arXiv subjects

Yafei Ren

Publications and source records attributed to Yafei Ren.

At least 19 recordsLinked to original sources

arXiv2026

Optical poling of a quantum ferroelectric metal across the order-disorder phase transition

Electron-doped strontium titanate has emerged as a prototypical quantum ferroelectric metal. It provides a fertile ground to explore how ferroelectric instability intertwined with itinerant electrons creates quantum phenomena, including unconventional superconductivity. Despite extensive studies, the microscopic origin of the ferroelectric transition remains unsettled, with distinct interpretations based on displacive mechanism driven by soft mode and order-disorder alignment of local dipoles. In particular, the local dipoles form nanoscale, spatially heterogeneous clusters, termed polar nanoregions, posing a significant challenge for probing or manipulating them. Here, using rotational anisotropy second harmonic generation, a symmetry-resolved probe, we quantify the orientational statistics of polar nanoregions in dilute electron-doped Sr$_{0.95}$Ba$_{0.05}$Ti$_{1-x}$Nb$_x$O$_3$. By tracking the alignment and meltdown of polar nanoregions in thermal cycles, we unambiguously demonstrate the order-disorder nature of the ferroelectric transition. We further show that, above transition temperature, femtosecond optical fields enable deterministic control of otherwise disordered polar nanoregions, realizing reversible write and readout of polar textures on ultrafast timescales. Our findings provide new insight into ferroelectric instability and establish an all-optical route of controlling polar metal systems where conventional electrical approaches are not feasible.

cond-mat.mtrl-sci↗

arXiv2026

Antiferro-Chiral Phonons in $\mathcal{P}\mathcal{T}$-Symmetric Antiferromagnets

Chiral phonons provide a route to couple lattice motion to magnetic order, but conventional chiral phonons carry a net angular momentum and thus couple naturally to net magnetization rather than to compensated N\'eel order. Here we show that $\mathcal{P}\mathcal{T}$-symmetric antiferromagnets can host \emph{antiferro-chiral phonons} (AFCPs): phonon modes with vanishing total angular momentum but finite sublattice-staggered angular momentum. Symmetry enforces this distinction because $\mathcal{P}\mathcal{T}$ forbids a net phonon angular momentum while allowing counter-rotating local motion on inversion-related sublattices. AFCPs arise from a N\'eel-vector-locked coupling between Raman and infrared-active phonons. The coupling is odd under both $\mathcal{P}$ and $\mathcal{T}$ while preserving their product. Through this hybridization, the normal modes acquire both Raman and infrared character and carry a sublattice-staggered phonon angular momentum that acts as a conjugate field to the N\'eel vector. This coupling is microscopically generated by the molecular Berry curvature, which is demonstrated in a prototype lattice model. Reversing the N\'eel vector reverses the staggered phonon chirality. These results indicate AFCPs as probes of antiferromagnetic order and suggest coherent phonon excitation as a route to its dynamical control.

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

Dissipative Nonlinear Phononics: Nonequilibrium Quasiperiodic Order in Light-Driven Spin-Phonon System

Nonlinear phononics has emerged as a powerful paradigm for the nonthermal control of quantum materials by engineering a conservative potential energy landscape. Here, we show that dissipation can serve as an additional control knob for nonequilibrium states in nonlinear phononics. We reveal a nontrivial role of dissipation by investigating a spin-phonon coupled system driven by circularly polarized light. By tuning the spin relaxation time $\tau_s$, the steady state undergoes a transition from a trivial limit cycle to a temporally ordered state, which spontaneously breaks the discrete time-translation symmetry imposed by the drive. In this state, both the spin and phonon angular momentum exhibit persistent oscillations at an emergent frequency $\Omega_s$, which is generally incommensurate with the driving frequency. This state is stabilized by a dissipation-induced phase lag between spin and phonon angular momentum that generates a feedback loop sustaining the oscillation. The dissipation-controlled transition can be described within a Landau-type framework using a pseudo-potential, where the order parameter has a $U(1)$ phase symmetry, and its amplitude is proportional to the oscillation amplitude of the phonon angular momentum.

cond-mat.mes-hall↗

arXiv2025

Nonadiabatic Wave-Packet Dynamics: Nonadiabatic Metric, Quantum Geometry, and Gravitational Analogy

We develop a unified theory for the nonadiabatic wave-packet dynamics of Bloch electrons subject to slowly varying spatial and temporal perturbations. Extending the conventional wave-packet ansatz to include interband contributions, we derive equations for the interband coefficients using the time-dependent variational principle, referred to as the wave-packet coefficient equation. Solving these equations and integrating out interband contributions yields the leading-order nonadiabatic corrections to the wave-packet Lagrangian. These corrections appear in three forms: (i) a nonadiabatic metric in real and momentum space, which we identify with the energy-gap-renormalized quantum metric, (ii) modified Berry connections associated with the motion of the wave-packet center, and (iii) an energy correction arising from spatial and temporal variations of the Hamiltonian. This metric reformulates the wave-packet dynamics as geodesic motion in phase space, enabling an analogue-gravity perspective in condensed matter systems. As an application, we analyze one-dimensional Dirac electron systems under a slowly varying exchange field $\bm{m}$. Our results demonstrate that variations in the magnitude of $\bm{m}$ are important to nonadiabatic dynamics, in sharp contrast to the adiabatic regime where directional variations of $\bm{m}$ are crucial.

cond-mat.mtrl-sci↗

arXiv2025

Light-induced ultrafast magnetization dynamics in van der Waals antiferromagnetic CrSBr

We investigate the ultrafast magnetization dynamics of semiconducting antiferromagnetic CrSBr using real-time time-dependent density functional theory. In zero magnetic field, laser excitation modifies only the magnetization along the easy axis, leaving transverse components unaffected. We find that below-gap, low-fluence pulses enhance the local magnetic moments via spin transfer from nonmagnetic to magnetic atoms, increasing the Neel vector. In contrast, high-fluence pulses drive interlayer spin transfer between magnetic atoms, producing strong demagnetization and reducing the Neel vector, while S and Br atoms exhibit primarily charge transfer with weak opposite contribution to the demagnetization. An applied magnetic field qualitatively alters the response, enabling both magnitude changes and ultrafast reorientation of the magnetization. We show that the resulting layer-resolved reorientation respects a twofold rotation about the x-axis, exciting coherent optical magnons even under this symmetry, which modulate the relative angle between neighboring layers and periodically tune electronic properties. These results reveal a microscopic pathway for coherent magnon excitation in van der Waals magnets and establish a framework for controlling their coupled spin-charge dynamics on femtosecond timescales.

cond-mat.mtrl-sci↗

arXiv2025

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

Nonadiabatic Origin of Quantum-Metric Effects via Momentum-Space Metric Tensor

We reveal a fundamental geometric structure of momentum space arising from the nonadiabatic evolution of Bloch electrons. By extending semiclassical wave packet theory to incorporate nonadiabatic effects, we introduce a momentum-space metric tensor -- the nonadiabatic metric. This metric gives rise to two velocity corrections, dubbed geometric and geodesic velocities, providing a unified and intuitive framework for understanding nonlinear and nonadiabatic transport phenomena beyond Berry phase effects. The geometric velocity is related to the nonadiabatic metric itself, whereas the geodesic velocity is a Christoffel symbol of the nonadiabatic metric. As the nonadiabatic metric is related to the energy-gap renormalized quantum metric, it unifies the broad quantum metric effects in electronic responses. When the nonadiabatic metric is constant, it reduces to an effective mass, modifying flat-band electron dynamics in confining potentials. In a flat Chern band with harmonic attractive interactions, the two-body wave functions mirror the Landau-level wave functions on a torus. Furthermore, we show that the nonadiabatic metric endows momentum space with a curved geometry, recasting wave packet dynamics as forced geodesic motion.

cond-mat.mtrl-sci↗

arXiv2025

Adiabatic Pumping of Orbital Magnetization by Spin Precession

We propose adiabatic pumping of orbital magnetization driven by coherent spin precession, facilitating the rectification of this precession. The orbital magnetization originates from the adiabatic evolution of valence electrons with a topological bulk contribution expressed as a Chern-Simons form. When the precession cone angle of spin $\mathbf{S}$ is small, the resulting magnetization is proportional to $\mathbf{S}\times \dot{\mathbf{S}}$, contributing to the magnon Zeeman effect. With a large cone angle, the magnetization can reach its natural unit, $e/T$, in an antiferromagnetic topological insulator with $e$ as the elementary charge and $T$ as the precession period. This significant magnetization is related to the global properties of the electronic geometric phases in the parameter space spanned by $\mathbf{S}$ and momentum $\mathbf{k}$. When the pumped magnetization is inhomogeneous, induced by spin textures or electronic topological phase domains, a dissipationless charge current is also pumped. At last, we discuss the boundary contributions from the spin-driving edge states, which are intricately linked to the gauge-dependent quantum uncertainty of the Chern-Simons form.

cond-mat.mtrl-sci↗

arXiv2024

Exciton Dressing by Extreme Nonlinear Magnons in a Layered Semiconductor

Collective excitations presenting nonlinear dynamics are fundamental phenomena with broad applications. A prime example is nonlinear optics, where diverse frequency mixing processes are central to communication, sensing, wavelength conversion, and attosecond physics. Leveraging recent progress in van der Waals magnetic semiconductors, we demonstrate nonlinear opto-magnonic coupling by presenting exciton states dressed by up to 20 harmonics of magnons, resulting from their nonlinearities, in the layered antiferromagnetic semiconductor CrSBr. We also create tunable optical side bands from sum- and difference-frequency generation between two optically bright magnon modes under symmetry breaking magnetic fields. Moreover, the observed difference-frequency generation mode can be continuously tuned into resonance with one of the fundamental magnons, resulting in parametric amplification of magnons. These findings realize the modulation of the optical frequency exciton with the extreme nonlinearity of magnons at microwave frequencies, which could find applications in magnonics and hybrid quantum systems, and provide new avenues for implementing opto-magnonic devices.

cond-mat.mes-hall↗

arXiv2024

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↗

arXiv2024

Magnetic proximity coupling to defects in a two-dimensional semiconductor

The ultrathin structure and efficient spin dynamics of two-dimensional (2D) antiferromagnetic (AFM) materials hold unprecedented opportunities for ultrafast memory devices, artificial intelligence circuits, and novel computing technology. For example, chromium thiophosphate (CrPS4) is one of the most promising 2D A-type AFM materials due to its robust stability in diverse environmental conditions and net out-of-plane magnetic moment in each layer, attributed to anisotropy in crystal axes (a and b). However, their net zero magnetic moment poses a challenge for detecting the Neel state that is used to encode information. In this study, we demonstrate the detection of the Neel vector by detecting the magnetic order of the surface layer by employing defects in tungsten diselenide (WSe2). These defects are ideal candidates for optically active transducers to probe the magnetic order due to their narrow linewidth and high susceptibility to magnetic fields. We observed spin-polarized charge transfer in the heterostructure of bulk CrPS4 and single-layer WSe2 indicating type-II band alignment as supported by density functional theory (DFT) calculations. In the A-type AFM regime, the intensity of both right-handed and left-handed circularly polarized light emanating from the sample remains constant as a function of the applied magnetic field, indicating a constant polarized transition behavior. Our results showcase a new approach to optically characterizing the magnetic states of 2D bulk AFM material, highlighting avenues for future research and technological applications.

cond-mat.mtrl-sci↗

arXiv2024

Electric polarization induced by magnons and magnon Nernst effects

Magnons offer a promising path toward energy-efficient information transmission and the development of next-generation classical and quantum computing technologies. However, methods to efficiently excite, manipulate, and detect magnons remain a critical need. Here, we show that magnons, despite their charge-neutrality, can induce electric polarization as a result of both their spin and orbital moments. We demonstrate this by calculating the electric polarization induced by magnons in two-dimensional (2D) honeycomb antiferromagnets. The electric polarization becomes finite when the Dzyaloshinskii-Moriya Interaction (DMI) is present and its magnitude can be increased by symmetries of the system. We illustrate this by computing and comparing the electric polarizations induced by the magnon Nernst effects in 2D materials with N\'eel and Zigzag ordering. Our findings show that in the Zigzag order, where the effect is dominated by the magnon orbital moment, the induced electric polarization is approximately three orders of magnitude greater than in the N\'eel phase. These findings reveal that electric fields could enable both detection and manipulation of magnons under certain conditions by leveraging their spin and orbital angular moment. They also suggest that the discovery or engineering of materials with substantial magnon orbital moments could lead to more practical use of magnons for future computing and information transmission device applications.

cond-mat.mes-hall↗

arXiv2024

Nonreciprocal phonons in PT-symmetric antiferromagnet

Phonon nonreciprocity, indicating different transport properties along opposite directions, has been observed in experiments under a magnetic field. We show that nonreciprocal acoustic phonons can also exist without a magnetic field nor net magnetization. We focus on PT symmetric antiferromagnets that break both time-reversal T and inversion symmetry P. We identify crucial contributions in phenomenological elastic theory, dubbed flexo-viscosity and flexo-torque, that induce phonon nonreciprocity without changing the phonon polarization. The microscopic origin of these contributions is the molecular Berry curvature, manifested as emergent nonlocal magnetic fields on phonons. The symmetry breaking originated from spin order is transferred to the phonon system through spin-orbit coupling, where the orbital degree of freedom affects the lattice dynamics directly. By electrically modifying the spin-orbit coupling, we show that both the phonon nonreciprocity and helicity can be controlled and enhanced. Importantly, the phonon nonreciprocity is an odd function of the N\'eel vector, serving as an indicator of the order parameter.

cond-mat.mes-hall↗

arXiv2024

Engineering second-order topological insulators via coupling two first-order topological insulators

We theoretically investigate the engineering of two-dimensional second-order topological insulators with corner states by coupling two first-order topological insulators. We find that the interlayer coupling between two topological insulators with opposite topological invariants results in the formation of edge-state gaps, which are essential for the emergence of the corner states. Using the effective Hamiltonian framework, We elucidate that the formation of topological corner states requires either the preservation of symmetry in the crystal system or effective mass countersigns for neighboring edge states. Our proposed strategy for inducing corner state through interlayer coupling is versatile and applicable to both $\mathbb{Z}_2$ topological insulators and quantum anomalous Hall effects. We demonstrate this approach using several representative models including the seminal Kane-Mele model, the Bernevig-Hughes-Zhang model, and the Rashba graphene model to explicitly exhibit the formation of corner states via interlater coupling. Moreover, we also observe that the stacking of the coupled $\mathbb{Z}_2$ topological insulating systems results in the formation of the time-reversal invariant three-dimensional second-order nodal ring semimetals. Remarkably, the three-dimensional system from the stacking of the Bernevig-Hughes-Zhang model can be transformed into second-order Dirac semimetals, characterized by one-dimensional hinge Fermi arcs. Our strategy of engineering second-order topological phases via simple interlayer coupling promises to advance the exploration of higher-order topological insulators in two-dimensional spinful systems.

cond-mat.mes-hall↗

arXiv2024

Interlayer Coupling Induced Topological Phase Transition to Higher Order

We theoretically find that the second-order topological insulator, i.e., corner states, can be engineered by coupling two copies of two-dimensional $\mathbb{Z}_2$ topological insulators with opposite spin-helicities. As concrete examples, we utilize Kane-Mele models (i.e., graphene with intrinsic spin-orbit coupling) to realize the corner states by setting the respective graphenes to be $\mathbb{Z}_2$ topological insulators with opposite intrinsic spin-orbit couplings. To exhibit its universality, we generalize our findings to other representative $\mathbb{Z}_2$ topological insulators, e.g., the Bernevig-Hughes-Zhang model. An effective model is presented to reveal the physical origin of corner states. We further show that the corner states can also be designed in other topological systems, e.g., by coupling quantum anomalous Hall systems with opposite Chern numbers. Our work suggests that interlayer coupling can be treated as a simple and efficient strategy to drive lower-order topological insulators to the higher-order ones.

cond-mat.mes-hall↗

arXiv2024

Chern Number Tunable Quantum Anomalous Hall Effect in Compensated Antiferromagnets

We propose to realize the quantum anomalous Hall effect (QAHE) in two-dimensional compensated antiferromagnets without net spin magnetization.} We consider antiferromagnetic MnBi$_2$Te$_4$ as a concrete example. \textcolor{blue}{By breaking the parity-time ($\mathcal{PT}$) symmetry of even-layer MnBi$_2$Te$_4$, we find that the system can host the QAHE with a nonzero Chern number.} We show that by controlling the antiferromagnetic spin configuration, for example, down/up/up/down that breaks $\mathcal{PT}$ symmetry, tetralayer MnBi$_2$Te$_4$ can host a Chern number $\mathcal{C}=-1$. Such spin configuration can be stabilized by pinning the spin orientations of the surfaces. \textcolor{blue}{Furthermore, via tuning the on-site orbital energy and vertical electric fields, we find rich QAHE phases with tunable Chern number of $|\mathcal{C}|=1, 2, 3$. In addition, we reveal that the edge states are layer-selective and primarily locate at the boundaries of the bottom and top layers. Our work not only proposes a scheme to realize Chern number tunable QAHE in antiferromagnets without net spin magnetization, but also provides a platform for layer-selective dissipationless transport devices.

cond-mat.mes-hall↗

arXiv2023

Light Driven Spontaneous Phonon Chirality and Magnetization in Paramagnets

Spin-phonon coupling enables the mutual manipulation of phonon and spin degrees of freedom in solids. In this study, we reveal the inherent nonlinearity within this coupling. Using a paramagnet as an illustration, we demonstrate the nonlinearity by unveiling spontaneous symmetry breaking under a periodic drive. The drive originates from linearly polarized light, respecting a mirror reflection symmetry of the system. However, this symmetry is spontaneously broken in the steady state, manifested in the emergence of coherent chiral phonons accompanied by a nonzero magnetization. We establish an analytical self-consistent equation to find the parameter regime where spontaneous symmetry breaking occurs. Furthermore, we estimate realistic parameters and discuss potential materials that could exhibit this behavior. Our findings shed light on the exploration of nonlinear phenomena in magnetic materials and present possibilities for on-demand control of magnetization.

cond-mat.mes-hall↗