arXiv subjects

Xi-guang Wang

Publications and source records attributed to Xi-guang Wang.

18 recordsLinked to original sources

arXiv2026

Pseudo-Hermitian Magnon Dynamics

A defining quantity of a physical system is its energy which is represented by the Hamiltonian. In closed quantum mechanical or/and coherent wave-based systems the Hamiltonian is introduced as a Hermitian operator which ensures real energy spectrum and secures the decomposition of any state over a complete basis set spanning the space where the states live. Pseudo-Hermitian, or PT symmetric, systems are a special class of non-Hermitian ones. They describe open systems but may still have real energy spectrum. The eigenmodes are however not orthogonal in general. This qualitative difference to Hermitian physics has a range of consequences for the physical behaviour of the system in the steady state or when it is subjected to external perturbations. This overview reviews the recent progress in the field of pseudo-Hermitian physics as it unfolds when applied to low-energy excitations of magnetically ordered materials. The focus is mainly on long wave length spin excitations (spin waves) with magnons being the energy quanta of these excitations. Various setups including ferromagnetic, antiferromagnetic, magnonic crystals, and hybride structures with different types of coupling to the environments as well as spatio-temporally engineered systems will be discussed with a focus on the particular aspects that are brought about by the pseudo-Hermiticity such as mode amplifications, non-reciprocal propagation, magnon cloaking, non-Hermitian skin effect, PT-symmetric assisted Floquet engineering, topological energy transfer, and field-induced enhanced sensitivity.

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arXiv2025

PT-Symmetric Magnon Lasing and Anti-Lasing

A mechanism for electrically tunable PT-symmetric magnonic lasing and anti-lasing is proposed along with a device consisting of a current-biased region in a magnetically ordered planar waveguide. Within the bias area, several heavy-metal wires carrying dc charge current are periodically attached to the waveguide and exert so spatially periodic spin-orbit torques, producing current-controllable modulated magnon gain and loss. It is demonstrated that this decorated waveguide can emit a strong, single frequency magnon mode at the Bragg point (lasing) and also absorb at the same frequency phase-matched incoming coherent magnons (anti-lasing). The underlying physics is captured by an analytical model and validated with full material and device-specific numerical simulations. The magnonic laser absorber response is tunable via the current density in the wires, the extent of the biased region, and the intrinsic damping, enabling the control of lasing frequency and emission power. The structure is shown to amplify thermal magnons, offering a route to low-noise on-chip microwave sources. The concept is compatible with planar waveguides, ring geometries, and antiferromagnets. The results establish an experimentally realistic platform where a single element functions simultaneously as both magnon laser and absorber, opening opportunities for reconfigurable non-Hermitian magnonics and integrated magnon signal processing.

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arXiv2025

Electrically Tunable Magnonic Bound States in the Continuum

Low energy excitations of a magnetically ordered system are spin waves with magnon being their excitation quanta. Magnons are demonstrated to be useful for data processing and communication. To achieve magnon transport across extended distances, it is essential to minimize magnonic dissipation which can be accomplished by material engineering to reduce intrinsic damping or by spin torques that can counteract damping. This study introduces an alternative methodology to effectively reduce magnon dissipation based on magnonic bound states in the continuum (BIC). We demonstrate the approach for two antiferromagnetically coupled magnonic waveguides, with one waveguide being attached to a current carrying metallic layer. The current acts on the attached waveguide with a spin-orbit torque effectively amplifying the magnonic signal. The setup maps on a non-Hermitian system with coupled loss and more loss, enabling the formation of dissipationless magnon BIC. We investigate the necessary criteria for the formation of magnon BIC through electric currents. The influences of interlayer coupling constant, anisotropy constants and applied magnetic field on the current-induced magnon BIC are analyzed. The identified effect can be integrated in the design of magnon delay lines, offering opportunities for the enhancement of magnonic devices and circuits.

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arXiv2024

Active nonreciprocal cloaking for pseudo-Hermitian magnons

Cloaking has important applications but entails sophisticated control of signal propagation and scattering characteristics. Here, we show that invisibility for magnon signals is achievable in a non-reciprocal and electrically controlled way by engineering the magnonic channels such that they exhibit PT-symmetry. This is accomplished by attaching current-carrying heavy metal contacts to the magnon waveguides and exerting fields from an attached bias layer. Tuning the current density in the metal layer, the magnons in this setup experience electrically controlled, compensated gain and loss due to spin-orbit torque which renders the setup PT-symmetric. The magnon dynamics is then shown to be pseudo-Hermitian with exceptional points (EPs) determined actively by an external electric field. We analyze the magnon scattering from single and periodic PT-symmetric regions and identify the conditions necessary for the formation of unidirectional invisibility which can be steered by specific combinations of bias layers and current amplitudes in the heavy metal as to reach the EP. The unidirectional invisibility at EP is found to be extended for a periodic PT-symmetric region. Intrinsic damping on PT-symmetric unidirectional invisibility is shown to be marginal confirming the experimental feasibility. It is shown how the unidirectional magnons can be utilized to amplify and generate magnonic orbital angular momentum states in coupled magnetic rings demonstrating a new path for manipulating magnon propagation and processing.

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arXiv2023

Floquet-engineering the exceptional points in parity-time-symmetric magnonics

Magnons serve as a testing ground for fundamental aspects of Hermitian and non-Hermitian wave mechanics and are of high relevance for information technology. This study presents setups for realizing spatio-temporally driven parity-time (PT) symmetric magnonics based on coupled magnetic waveguides and magnonic crystals. A charge current in a metal layer with strong spin-orbit coupling sandwiched between two insulating magnetic waveguides leads to gain or loss in the magnon amplitude depending on the directions of the magnetization and the charge currents. When gain in one waveguide is balanced by loss in the other waveguide a PT-symmetric system hosting non-Hermitian degeneracies (or exceptional points (EPs)) is realized. For AC current multiple EPs appear for a certain gain/loss strength and mark the boundaries between the preserved PT-symmetry and the broken PT-symmetry phases. The number of islands of broken PT-symmetry phases and their extensions is tunable by the frequency and the strength of the spacer current. At EP and beyond, the induced and amplified magnetization oscillations are strong and self-sustained. In particular, these magnetization auto-oscillations in broken PT-symmetry phase occur at low current densities and do not require further adjustments such as tilt angle between electric polarization and equilibrium magnetization direction in spin-torque oscillators, pointing to a new design of these oscillators and their utilization in computing and sensoric. It is also shown how the periodic gain/loss mechanism allows for the generation of high-frequency spin waves with low-frequency currents. For spatially-periodic gain/loss acting on a magnonic crystal, magnon modes approaching each other at the Brillouin-zone boundaries are highly susceptible to PT-symmetry, allowing for a wave-vector-resolved experimental realization at very low currents.

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arXiv2023

PT-symmetry enabled spintronic thermal diode and logic gates

Devices for performing computation and logic operations with low-energy consumption are of key importance for environmentally friendly data-processing and information technology. Here, we present a design for magnetic elements that use excess heat to perform logic operations. The basic information channel is coupled non-conductive magnetic stripes with a normal metal spacer. The thermal information signal is embodied in magnetic excitations and it can be transported, locally enhanced, and controllably steered by virtue of charge current pulses in the spacer. Functionality of essential thermal logic gates is demonstrated by material-specific simulations. The operation principle takes advantage of the special material architecture with a balanced gain/loss mechanism for magnetic excitation which renders the circuit parity-time symmetric with exceptional points tunable by the current strength in the spacer. Heat flow at these points can be enhanced, be non-reciprocal, or may oscillate between the information channels enabling so controlled thermal diode and thermal gate operations. The findings point to a new route for exploiting heat for useful work on the nanoscale.

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arXiv2023

Steering skyrmions with microwave and THz electric pulses

Tools for controlling electrically the motion of magnetic skyrmions are important elements towards their use in spintronic devices. Here, we propose and demonstrate the transport of skyrmions via GHz and THz electric pulses. The method relies on using polarization textured pulses such that the skyrmion experiences (via its inherent magnetoelectricity) the out-of-plane and in-plane components of the pulse electric field. It is shown how the electric field drags efficiently the skyrmion. The control of the skyrmion motion depends solely on the amplitude of electric fields, frequency, polarization, or phase in case two pulses are applied. Micromagnetic calculations supported by analytic modeling and analysis indicate the experimental feasibility of the control scheme.

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arXiv2022

Magnon dynamics in parity-time-symmetric dipolarly coupled waveguides and magnonic crystals

We consider the magnonic properties of two dipolarly coupled magnetic stripes, both deposited on a normal conductive substrate with strong spin-orbit coupling. A charge current in the substrate acts on the adjacent magnets with spin-orbit torques, which result in magnonic damping or antidamping of the spin waves, and hence a gain-loss coupling of the two magnetic stripes. The whole setup is demonstrated to exhibit features typical for parity-time (PT) symmetric systems. Phenomena are demonstrated that can be functionalized in magnonic devices, including reconfigurable magnonic diodes and logic devices. Alternative stripes designs and PT-symmetric, periodic, coupled magnonic textures are studied. Analytical and full numerical analysis identify the conditions for the appearance of exceptional points (EPs), where magnonic gain and loss are balanced and evidence nonreciprocal magnon propagation and enhanced magnon excitation around EPs. Furthermore, the dipolar coupling is shown to bring in a wave vector-dependent PT-symmetric behavior. Proposing and simulating a PT-symmetric magnonic crystal, we show how EPs and hence associated phenomena can be steered to a particular wave vector in a gaped spectrum via material design. The phenomena offer additional tools for magnonic-based communication and computational devices.

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arXiv2022

Rectification of the spin Seebeck current in noncollinear antiferromagnets

In the absence of an external magnetic field and a spin-polarized charge current, an antiferromagnetic system supports two degenerate magnon modes. An applied thermal bias activates the magnetic dynamics, leading to a magnon flow from the hot to the cold edge (magnonic spin Seebeck current). Both degenerate bands contribute to the magnon current but the orientations of the magnetic moments underlying the magnons are opposite in different bands. Therefore, while the magnon current is nonzero, the net spin current is zero.

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arXiv2020

Electron-magnon spin conversion and magnonic spin pumping in antiferromagnet/heavy metal heterostructure

We study the exchange between electron and magnon spins at the interface of an antiferromagnet and a heavy metal at finite temperatures. The underlying physical mechanism is based on spin torque associated with the creation/annihilation of thermal magnons with right-hand and left-hand polarization. The creation/annihilation process depends strongly on the relative orientation between the polarization of the electron and the magnon spins. For a sufficiently strong spin transfer torque (STT), the conversion process becomes nonlinear, generating a nonzero net spin pumping current in the AFM that can detected in the neighboring metal layer. Applying an external magnetic field renders possible the manipulation of the STT driving thermal spin pumping. Our theoretical results are experimentally feasible and are of a direct relevance to antiferromagnet-based spintronic devices.

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arXiv2019

Steering magnonic dynamics and permeability at exceptional points in a parity-time symmetric waveguide

Tuning the low-energy magnetic dynamics is a key element in designing novel magnetic metamaterials, spintronic devices and magnonic logic circuits. This study uncovers a new, highly effective way of controlling the magnetic permeability via shaping the magnonic properties in coupled magnetic waveguides separated by current carrying spacer with strong spin-orbit coupling. The spin-orbit torques exerted on the waveguides leads to an externally tunable enhancement of magnetic damping in one waveguide and a decreased damping in the other, constituting so a magnetic parity-time (PT) symmetric system with emergent magnetic properties at the verge of the exceptional point where magnetic gains/losses are balanced. In addition to controlling the magnetic permeability, phenomena inherent to PT-symmetric systems are identified, including the control on magnon power oscillations, nonreciprocal magnon propagation, magnon trapping and enhancement as well as the increased sensitivity to magnetic perturbation and abrupt spin reversal. These predictions are demonstrated analytically and confirmed by full numerical simulations under experimentally feasible conditions. The position of the exceptional points and the strength of the spontaneous PT symmetry breaking can be tuned by external electric and/or magnetic fields. The roles of the intrinsic magnetic damping, and the possibility of an electric control via Dzyaloshinskii-Moriya interaction are exposed and utilized for mode dispersion shaping and magnon amplification and trapping. The results point to a new route to designing optomagnonic waveguides, traps, sensors, and circuits.

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arXiv2019

High-fidelity magnonic gates for surface spin waves

We study the propagation of surface spin waves in two wave guides coupled through the dipole-dipole interaction. Essential for the observations made here is the magneto-electric coupling between the spin waves and the effective ferroelectric polarization. This allows an external electric field to act on spin waves and to modify the band gaps of magnonic excitations in individual layers. By an on/off switching of the electric field and/or varying its strength or direction with respect to the equilibrium magnetization, it is possible to permit or ban the propagation of the spin waves in selected waveguide. We propose experimentally feasible nanoscale device operating as a high fidelity surface wave magnonic gate.

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arXiv2019

Thermally assisted Skyrmion drag in a nonuniform electric field

Magnetic skyrmions are topologically protected excitations of the magnetization vector field with promising applications in spintronics and spin-caloritronics, particularly due to their high mobility. Skyrmions can be steered by a spin-polarized charge current or by exposure to a magnonic spin current. Here, we propose a further method for driving skyrmions by applying an inhomogeneous electric field and a homogeneous thermal bias. We show that the inhomogeneous electric torque leads to an efficient skyrmionic drag which can be thermally assisted as to enhance the skyrmion velocity. The calculations and analysis are limited to insulating samples; for conducting materials the influence of the inhomogeneous electric field on the charge carriers need to be taken also into account.

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arXiv2018

Left-handed polarized spin waves in ferromagnets induced by spin-transfer torque

Polarization is a fundamental property of waves that refers to the orientation of the oscillations. It has been widely used to encode information in photonics and phononics. However, the polarization of spin waves is rarely used yet in magnonics. The reason for this is that only the right-handed polarized spin waves can be accommodated in ferromagnets. Here, we report that stable left-handed polarized spin waves can be introduced into ferromagnets if a spin-polarized electrical current is presented. The right-handed and left-handed polarized spin waves coexist when the current density is larger than a critical value while the system keeps stable. The results are confirmed by micromagnetic simulations. This work provides new playgrounds to study spin waves and points to new findings for future experimental studies.

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arXiv2018

Electric control of emergent magnonic spin current and dynamic multiferroicity in magnetic insulators at finite temperatures

Conversion of thermal energy into magnonic spin currents and/or effective electric polarization promises new device functionalities. A versatile approach is presented here for generating and controlling open circuit magnonic spin currents and an effective multiferroicity at a uniform temperature with the aid of spatially inhomogeneous, external, static electric fields. This field applied to a ferromagnetic insulator with a Dzyaloshinskii-Moriya type coupling changes locally the magnon dispersion and modifies the density of thermally excited magnons in a region of the scale of the field inhomogeneity. The resulting gradient in the magnon density can be viewed as a gradient in the effective magnon temperature. This effective thermal gradient together with local magnon dispersion result in an open-circuit, electric field controlled magnonic spin current. In fact, for a moderate variation in the external electric field the predicted magnonic spin current is on the scale of the spin (Seebeck) current generated by a comparable external temperature gradient. Analytical methods supported by full-fledge numerics confirm that both, a finite temperature and an inhomogeneous electric field are necessary for this emergent non-equilibrium phenomena. The proposal can be integrated in magnonic and multiferroic circuits, for instance to convert heat into electrically controlled pure spin current using for example nanopatterning, without the need to generate large thermal gradients on the nanoscale.

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arXiv2017

Conversion of electronic to magnonic spin current at heavy-metal magnetic-insulator interface

Electronic spin current is convertible to magnonic spin current via the creation or annihilation of thermal magnons at the interface of a magnetic insulator and a metal with a strong spin-orbital coupling. So far this phenomenon was evidenced in the linear regime. Based on analytical and fulledged numerical results for the non-linear regime we demonstrate that the generated thermal magnons or magnonic spin current in the insulator is asymmetric with respect to the charge current direction in the metal and exhibits a nonlinear dependence on the charge current density, which is explained by the tuning effect of the spin Hall torque and the magnetization damping. The results are also discussed in light of and are in line with recent experiments pointing to a new way of non-linear manipulation of spin with electrical means.

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arXiv2016

Magnonic band structure of domain wall magnonic crystals

Magnonic crystals are prototype magnetic metamaterials designed for the control of spin wave propagation. Conventional magnonic crystals are composed of single domain elements. If magnetization textures, such as domain walls, vortices and skyrmions, are included in the building blocks of magnonic crystals, additional degrees of freedom over the control of the magnonic band structure can be achieved. We theoretically investigate the influence of domain walls on the spin wave propagation and the corresponding magnonic band structure. It is found that the rotation of magnetization inside a domain wall introduces a geometric vector potential for the spin wave excitation. The corresponding Berry phase has quantized value $4 n_w π$, where $n_w$ is the winding number of the domain wall. Due to the topological vector potential, the magnonic band structure of magnonic crystals with domain walls as comprising elements differs significantly from an identical magnonic crystal composed of only magnetic domains. This difference can be utilized to realize dynamic reconfiguration of magnonic band structure by a sole nucleation or annihilation of domain walls in magnonic crystals.

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arXiv2013

Magnonic momentum transfer force on domain walls confined in space

Momentum transfer from incoming magnons to a Bloch domain wall is calculated using one dimensional continuum micromagnetic analysis. Due to the confinement of the wall in space, the dispersion relation of magnons is different from that of a single domain. This mismatch of dispersion relations can result in reflection of magnons upon incidence on the domain wall, whose direct consequence is a transfer of momentum between magnons and the domain wall. The corresponding counteraction force exerted on the wall can be used for the control of domain wall motion through magnonic linear momentum transfer, in analogy with the spin transfer torque induced by magnonic angular momentum transfer.

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