arXiv subjects

Yunshan Cao

Publications and source records attributed to Yunshan Cao.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear Dynamics of Hopfion for Frequency Multiplication

Hopfions, associated with higher-dimensional topology through the Hopf fibration, exhibit {exotic} features like {complex knot} and improved stability compared to skyrmions, enhancing their appeal for innovative applications. In this paper, we study the nonlinear response of magnetic Hopfion to microwave fields. {We observe the emergence of higher-order harmonics of the driving microwave field as it interacts with the Hopfion}. By carefully selecting the {driving frequency, the corresponding harmonic can efficiently excite localized magnon state of a Hopfion. Our results demonstrate the promising potential of hopfions in nonlinear magnonics.

cond-mat.mes-hall↗

arXiv2025

Strong coupling of chiral magnons in altermagnets

Altermagnets recently are identified as a new class of magnets that break the time-reversal symmetry without exhibiting net magnetization. The role of the dipole-dipole interaction (DDI) on their dynamical properties however is yet to be addressed. In this work, we show that the DDI can induce the strong coupling between exchange magnons with opposite chiralities in altermagnets, manifesting as a significant level repulsion in the magnon spectrum. Crucially, the predicted magnon-magnon coupling is highly anisotropic, and observable in practical experiments. These exotic features are absent in conventional antiferromagnets. Our findings open a new pathway for quantum magnonic information processing based on altermagnetism.

cond-mat.mtrl-sci↗

arXiv2024

Quantum entanglement and Einstein-Podolsky-Rosen steering in magnon frequency comb

Significant progress has been made for the emerging concept of magnon frequency comb (MFC) but mainly in the classical region. The quantum property of the comb structure is yet to be explored. Here we theoretically investigate the quantum fluctuations of frequency combs and demonstrate the continuous-variable quantum entanglement and Einstein-Podolsky-Rosen (EPR) steering between different teeth of MFC. Without loss of generality, we address this issue in a hybrid magnon-skyrmion system. We observe a strong two-mode squeezed entanglement and asymmetric steering between the sum- and difference-frequency magnon teeth mediated by the skyrmion that acts as an effective reservoir to cool the Bogoliubov mode delocalized over the first-order magnon pair in MFC. Our findings show the prominent quantum nature of MFC, which has the potential to be utilized in ultrafast quantum metrology and multi-task quantum information processing.

quant-ph↗

arXiv2024

Simulating Thiele's Equation and Collective Skyrmion Dynamics in Circuit Networks

Developed half a century ago, Thiele's equation has played a crucial role in describing the motion of magnetic textures, ranging from simple magnetic domains to exotic magnetic solitons like skyrmions and hopfions. However, it remains a challenge to fully understand the collective dynamics of magnetic texture crystals, due to the complex many-body interactions. On the other hand, electrical circuits have recently been proved to be a powerful platform to realize rich physical phenomena in solids. In this Letter, we first construct a circuit unit to simulate Thiele's equation, which enables us to experimentally observe the "skyrmion Hall effect". By coupling a pair of such circuit units, we find an emerging frequency comb under strong pumpings, a long-sought nonlinear effect in interacting skyrmion systems. By extending our strategy to circuit arrays, we experimentally observe the topological edge state, thus confirming the theoretical prediction of topological solitonic insulators years ago. Our work builds a faithful connection between electric circuits and magnetic solitons, two seemingly unrelated areas, and opens the door for exploring collective magnetization dynamics in circuit networks.

cond-mat.mes-hall↗

arXiv2024

Low-lying magnon frequency comb in skymion crystals

A stable, low-power and tunable magnon frequency comb (MFC) is crucial for magnon-based precision measurements, quantum information processing and chip integration. Original method for creating MFC utilizes the nonlinear interactions between propagating spin waves and localized oscillations of an isolated magnetic texture, e.g., skyrmion. It requires a driving frequency well above the ferromagnetic resonance (FMR) and the spectrum frequency of MFC will quickly approach to the detection limit of conventional microwave technique after only tens of comb teeth. In addition, the detection and manipulation of a single skyrmion is challenging in experiments due to its high degree of locality. These issues hinder the applications of MFC. In this work, we report the low-lying MFC with comb frequencies below the FMR in a skyrmion crystal (SkX). We show that the MFC originates from the three-wave mixing between the collective skyrmion gyration and breathing in the SkX. Our findings significantly improve the efficiency of the nonlinear frequency conversion from a single-frequency mircowave input, and establish a synergistic relationship between the SkX and MFC, which paves the way to coherent information processing and ultra-sensitive metrology based on MFC.

cond-mat.mtrl-sci↗

arXiv2024

Skyrmion Hall effect in altermagnets

It is widely believed that the skyrmion Hall effect is absent in antiferromagnets because of the vanishing topological charge. However, the Aharonov-Casher theory indicates the possibility of topological effects for neutral particles. In this work, we predict the skyrmion Hall effect in emerging altermagnets with zero net magnetization and zero skyrmion charge. We first show that the neutral skyrmion manifests as a magnetic quadrupole in altermagnets. We reveal a hidden gauge field from the magnetic quadrupole, which induces the skyrmion Hall effect when driven by spin transfer torque. Interestingly, we identify a sign change of the Hall angle when one swaps the anisotropic exchange couplings in altermagnets. Furthermore, we demonstrate that both the velocity and Hall angle of altermagnetic skyrmions sensitively depend on the current direction. Our findings real the critical role of magnetic quadrupole in driving the skyrmion Hall effect with vanishing charge, and pave the way to discovering new Hall effect of neutral quasiparticles beyond magnetic skyrmions.

cond-mat.mtrl-sci↗

arXiv2024

Circuit realization of topological physics

Recently, topolectrical circuits (TECs) boom in studying the topological states of matter. The resemblance between circuit Laplacians and tight-binding models in condensed matter physics allows for the exploration of exotic topological phases on the circuit platform. In this review, we begin by presenting the basic equations for the circuit elements and units, along with the fundamentals and experimental methods for TECs. Subsequently, we retrospect the main literature in this field, encompassing the circuit realization of (higher-order) topological insulators and semimetals. Due to the abundant electrical elements and flexible connections, many unconventional topological states like the non-Hermitian, nonlinear, non-Abelian, non-periodic, non-Euclidean, and higher-dimensional topological states that are challenging to observe in conventional condensed matter physics, have been observed in circuits and summarized in this review. Furthermore, we show the capability of electrical circuits for exploring the physical phenomena in other systems, such as photonic and magnetic ones. Importantly, we highlight TEC systems are convenient for manufacture and miniaturization because of their compatibility with the traditional integrated circuits. Finally, we prospect the future directions in this exciting field, and connect the emerging TECs with the development of topology physics, (meta)material designs, and device applications.

cond-mat.mes-hall↗

arXiv2023

Cavity-Induced Strong Magnon-Magnon Coupling in Altermagnets

Long-distance strong coupling between short-wavelength magnons remains an outstanding challenge in quantum magnonics, an emerging interdiscipline between magnonics and quantum information science. Recently, altermagnets are identified as the third elementary class of magnets that break the time-reversal symmetry without magnetization and thus combine characteristics of conventional collinear ferromagnets and antiferromagnets. In this work, we show that cavity photons can mediate the long-distance strong coupling of exchange magnons with opposite chiralities in altermagnets, manifesting as an anticrossing of the magnon-polariton spectrum in the extremely dispersive regime. The predicted effective magnon-magnon coupling strongly depends on the magnon propagation direction, and is thus highly anisotropic. Our findings are intimately connected to the intrinsic nature of altermagnetic magnons, i.e., chirality-splitting-induced crossing of exchange magnons, which has no counterpart in conventional ferromagnets or antiferromagnets, and may open a new path way for magnon-based quantum information processing in altermagnets.

cond-mat.mtrl-sci↗

arXiv2023

Competition of non-Hermitian skin effect and topological localization of corner states observed in circuits

Exploring topological phases in non-Hermitian systems has attracted significant recent attention. One intriguing question is how topological edge states compete with the non-Hermitian skin effect. Here, we report the experimental observation of corner states in a two-dimensional non-reciprocal rhombus honeycomb electric circuit. We construct non-reciprocal and non-Hermitian circuits by introducing current-direction resolved capacitance between two nodes depends on the current direction. Skin effect thus emerges due to the non-reciprocity and prevails in dragging the corner state into the bulk. The non-Bloch winding number defined in generalized Brillouin zone is adopted to characterize the topological phase transition. Interestingly, we find that the non-Bloch $Z_2$ Berry phase can serve as an invariant to describe the non-Hermitian topology. By tuning the non-reciprocal parameter, we observe unbalanced distribution of corner states emerging on two acute angles of the rhombus lattice, with the localization length of the left corner state increasing exponentially with the degree of non-reciprocity.

cond-mat.mes-hall↗

arXiv2023

Realization of Wilson fermions in topolectrical circuits

Wilson fermion (WF) is a fundamental particle in the theory of quantum chromodynamics, originally proposed by Kenneth Wilson to solve the fermion doubling problem, i.e., more fermions than expected when one puts fermionic fields on a lattice. In this Letter, we report a direct observation of the WF in circuit systems. It is found that WFs manifest as topological spin textures analogous to the half skyrmion, half-skyrmion pair, and Néel skyrmion structures, depending on their mass. Transformations of different WF states are realized by merely tuning the electric elements. Theoretical calculations have shown that the WF with a half-skyrmion profile represents a novel quantum anomalous semimetal phase supporting a chiral edge current [B. Fu et al. npj Quantum Mater. 7, 94 (2022)], but the experimental evidence is still lacking. We experimentally observe the propagation of chiral edge current along the domain-wall separating two circuits with contrast fractional Chern numbers. Our work presents the first experimental evidence for WFs in topolectrical circuits. The nontrivial analogy between the WF state and the skyrmionic structure builds an intimate connection between the two burgeoning fields.

cond-mat.mes-hall↗

arXiv2023

Nonlinear Topological Magnon Spin Hall Effect

When a magnon passes through two-dimensional magnetic textures, it will experience a fictitious magnetic field originating from the $3\times 3$ skew-symmetric gauge fields. To date, only one of the three independent components of the gauge fields has been found to play a role in generating the fictitious magnetic field while the rest two are perfectly hidden. In this work, we show that they are concealed in the nonlinear magnon transport in magnetic textures. Without loss of generality, we theoretically study the nonlinear magnon-skyrmion interaction in antiferromagnets. By analyzing the scattering features of three-magnon processes between the circularly-polarized incident magnon and breathing skyrmion, we predict a giant Hall angle of both the confluence and splitting modes. Furthermore, we find that the Hall angle reverses its sign when one switches the handedness of the incident magnons. We dub it nonlinear topological magnon spin Hall effect. Our findings are deeply rooted in the bosonic nature of magnons that the particle number is not conserved, which has no counterpart in low-energy fermionic systems, and may open the door for probing gauge fields by nonlinear means.

cond-mat.mes-hall↗

arXiv2022

Nonreciprocal Spin Waves Driven by Left-Hand Microwaves

It is a conventional wisdom that a left-hand microwave cannot efficiently excite the spin wave (SW) in ferromagnets, due to the constraint of angular momentum conservation. In this work, we show that the left-hand microwave can drive nonreciprocal SWs in the presence of a strong ellipticity-mismatch between the microwave and precessing magnetization. A critical frequency is predicted, at which the left-hand microwave cannot excite SWs. Away from it the SW amplitude sensitively depends on the ellipticity of left-hand microwaves, in sharp contrast to the case driven by right-hand ones. By tuning the microwave frequency, we observe a switchable SW non-reciprocity in a ferromagnetic single layer. A mode-dependent mutual demagnetizing factor is proposed to explain this finding. Our work advances the understanding of the photon-magnon conversion, and paves the way to designing diode-like functionalities in nano-scaled magnonics.

cond-mat.mes-hall↗

arXiv2022

Observation of type-III corner states induced by long-range interactions

Long-range interactions (LRIs) are ubiquitous in nature. Higher-order topological (HOT) insulator represents a new phase of matter. A critical issue is how LRI dictates the HOT phases. In this work, we discover four topologically distinct phases, i.e., HOT phase, the bound state in the continuum, Dirac semimetal (DSM) phase, and trivial insulator phase in a breathing kagome circuit with tunable LRIs. We find an emerging type-III corner state in the HOT phase, which splits from the edge state continuum and originates from the strong couplings between nodes at different edges. We experimentally detect this novel state by impedance and voltage measurements. In the DSM phase, the Dirac cone exhibits an itinerant feature with a tunable position that depends on the LRI strength. Our findings provide a deeper understanding of the LRI effect on exotic topological states and pave the way for regulating interactions in topolectrical circuits.

cond-mat.mes-hall↗

arXiv2022

Twisted Magnon Frequency Comb and Penrose Superradiance

Quantization effects of the nonlinear magnon-vortex interaction in ferromagnetic nanodisks are studied. We show that the circular geometry twists the spin-wave fields with spiral phase dislocations carrying quantized orbital angular momentum (OAM). Meanwhile, the confluence and splitting scattering of twisted magnons off the gyrating vortex core (VC) generates a frequency comb consisting of discrete and equally spaced spectral lines, dubbed as twisted magnon frequency comb (tMFC). It is found that the mode spacing of the tMFC is equal to the gyration frequency of the VC and the OAM quantum numbers between adjacent spectral lines differ by one. By applying a magnetic field perpendicular to the plane of a thick nanodisk, we observe a magnonic Penrose superradiance inside the cone vortex state, which mimics the amplification of waves scattered from a rotating black hole. It is demonstrated that the higher-order modes of tMFC are significantly amplified while the lower-order ones are trapped within the VC gyrating orbit which manifests as the ergoregion. These results suggest a promising way to generate twisted magnons with large OAM and to drastically improve the flatness of the magnon comb.

cond-mat.mes-hall↗

arXiv2022

All-magnonic Stern-Gerlach effect in antiferromagnets

The Stern-Gerlach (SG) effect is well known as the spin-dependent splitting of a beam of atoms carrying magnetic moments by a magnetic-field gradient, leading to the concept of electron spin. Antiferromagnets can accommodate two magnon modes with opposite spin polarizations, which is equivalent to the spin property of electrons. Here, we propose the existence of an all-magnonic SG effect in antiferromagnetic magnonic system, where a linearly polarized spin-wave beam is deflected by a straight Dzyaloshinskii-Moriya interaction (DMI) interface into two opposite polarized spin-wave beams propagating in two discrete directions. Moreover, we observe bi-focusing of antiferromagnetic spin waves induced by a curved DMI interface, which can also spatially separate thermal magnons with opposite polarizations. Our findings provide a unique perspective to understand the rich phenomena associated with antiferromagnetic magnon spin and would be helpful for polarization-dependent application of antiferromagnetic spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Generation of twisted magnons via spin-to-orbital angular momentum conversion

Twisted magnons (TMs) carrying orbital angular momentum (OAM) have attracted much attention from the magnonic community. The fabrication of such novel magnon state however is still challenging. Here we present a simple method to generate TMs with arbitrary radial and azimuthal quantum numbers through the spin-to-orbital angular momentum conversion. The conversion rate from plane-wave magnons to twisted ones is shown to be insensitive to the quantum index. The spectrum of TMs in thin nanodisks is solved analytically, showing a good agreement with micromagnetic simulations. Moreover, we numerically study the propagation of TMs in magnetic nanodisk arrays and obtain the quantitative dependence of the decay length on quantum indexes. Our results are helpful for realizing TMs with large OAMs that are indispensable for future high-capacity magnonic communications and computings.

cond-mat.mes-hall↗

arXiv2022

Quantum magnonics: when magnon spintronics meets quantum information science

Spintronics and quantum information science are two promising candidates for innovating information processing technologies. The combination of these two fields enables us to build solid-state platforms for studying quantum phenomena and for realizing multi-functional quantum tasks. For a long time, however, the intersection of these two fields was limited. This situation has changed significantly over the last few years because of the remarkable progress in coding and processing information using magnons. On the other hand, significant advances in understanding the entanglement of quasi-particles and in designing high-quality qubits and photonic cavities for quantum information processing provide physical platforms to integrate magnons with quantum systems. From these endeavours, the highly interdisciplinary field of quantum magnonics emerges, which combines spintronics, quantum optics and quantum information science.Here, we give an overview of the recent developments concerning the quantum states of magnons and their hybridization with mature quantum platforms. First, we review the basic concepts of magnons and quantum entanglement and discuss the generation and manipulation of quantum states of magnons, such as single-magnon states, squeezed states and quantum many-body states including Bose-Einstein condensation and the resulting spin superfluidity. We discuss how magnonic systems can be integrated and entangled with quantum platforms including cavity photons, superconducting qubits, nitrogen-vacancy centers, and phonons for coherent information transfer and collaborative information processing. The implications of these hybrid quantum systems for non-Hermitian physics and parity-time symmetry are highlighted, together with applications in quantum memories and high-precision measurements. Finally, we present an outlook on the opportunities in quantum magnonics.

quant-ph↗

arXiv2022

Experimental realization of two-dimensional weak topological insulators

We report the experimental realization of two-dimensional (2D) weak topological insulator (WTI) in spinless Su-Schrieffer-Heeger circuits with parity-time and chiral symmetries. Strong and weak $\mathbb{Z}_2$ topological indexes are adopted to explain the experimental findings that a Dirac semimetal (DSM) phase and four WTI phases emerge in turn when we modulate the centrosymmetric circuit deformations. In DSM phase, it is found that the Dirac cone is highly anisotropic and not pinned to any high-symmetry points but can widely move within the Brillouin zone, which eventually leads to the phase transition between WTIs. In addition, we observe a pair of flat-band domain wall states by designing spatially inhomogeneous node connections. Our work provides the first experimental evidence for 2D WTIs, which significantly advances our understanding on the strong and weak nature of topological insulators, the robustness of flat bands, and the itinerant and anisotropic feature of Dirac cones.

cond-mat.mes-hall↗