arXiv subjects

Yan Zhou

Publications and source records attributed to Yan Zhou.

At least 145 records · Page 8Linked to original sources

arXiv2022

Universal quantum computation based on nanoscale skyrmion helicity qubits in frustrated magnets

Skyrmions in frustrated magnets have the helicity degree of freedom, where two different configurations of Bloch-type skyrmions are energetically favored by the magnetic dipole-dipole interaction and characterized by opposite helicities. A skyrmion must become a quantum-mechanical object when its radius is of the order of nanometer. We construct a qubit based on the two-fold degeneracy of the Bloch-type nanoscale skyrmions in frustrated magnets. It is shown that the universal quantum computation is possible based on nanoscale skyrmions in a multilayered system. We explicitly show how to construct the $π/4$ phase-shift gate, the Hadamard gate, and the CNOT gate. The one-qubit quantum gates are materialized by temporally controlling the electric field and the spin current. The two-qubit gate is materialized with the use of the Ising-type exchange coupling by controlling the distance between the skyrmion centers in two adjacent layers. The merit of the present mechanism is that an external magnetic field is not necessary. Our results may open a possible way toward universal quantum computations based on nanoscale topological spin textures.

cond-mat.mes-hall↗

arXiv2022

Deep Q-learning of global optimizer of multiply model parameters for viscoelastic imaging

Objective: Estimation of the global optima of multiple model parameters is valuable in imaging to form a reliable diagnostic image. Given non convexity of the objective function, it is challenging to avoid from different local minima. Methods: We first formulate the global searching of multiply parameters to be a k-D move in the parametric space, and convert parameters updating to be state-action decision-making problem. We proposed a novel Deep Q-learning of Model Parameters (DQMP) method for global optimization of model parameters by updating the parameter configurations through actions that maximize a Q-value, which employs a Deep Reward Network designed to learn global reward values from both visible curve fitting errors and hidden parameter errors. Results: The DQMP method was evaluated by viscoelastic imaging on soft matter by Kelvin-Voigt fractional derivative (KVFD) modeling. In comparison to other methods, imaging of parameters by DQMP yielded the smallest errors (< 2%) to the ground truth images. DQMP was applied to viscoelastic imaging on biological tissues, which indicated a great potential of imaging on physical parameters in diagnostic applications. Conclusions: DQMP method is able to achieve global optima, yielding accurate model parameter estimates in viscoelastic imaging. Assessment of DQMP by simulation imaging and ultrasound breast imaging demonstrated the consistency, reliability of the imaged parameters, and powerful global searching ability of DQMP. Significance: DQMP method is promising for imaging of multiple parameters, and can be generalized to global optimization for many other complex nonconvex functions and imaging of physical parameters.

cs.LG↗

arXiv2022

Nonlinear dynamics of topological helicity wave in a frustrated skyrmion string

A skyrmion in frustrated magnetic system has the helicity degree of freedom. A skyrmion string is formed in a frustrated layered system, which is well described by the $XY$ model owing to the exchange coupling between adjacent layers. We consider a system where the interlayer exchange couplings are alternating, where the dimerized $XY$ model is materialized, whose linear limit is the Su-Schrieffer-Heeger model. We argue that it is a nonlinear topological system. We study the quench dynamics of the helicity wave under the initial condition that the helicity of the skyrmion in the bottommost layer is rotated. It yields a good signal to detect whether the system is topological or trivial. Our results show that the helicity dynamics of the skyrmion string have a rich physics in the modulated exchange-coupled system.

cond-mat.mes-hall↗

arXiv2022

Decay properties of Axially Symmetric D-solutions to the Steady Incompressible Magnetohydrodynamic Equations

In this paper, we investigate the decay properties of axially symmetric solutions to the steady incompressible magnetohydrodynamic equations in $\mathbb{R}^3$ with finite Dirichlet integral. We first derive the decay rates of general D-solutions to the axisymmetric MHD equations. In the special case where the magnetic field only has the swirl component ${\textbf {h}}(r,z)= h_θ(r,z) {\mathbf e_θ}$, we obtain better decay rates. The last result examines the decay rates along the axis $Oz$ also within the special class of D-solutions with only swirl magnetic field. The main tool in this paper is the combination of the scaling argument, the \textit{Brezis-Gallouet inequality} and the weighted energy estimate.

math.AP↗

arXiv2022

Fully on-chip microwave photonics system

Microwave photonics (MWP), harnessing the tremendous bandwidth of light to generate, process and measure wideband microwave signals, are poised to spark a new revolution for the information and communication fields. Within the past decade, new opportunity for MWP has emerged driven by the advances of integrated photonics. However, despite significant progress made in terms of integration level, a fully on-chip MWP functional system comprising all the necessary photonic and electronic components, is yet to be demonstrated. Here, we break the status quo and provide a complete on-chip solution for MWP system, by exploiting hybrid integration of indium phosphide, silicon photonics and complementary metal-oxide-semiconductor (CMOS) electronics platforms. Applying this hybrid integration methodology, a fully chip-based MWP microwave instantaneous frequency measurement (IFM) system is experimentally demonstrated. The unprecedented integration level brings great promotion to the compactness, reliability, and performances of the overall MWP IFM system, including a wide frequency measurement range (2-34 GHz), ultralow estimation errors (10.85 MHz) and a fast response speed (0.3 ns). Furthermore, we deploy the chip-scale MWP IFM system into realistic application tasks, where diverse microwave signals with rapid-varying frequencies at X-band (8-12 GHz) are accurately identified in real-time. This demonstration marks a milestone for the development of integrated MWP, by providing the technology basis for the miniaturization and massive implementations of various MWP functional systems.

physics.app-ph↗

arXiv2022

Mutual conversion between a magnetic Neel hopfion and a Neel toron

Three-dimensional (3D) magnetic textures attract great attention from researchers due to their fascinating structures and dynamic behaviors. Magnetic hopfion is a prominent example of 3D magnetic textures. Here, we numerically study the mutual conversion between a Neel-type hopfion and a Neel-type toron under an external magnetic field. We also investigate the excitation modes of hopfions and torons in a film with strong perpendicular magnetic anisotropy. It is found that the Neel-type hopfion could be a stable state in the absence of the external magnetic field, and its diameter varies with the out-of-plane magnetic field. The Neel-type hopfion may transform to a Neel-type toron at an out-of-plane magnetic field of about 20 mT, where the cross section structure is a Neel-type skyrmion. The hopfion and toron show different excitation modes in the presence of an in-plane microwave magnetic field. Our results provide a method to realize the conversion between a Neel-type hopfion and a Neel-type toron, which also gives a way to distinguish Bloch-type and Neel-type hopfions.

cond-mat.mes-hall↗

arXiv2022

Field-driven skyrmion motion through velocity equipartition between skyrmions and a domain wall

Magnetic skyrmions, as a whirling spin texture with axisymmetry, cannot be propelled by a uniform magnetic field. Therefore, reported skyrmion motions have been induced using other sorts of stimuli; typically, electric currents in magnetic metals as well as spin waves, thermal gradient, and field gradient have manifested their ability to drive skyrmion motion. Here, we demonstrate, through micromagnetic simulations and analytically, that magnetic skyrmions can be displaced by a uniform perpendicular magnetic field via a domain-wall (DW) mediator. For a fixed field strength, the velocity of a skyrmion train evolves in terms of 1 / (1 + Ns) with Ns denoting the number of skyrmions. Based on Thiele's model, we reproduce the velocity-Ns relation first identified from numerical results and reveal that the skyrmion-DW and inter-skyrmion repulsions offer the direct driving force for skyrmion motion. This study underlines the role of spin textures' interaction in skyrmion dynamics, and opens an alternative route for skyrmion manipulation especially relevant to insulating magnets. Given the correspondence between magnetism and electricity, we anticipate that the scheme should also work for polar skyrmions in ferroelectrics.

cond-mat.mes-hall↗

arXiv2022

Amplifying spin waves along Néel domain wall by spin-orbit torque

Traveling spin waves in magnonic waveguides undergo severe attenuation, which tends to result in a finite propagation length of spin waves, even in magnetic materials with the accessible lowest damping constant, heavily restricting the development of magnonic devices. Compared with the spin waves in traditional waveguides, propagating spin waves along strip domain wall are expected to exhibit enhanced transmission. Here, we demonstrate, theoretically and through micromagnetic simulations, that spin-orbit torque associated with a ferromagnet/heavy metal bilayer can efficiently control the attenuation of spin waves along a Néel-type strip domain wall, despite the complexity in the ground-state magnetization configuration. The direction of the electric current applied to the heavy-metal layer determines whether these spin waves are amplified or further attenuated otherwise. Remarkably, our simulations reveal that the effective current densities required to efficiently tune the decay of such spin waves are just ~10^10 Am-2, roughly an order smaller than those required in conventional spin waveguides. Our results will enrich the toolset for magnonic technologies.

cond-mat.mes-hall↗

arXiv2022

Spectroscopy on the eEDM-sensitive states of ThF$^+$

An excellent candidate molecule for the measurement of the electron's electric dipole moment (eEDM) is thorium monofluoride (ThF$^+$) because the eEDM-sensitive state, $^3Δ_1$, is the electronic ground state, and thus is immune to decoherence from spontaneous decay. We perform spectroscopy on $X\,^3Δ_1$ to extract three spectroscopic constants crucial to the eEDM experiment: the hyperfine coupling constant, the molecular frame electric dipole moment, and the magnetic $g$-factor. To understand the impact of thermal blackbody radiation on the vibrational ground state, we study the lifetime of the first excited vibrational manifold of $X\,^3Δ_1$. We perform ab initio calculations, compare them to our results, and discuss prospects for using ThF$^+$ in a new eEDM experiment at JILA.

physics.atom-ph↗

arXiv2022

Three-dimensional structure and formation mechanism of biskyrmions in uniaxial ferromagnets

Magnetic biskyrmions are observed in experiments but their existences are still under debate. In this work, we present the existence of biskyrmions in a magnetic film with tilted uniaxial anisotropy via micromagnetic simulations. We find biskyrmions and bubbles share a unified three-dimensional structure, in which the relative position of two intrinsic Bloch points dominates the two-dimensional topological property in the film middle. The film edge can drive Bloch points and transform bubbles into biskyrmions via the demagnetizing field. This mechanism is found in the formation process of biskyrmions in confined geometry under zero field. Our work clarifies the structure and formation mechanism of biskyrmions, emphasizing the three-dimensional aspect of skyrmion-related nanostructures.

cond-mat.str-el↗

arXiv2022

Nonreciprocal dynamics of ferrimagnetic bimerons

Magnetic bimerons are topologically nontrivial spin textures in in-plane easy-axis magnets, which can be used as particle-like information carriers. Here, we report a theoretical study on the nonreciprocal dynamics of asymmetrical ferrimagnetic (FiM) bimerons induced by spin currents. The FiM bimerons have the ability to move at a speed of kilometers per second and do not show the skyrmion Hall effect at the angular momentum compensation point. Our micromagnetic simulations and analytical results demonstrate that spin currents are able to induce the nonreciprocal transport and a drift motion of the FiM bimeron even if the system is at the angular momentum compensation point. By analyzing the current-induced effective fields, we find that the nonreciprocal transport is attributed to the asymmetry of the bimeron structure. Our results are useful for understanding the physics of bimerons in ferrimagnets and may provide guidelines for building bimeron-based spintronic devices.

cond-mat.mes-hall↗

arXiv2021

Disproof of a conjecture by Erdős and Guy on the crossing number of hypercubes

Let $Q_n$ be the $n$-dimensional hypercube, and let ${\rm cr}(Q_n)$ be the \textit{crossing number} of $Q_n$. Erdős and Guy in 1973 conjectured the following equality: ${\rm cr}(Q_n)=\frac{5}{32}4^n-\lfloor\frac{n^2+1}{2}\rfloor 2^{n-2}$. In this paper, we construct a drawing of $Q_n$ with less crossings when $n>6$, which implies that for $n>6$ we have a strict inequality.

math.CO↗

arXiv2021

Dynamic transformation between a skyrmion string and a bimeron string in a layered frustrated system

Frustrated topological spin textures have unique properties that may enable novel spintronic applications, such as helicity-based information storage and computing. Here, we report the statics and current-induced dynamics of two-dimensional (2D) pancake skyrmions in a stack of weakly coupled frustrated magnetic monolayers, which form a three-dimensional (3D) skyrmion string. The Bloch-type skyrmion string is energetically more stable than its Néel-type counterpart. It can be driven into translational motion by the dampinglike spin-orbit torque and shows the damping-dependent skyrmion Hall effect. Most notably, the skyrmion string can be transformed to a dynamically stable bimeron string by the dampinglike spin-orbit torque. The current-induced bimeron string rotates stably with respect to its center, which can spontaneously transform back to a skyrmion string when the current is switched off. Our results reveal unusual physical properties of 3D frustrated spin textures, and may open up different possibilities for spintronic applications based on skyrmion and bimeron strings.

cond-mat.mes-hall↗

arXiv2021

Skyrmion Dynamics in the Presence of Deformation

Magnetic skyrmions are topological spin textures promising for future high-density and nonvolatile memory. It is crucial to understand the current-driven skyrmion dynamics in the presence of deformation, of which an analytical model, however, remains elusive. Here we extend Thiele's model by considering both the radial and tangential forces. Our model attributes the skyrmion deformation to the current-induced rotational symmetry breaking, which includes magnetization canting and domain wall width variation. Our predictions of skyrmion radius and nonlinear dynamics are consistent with micromagnetic simulation results. Besides, we show that by applying an in-plane magnetic field, the deformation of a skyrmion can be suppressed, and even the compression of a skyrmion can be achieved. Our model provides a generic way to analyze the skyrmion deformation and may inspire applications based on nonlinear skyrmion dynamics.

cond-mat.mes-hall↗

arXiv2021

Configurable pixelated skyrmions on nanoscale magnetic grids

Topological spin textures can serve as non-volatile information carriers. Here we study the current-induced dynamics of an isolated magnetic skyrmion on a nanoscale square-grid pinning pattern formed by orthogonal defect lines with reduced magnetic anisotropy. The skyrmion on the square grid can be pixelated with a quantized size of the grid. We demonstrate that the position, size, and shape of skyrmion on the square grid are electrically configurable. The skyrmion center is quantized to be on the grid and the skyrmion may show a hopping motion instead of a continuous motion. We find that the skyrmion Hall effect can be perfectly prohibited due to the pinning effect of the grid. The pixelated skyrmion can be harnessed to build future programmable racetrack memory, multistate memory, and logic computing device. Our results will be a basis for digital information storage and computation based on pixelated topological spin textures on artificial pinning patterns.

cond-mat.mes-hall↗

arXiv2021

Dynamics of ferrimagnetic skyrmionium driven by spin-orbit torque

Magnetic skyrmionium is a skyrmion-like spin texture with nanoscale size and high mobility. It is a topologically trivial but dynamically stable structure, which can be used as a non-volatile information carrier for next-generation spintronic storage and computing devices. Here, we study the dynamics of a skyrmionium driven by the spin torque in a ferrimagnetic nanotrack. It is found that the direction of motion is jointly determined by the internal configuration of a skyrmionium and the spin polarization vector. Besides, the deformation of a skyrmionium induced by the intrinsic skyrmion Hall effect depends on both the magnitude of the driving force and the net angular momentum. The ferrimagnetic skyrmionium is most robust at the angular momentum compensation point, whose dynamics is quite similar to the skyrmionium in antiferromagnet. The skyrmion Hall effect is perfectly prohibited, where it is possible to observe the position of the skyrmionium by measuring the magnetization. Furthermore, the current-induced dynamics of a ferrimagnetic skyrmionium is compared with that of a ferromagnetic and antiferromagnetic skyrmionium. We also make a comparison between the motion of a ferrimagnetic skyrmionium and a skyrmion. Our results will open a new field of ferrimagnetic skyrmioniums for future development of ferrimagnetic spintronics devices.

cond-mat.mes-hall↗

arXiv2021

Weyl groups and cluster structures of families of log Calabi-Yau surfaces

Given a generic Looijenga pair $(Y,D)$ together with a toric model $ρ:(Y,D)\rightarrow(\overline{Y},\overline{D})$, one can construct a seed ${\bf s}$ such that the corresponding $\mathcal{X}$-cluster variety $\mathcal{X}_{\bf s}$ can be viewed as the universal family of the log Calabi-Yau surface $U=Y\setminus D$. In cases where $(Y,D)$ is positive and $\mathcal{X}_{\bf s}$ is not acyclic, we describe the action of the Weyl group of $(Y,D)$ on the scattering diagram $\mathfrak{D}_{\bf s}$. Moreover, we show that there is a Weyl group element ${\bf w}$ of order $2$ that either agrees with or approximates the Donaldson-Thomas transformation ${\rm DT}_{\mathcal{X}_{\bf s}}$ of $\mathcal{X}_{\bf s}$. As a corollary, ${\rm DT}_{\mathcal{X}_{\bf s}}$ is cluster. In positive non-acyclic cases, we also apply the folding technique as developed in \cite{YZ} and construct a maximally folded new seed $\overline{\bf s}$ from ${\bf s}$. The $\mathcal{X}$-cluster variety $\mathcal{X}_{\overline{\bf s}}$ is a locally closed subvariety of $\mathcal{X}_{\bf s}$ and corresponds to the maximally degenerate subfamily in the universal family. We show that the action of the special Weyl group element ${\bf w}$ on $\mathfrak{D}_{\bf s}$ descends to $\mathfrak{D}_{\overline{\bf s}}$ and permutes distinct subfans in $\mathfrak{D}_{\overline{\bf s}}$ , generalizing the well-known case of the Markov quiver.

math.AG↗

arXiv2021

Write asymmetry of spin-orbit torque memory induced by in-plane magnetic fields

Write asymmetry, the significantly different write current for high-to-low and low-to-high resistance switching because of natural stochastic behaviors of magnetization, is a fundamental issue in magnetic random-access memory (MRAM). For high-performance spin transfer torque (STT) MRAM, it can be eliminated by precisely controlling atomically thin magnetic multilayers or by introducing compensation techniques in circuit-level designs, while for spin-orbit torque (SOT) MRAM, it has not been addressed. Here we systematically investigated the write asymmetry of SOT-MRAM as a function of applied magnetic fields (H) and demonstrated that the write currents are intrinsically asymmetric due to different SOT efficiencies for high-to-low and low-to-high switching. Furthermore, we found that the SOT efficiency is very sensitive to the tilt angle between H and write current, which can be tuned through H to achieve symmetric SOT switching. These results provide an additional guideline for designing SOT devices and suggest that the write asymmetry can be eliminated by adjusting the introduced effective magnetic fields within a field-free SOT-MRAM architecture.

physics.app-ph↗