approach to extract spin current between Co and NM=Pt or NM=W layer where Co is driven by fsLP responsible for its demagnetization, i.e., shrinking of its magnetization vector, $M^y(t)/M^y(t=0)<1$. By comparing time dependence of spin current with those of other relevant quantities, we find that: ({\em i}) spin current is generated by demagnetization dynamics because it {\em follows} closely $dM^y/dt$, thus it is an example of quantum pumping phenomenon that cannot be captured by phenomenological notions (such as ``spin voltage'') and related semiclassical transport theories; ({\em ii}) time dependence of pumped spin current {\em does not follow} closely that of charge current emerging within NM layer via spin-to-charge conversion mechanisms; ({\em iii}) THz emission can be governed by {\em both} charge current (i.e., its time derivative entering the Jefimenko equations) within Co layer or NM layer, but in different times frames. We also unravel a special case of NM=W where spin-to-charge conversion by the inverse spin Hall effect and its contribution to THz emission is suppressed, despite large spin Hall angle of W, because of localization of excited electrons onto the outer unfilled $d$-orbitals of W.

cond-mat.mes-hall↗

arXiv2025

Nonclassical dynamics of Néel vector and magnetization accompanied by THz and high-harmonic radiation from ultrafast-light-driven NiO antiferromagnet insulator

Ultrafast-light-driven strongly correlated antiferromagnetic insulators, such as prototypical NiO with large energy gap 4 eV, have recently attracted experimental attention using either above-gap [K. Gillmeister et al., Nat. Commun. 11, 4095 (2020)] or subgap [H. Qiu et al., Nat. Phys. 17, 388 (2021)] energy photons that are of fundamental interest in far-from-equilibrium quantum matter or spintronic applications, respectively. In the latter context, emission of THz radiation is also observed from NiO/Pt bilayers, where heavy metal (HM) Pt introduces strong spin-orbit coupling (SOC). However, microscopic mechanisms of such emission remain obscure because spintronic THz emitters have been amply studied using FM/HM (FM-ferromagnetic metal of conventional type) bilayers, where ultrafast demagnetization takes place and is directly related to THz emission. Conversely, in NiO total magnetization is zero prior to the fs laser pulse (fsLP) application. Here we employ the two-orbital Hubbard-Hund-Heisenberg model and study, via numerically exact nonequilibrium quantum many-body methods, the dynamics of its Neel vector and nonequilibrium magnetization. Additionally, we compute electromagnetic radiation by both time-dependent magnetization and local charge currents arising in either plain NiO or NiO with proximity SOC introduced by HM layer. Our analysis reveals nonclassical dynamics of Neel vector and nonequilibrium magnetization, changing only in length while not rotating, where the former is substantially reduced only in the case above-gap fsLP. In the plain NiO case, THz radiation of interest to applications is insignificant, but adding SOC enhances both current and magnetic dipole contributions to it. Above THz range, we find integer high-harmonic generation, as well as unusual noninteger harmonics for above-gap fsLP pump.

cond-mat.str-el↗

arXiv2025

Magnon spectrum of altermagnets beyond linear spin wave theory: Magnon-magnon interactions via time-dependent matrix product states vs. atomistic spin dynamics

The energy-momentum dispersion of magnons, as collective low-energy excitations of magnetic material, is computed from an effective quantum spin Hamiltonian but simplified via linearized Holstein-Primakoff transformations to describe noninteracting magnons. The dispersion produced by such linear spin wave theory (LSWT) is then plotted as ``sharp bands'' of infinitely long-lived quasiparticles. However, magnons are prone to many-body interactions with other quasiparticles -- such as electrons, phonons or other magnons -- which can lead to shifting (i.e., band renormalization) and broadening of ``sharp bands'' as the signature of finite quasiparticle lifetime. The magnon-magnon interactions can be particularly important in antiferromagnets (AFs), and, therefore, possibly in newly classified altermagnets sharing many features of collinear AFs. Here, we employ nonperturbative quantum many-body calculations, via time-dependent matrix product states (TDMPS), to obtain magnon spectral function for RuO$_2$ altermagnet whose effective quantum spin Hamiltonian is put onto 4-leg cylinder. Its upper band is shifted away from upper ``sharp band'' of LSWT, as well as broadened, which is explained as the consequence of {\em hybridization} of the latter with three-magnon continuum. This implies that two-magnon Raman scattering spectra {\em cannot} be computed from LSWT bands, which offers a litmus test for the relevance of magnon-magnon interactions. Finally, we employ atomistic spin dynamics (ASD) simulations, based on classical Landau-Lifshitz-Gilbert (LLG) equation, to obtain magnon spectrum at finite temperature and/or at a fraction of the cost of TDMPS calculations. Despite including magnon-magnon interactions via nonlinearity of LLG equation, ASD simulations {\em cannot} match the TDMPS-computed magnon spectrum, thereby signaling {\em nonclassical} effects harbored by AFs and altermagnets.

cond-mat.str-el↗

arXiv2024

Nonlocal damping of spin waves in a magnetic insulator induced by normal, heavy, or altermagnetic metallic overlayer: A Schwinger-Keldysh field theory approach

Understanding spin wave (SW) damping, and how to control it to the point of being able to amplify SW-mediated signals, is one of the key requirements to bring the envisaged magnonic technologies to fruition. Even widely used magnetic insulators with low magnetization damping in their bulk, such as yttrium iron garnet, exhibit 100-fold increase in SW damping due to inevitable contact with metallic layers in magnonic circuits, as observed in very recent experiments [I. Bertelli et al., Adv. Quantum Technol. 4, 2100094 (2021)] mapping SW damping in spatially-resolved fashion. Here, we provide microscopic and rigorous understanding of wavevector-dependent SW damping using extended Landau-Lifshitz-Gilbert equation with nonlocal damping tensor, instead of conventional local scalar Gilbert damping, as derived from Schwinger-Keldysh nonequilibrium quantum field theory. In this picture, the origin of nonlocal magnetization damping and thereby induced wavevector-dependent SW damping is interaction of localized magnetic moments of magnetic insulator with conduction electrons from the examined three different types of metallic overlayers -- normal, heavy, and altermagnetic. Due to spin-split energy-momentum dispersion of conduction electrons in the latter two cases, the nonlocal damping is anisotropic in spin and space, and it can be dramatically reduced by changing the relative orientation of the two layers when compared to the usage of normal metal overlayer.

cond-mat.mes-hall↗

arXiv2024

Reconciling Kubo and Keldysh Approaches to Fermi-Sea-Dependent Nonequilibrium Observables: Application to Spin Hall Current and Spin-Orbit Torque in Spintronics

Quantum transport studies of spin-dependent phenomena in solids commonly employ the Kubo or Keldysh formulas for the nonequilibrium density operator in the steady-state linear-response regime. Its trace with operators of interest, such as the spin density, spin current density, etc., gives expectation values of experimentally accessible observables. For local quantities, these formulas require summing over the manifolds of {\em both} Fermi-surface and Fermi-sea states. However, debates have been raging in the literature about the vastly different physics the two formulations can apparently produce, even when applied to the same system. Here, we revisit this problem using infinite-size graphene with proximity-induced spin-orbit and magnetic exchange effects as a testbed. By splitting this system into semi-infinite leads and central active region, in the spirit of Landauer formulation of quantum transport, we prove the {\em numerically exact equivalence} of the Kubo and Keldysh approaches via the computation of spin Hall current density and spin-orbit torque in both clean and disordered limits. The key to reconciling the two approaches are the numerical frameworks we develop for: ({\em i}) evaluation of Kubo(-Bastin) formula for a system attached to semi-infinite leads, which ensures continuous energy spectrum and evades the need for commonly used phenomenological broadening introducing ambiguity; and ({\em ii}) proper evaluation of Fermi-sea term in the Keldysh approach, which {\em must} include the voltage drop across the central active region even if it is disorder free.

cond-mat.mes-hall↗

arXiv2024

Charge and spin current pumping by ultrafast demagnetization dynamics

The surprising discovery of ultrafast demagnetization -- where electric field of femtosecond laser pulse couples to electrons of a ferromagnetic (FM) layer causing its magnetization vector {\em to shrink while not rotating}, is also assumed to be accompanied by generation of spin current in the direction orthogonal to electric field. However, understanding of the microscopic origin of such spin current and how efficiently it can be converted into charge current, as the putative source of THz radiation, is lacking despite nearly three decades of intense studies. Here we connect the standard pumping phenomena driven by microwave precession of magnetization vector replacing periodic time-dependence of magnetization precession with nonperiodic time-dependence of demagnetization, as obtained from experiments on ultrafast-light-driven Ni layer. Applying time-dependent nonequilibrium Green's functions, able to evolve such setup with arbitrary time dependence, reveals how demagnetization dynamics pumps both charge and spin currents in directions both parallel and orthogonal to electric field of laser pulse, even in the absence of spin-orbit coupling and thereby induced spin-to-charge conversion mechanisms. Although pumped currents follow $dM_z/dt$ in some setups, this becomes obscured when NM layers are disconnected and pumped currents start to reflect from FM boundaries (as is the case of experimental setups). Finally, we use the Jefimenko equations to compute electromagnetic radiation by charge current pumped in disconnected setup during demagnetization, or later during its slow recovery, unraveling that radiated electric field only in the former time interval exhibits features in 0.1--30 THz frequency range probed experimentally or explored for applications of spintronic THz emitters.

cond-mat.mes-hall↗

arXiv2024

Anisotropic skyrmion mass induced by surrounding conduction electrons: A Schwinger-Keldysh field theory approach

The current-driven motion of magnetic skyrmions, as topologically protected winding vector fields of local magnetization, has attracted considerable attention due to both fundamental interest in the dynamics of topological solitons, and potential spintronic applications. While widely-used Thiele equation with zero skyrmion mass captures its rigid motion under the assumption of preserved skyrmion shape, experiments on realistic skyrmions find deviation due to inertial mass with numerous attempts to account for it by invoking internal skyrmion dynamics, geometry of the nanowire, and interaction with magnons or phonons. However, experimentally observed skyrmion mass is often much larger than predicted by these theories. In this Letter, we employ Schwinger-Keldysh field-theoretical approach to study conduction electrons interacting with localized magnetic moments comprising a skyrmion to obtain a stochastic differential equation of its motion. By analyzing its time-retarded damping kernel, we extract an anisotropic mass governed by conduction electrons and demonstrate its substantial effect on the current-driven skyrmion motion, even in the absence of any skyrmion deformation.

cond-mat.mes-hall↗

arXiv2024

Gilbert damping in metallic ferromagnets from Schwinger-Keldysh field theory: Intrinsically nonlocal and nonuniform, and made anisotropic by spin-orbit coupling

Understanding the origin of damping mechanisms in magnetization dynamics of metallic ferromagnets is a fundamental problem for nonequilibrium many-body physics of systems where quantum conduction electrons interact with localized spins assumed to be governed by the classical Landau-Lifshitz-Gilbert (LLG) equation. It is also of critical importance for applications, as damping affects energy consumption and speed of spintronic and magnonic devices. Since the 1970s, a variety of linear-response and scattering theory approaches have been developed to produce widely used formulas for computation of spatially-independent Gilbert scalar parameter as the magnitude of the Gilbert damping term in the LLG equation. The largely unexploited for this purpose Schwinger-Keldysh field theory (SKFT) offers additional possibilities, such as to rigorously derive an extended LLG equation by integrating quantum electrons out. Here we derive such equation whose Gilbert damping for metallic ferromagnets is nonlocal, i.e., dependent on all localized spins at a given time, and nonuniform, even if all localized spins are collinear and spin-orbit coupling (SOC) is absent. This is in sharp contrast to standard lore, where nonlocal damping is considered to emerge only if localized spins are noncollinear; for such situations, direct comparison on the example of magnetic domain wall shows that SKFT-derived nonlocal damping is an order of magnitude larger than the previously considered one. Switching on SOC makes such nonlocal damping anisotropic, in contrast to standard lore where SOC is usually necessary to obtain nonzero Gilbert damping scalar parameter. Our analytical formulas, with their nonlocality being more prominent in low spatial dimensions, are fully corroborated by numerically exact quantum-classical simulations.

cond-mat.mes-hall↗

arXiv2024

Fate of entanglement in magnetism under Lindbladian or non-Markovian dynamics and conditions for their transition to Landau-Lifshitz-Gilbert classical dynamics

It is commonly assumed in spintronics and magnonics that localized spins within antiferromagnets are in the Néel ground state (GS), as well as that such state evolves, when pushed out of equilibrium by current or external fields, according to the Landau-Lifshitz-Gilbert (LLG) equation viewing localized spins as classical vectors of fixed length. On the other hand, the true GS of antiferromagnets is highly entangled, as confirmed by very recent neutron scattering experiments witnessing their entanglement. Although GS of ferromagnets is always unentangled, their magnonic low-energy excitation are superpositions of many-body spin states and, therefore, entangled. In this study, we initialize quantum Heisenberg ferro- or antiferromagnetic chains hosing localized spins $S=1/2$, $S=1$ or $S=5/2$ into unentangled pure state and then evolve them by quantum master equations (QMEs) of Lindblad or non-Markovian type, derived by coupling localized spins to a bosonic bath (such as due to phonons) or by using additional ``reaction coordinate'' in the latter case. The time evolution is initiated by applying an external magnetic field, and entanglement of time-evolving {\em mixed} quantum states is monitored by computing its logarithmic negativity. We find that non-Markovian dynamics maintains some degree of entanglement, which shrinks the length of the vector of spin expectation values, thereby making the LLG equation inapplicable. Conversely, Lindbladian (i.e., Markovian) dynamics ensures that entanglement goes to zero, thereby enabling quantum-to-classical (i.e., to LLG) transition in all cases -- $S=1/2$, $S=1$ and $S=5/2$ ferromagnet or $S=5/2$ antiferromagnet -- {\em except} for $S=1/2$ and $S=1$ antiferromagnet. We also investigate the stability of entangled antiferromagnetic GS upon suddenly coupling it to the bosonic bath.

cond-mat.str-el↗

arXiv2023

Origins of electromagnetic radiation from spintronic terahertz emitters: A time-dependent density functional theory plus Jefimenko equations approach

Microscopic origins of charge currents and electromagnetic (EM) radiation generated by them in spintronic THz emitters -- such as, femtosecond laser pulse-driven single magnetic layer or its heterostructures with a nonmagnetic layer hosting strong spin-orbit coupling (SOC) -- remain poorly understood despite nearly three decades since the discovery of ultrafast demagnetization. We introduce a first-principles method to compute these quantities, where the dynamics of charge and current densities is obtained from real-time time-dependent density functional theory (TDDFT), which are then fed into the Jefimenko equations for properly retarded electric and magnetic field solutions of the Maxwell equations. By Fourier transforming different time-dependent terms in the Jefimenko equations, we unravel that in 0.1--30 THz range the electric field of far-field EM radiation by Ni layer, chosen as an example, is {\em dominated by charge current pumped by demagnetization}, while often invoked magnetic dipole radiation from time-dependent magnetization of a single magnetic layer is a negligible effect. Such overlooked case of charge current pumping by time-dependent quantum system, whose magnetization is shrinking while its vector does not rotate, does not require any spin-to-charge conversion via SOC effects. In Ni/Pt bilayer, EM radiation remains dominated by charge current within Ni layer, whose magnitude is larger than in the case of single Ni layer due to faster demagnetization, while often invoked spin-to-charge conversion within Pt layer provides additional but smaller contribution. By using Poynting vector and its flux, we also quantify efficiency of conversion of light into emitted EM radiation, as well as the angular distribution of the latter.

cond-mat.mes-hall↗

arXiv2023

Quantum-classical approach to spin and charge pumping and the ensuing radiation in THz spintronics: Example of ultrafast-light-driven Weyl antiferromagnet Mn$_3$Sn

The interaction of fs light pulses with magnetic materials has been intensely studied for more than two decades in order to understand ultrafast demagnetization in single magnetic layers or THz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. Here we develop a multiscale quantum-classical formalism -- where conduction electrons are described by quantum master equation of the Lindblad type; classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation; and incoming light is described by classical vector potential while outgoing electromagnetic radiation is computed using Jefimenko equations for retarded electric and magnetic fields -- and apply it a bilayer of antiferromagnetic Weyl semimetal Mn$_3$Sn with noncollinear local magnetization in contact with SO-coupled nonmagnetic material. Our QME+LLG+Jefimenko scheme makes it possible to understand how fs light pulse generates directly spin and charge pumping and electromagnetic radiation by the latter, including both odd and even high harmonics (of the pulse center frequency) up to order $n \le 7$. The directly pumped spin current then exert spin torque on local magnetization whose dynamics, in turn, pumps additional spin and charge currents radiating in the THz range. By switching on and off LLG dynamics and SO couplings, we unravel which microscopic mechanism contribute the most to emitted THz radiation -- charge pumping by local magnetization of Mn$_3$Sn in the presence of its intrinsic SO coupling is far more important than standardly assumed (for other types of magnetic layers) spin pumping and subsequent spin-to-charge conversion within the neighboring nonmagnetic SO-coupled material.

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