arXiv subjects

Jalil Varela-Manjarres

Publications and source records attributed to Jalil Varela-Manjarres.

4 recordsLinked to original sources

arXiv2026

Benchmarking the generalized Kadanoff-Baym ansatz and second-order adiabatic expansion using time-dependent spintronic effects: Spin pumping, torque, and inertia

The generalized Kadanoff-Baym ansatz (GKBA) [P. Lipavský {\em et al.}, Phys. Rev. B {\bf 34}, 6933 (1986)] has emerged as a popular and numerically efficient algorithm for simplification of nonequilibrium Green's function (NEGF)-based calculations of time-dependent quantum transport. For systems that can be split into classical and quantum degrees of freedom, another popular simplifying strategy is adiabatic expansion (AE) of NEGF [N. Bode {\em et al.}, Phys. Rev. Lett. {\bf 107}, 036804 (2011); S. Deghi {\em et al.}, Phys. Rev. B {\bf 110}, 115409 (2024)] in terms of the velocity of classical degrees of freedom, such as localized magnetic moments (LMMs) in spintronics or coordinates of nuclei in nanoelectronics. Here we compare GKBA and second-order AE with numerically exact benchmarks for two-terminal junctions whose central region hosting quantum electrons and classical LMMs is attached to two semi-infinite normal metal leads. Three simple models are employed to exhibit cornerstone time-dependent effects in spintronics---spin pumping and spin-transfer torque (STT), as well as magnetic inertia as a recently explored phenomenon. We find that GKBA fails to describe pumping of spin current by precessing LMMs, or STT vectors, and thereby induced LMM dynamics. Conversely, the second-order AE matches numerically exact benchmarks for both effects remarkably well, thereby also revealing the essentially {\em nonadiabatic} nature of spin pumping. Thus, AE opens a path toward an accurate description of STT-driven magnetization dynamics, including combination with first-principles Hamiltonians, while incurring a fraction of the cost of time evolution of full NEGF. However, despite including terms up to the second time derivatives of LMMs into AE, this approach fails to capture fast nutational oscillations on top of the precessional motion of LMMs as the hallmark of magnetic inertia.

cond-mat.mes-hall↗

arXiv2026

FermiLink: A Unified Agent Framework for Multidomain Autonomous Scientific Simulations

Artificial-intelligence (AI) agent frameworks have been developed for autonomous scientific simulations, but most current agent frameworks are tailored to a single or a small set of software packages. Herein, FermiLink, a unified and extensible open-source agent framework is introduced for multidomain scientific simulations. Its key design principle is the separation of package knowledge bases from simulation workflows, so that simulation workflows in FermiLink, from figure-level simulations to full-paper-level research on high-performance computing clusters, operate uniformly among supported packages via a four-layer progressive disclosure mechanism. Using OpenAI Codex as the agent provider, the capabilities of FermiLink are demonstrated across approximately 50 scientific software packages spanning nine research domains from physics to engineering. Systematic benchmarks on 132 real-world figure-level reproduction tasks with 44 packages show that FermiLink reproduces 74 (56.1%) of published figures with simulations, among which 30 achieve high-fidelity agreement and 35 reach qualitative agreement with the target figures. A smaller set of human expert-guided reproduction benchmarks with 10 packages further highlights the importance of expert insights for improving the simulation fidelity. Beyond reproduction, a single-blinded study demonstrates that FermiLink can produce research-grade results on unpublished polariton physics problems when provided with sufficiently detailed research objectives and source code, even in the absence of external documentation or tutorials. Overall, FermiLink provides a scalable research infrastructure that may accelerate the path from scientific questions to computational results across diverse domains.

physics.chem-ph↗

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↗

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↗