arXiv subjects

Peng Yan

Publications and source records attributed to Peng Yan.

At least 109 records · Page 6Linked to original sources

arXiv2019

Eshelby tensors and overall properties of nano-composites considering both interface stretching and bending effects

In this study, the fundamental framework of analytical micromechanics is generalized to consider nano-composites with both interface stretching and bending effects. The interior and exterior Eshelby tensors for a spherical nano-inclusion, with an interface defined by the Steigmann-Ogden model, subjected to an arbitrary uniform eigenstrain are derived for the first time. Correspondingly, the stress/strain concentration tensors for a spherical nano-inhomogeneity subjected to arbitrary uniform far-field stress/strain loadings are also derived. Using the obtained concentration tensors, the effective bulk and shear moduli are derived by employing the dilute approximation and the Mori-Tanaka method, respectively, which can be used for both nano-composites and nano-porous materials. An equivalent interface curvature parameter reflecting the influence of the interface bending resistance is found, which can significantly simplify the complex expressions of the effective properties. In addition to size-dependency, the closed form expressions show that the effective bulk modulus is invariant to interface bending resistance parameters, in contrast to the effective shear modulus. We also put forward for the first time a characteristic interface curvature parameter, near which the effective shear modulus is affected significantly. Numerical results show that the effective shear moduli of nano-composites and nano-porous materials can be greatly improved by an appropriate surface modification. Finally, the derived effective modulus with the Steigmann-Ogden interface model is provided in the supplemental MATLAB code, which can be easily executed, and used as a benchmark for semi-analytical solutions and numerical solutions in future studies.

physics.comp-ph↗

arXiv2019

Spherical nano-inhomogeneity with the Steigmann-Ogden interface model under general uniform far-field stress loading

An explicit solution, considering the interface bending resistance as described by the Steigmann-Ogden interface model, is derived for the problem of a spherical nano-inhomogeneity (nanoscale void/inclusion) embedded in an infinite linear-elastic matrix under a general uniform far-field-stress (including tensile and shear stresses). The Papkovich-Neuber (P-N) general solutions, which are expressed in terms of spherical harmonics, are used to derive the analytical solution. A superposition technique is used to overcome the mathematical complexity brought on by the assumed interfacial residual stress in the Steigmann-Ogden interface model. Numerical examples show that the stress field, considering the interface bending resistance as with the Steigmann-Ogden interface model, differs significantly from that considering only the interface stretching resistance as with the Gurtin-Murdoch interface model. In addition to the size-dependency, another interesting phenomenon is observed: some stress components are invariant to interface bending stiffness parameters along a certain circle in the inclusion/matrix. Moreover, a characteristic line for the interface bending stiffness parameters is presented, near which the stress concentration becomes quite severe. Finally, the derived analytical solution with the Steigmann-Ogden interface model is provided in the supplemental MATLAB code, which can be easily executed, and used as a benchmark for semi-analytical solutions and numerical solutions in future studies.

physics.class-ph↗

arXiv2019

Goos-Hänchen effect of spin waves at heterochiral interfaces

We theoretically investigate the Goos-Hänchen (GH) effect of spin-wave beams reflected from the interface between two ferromagnetic films with different Dzyaloshinskii-Moriya interactions (DMIs). The formula of the GH shift as functions of the incident angle and material parameters is derived analytically. We show that the GH effect occurs only when spin waves are totally reflected at the interface and vanishes otherwise. We further explore the GH shift of spin waves by narrow DMI strips of different widths. It is found that the induced shift is independent of the strip width down to $10$ nm, offering a novel approach to measure the DMI strength of ultra-narrow magnetic strips which is out the scope of current technology. Full micromagnetic simulations compare well with our theoretical findings. Strong distortion of edge magnetizations for narrower strips however generates a width dependence of the GH shift. The results presented in this work are helpful for understanding the GH effect in chiral magnets and for quantifying the DMI parameter in magnetic strips of sub-$50$ nm scales.

cond-mat.mes-hall↗

arXiv2019

Exceptional magnetic sensitivity of PT-symmetric cavity magnon polaritons

Achieving magnetometers with ultrahigh sensitivity at room temperature is an outstanding problem in physical sciences and engineering. Recently developed non-Hermitian cavity spintronics offers new possibilities. In this work we predict an exceptional magnetic sensitivity of cavity magnon polaritons with the peculiar parity-time (PT) symmetry. Based on the input-output formalism, we demonstrate a "Z"-shape spectrum including two sideband modes and a dark-state branch with an ultranarrow linewidth in the exact PT phase. The spectrum evolves to a step function when the polariton touches the third-order exceptional point, accompanied by an ultrahigh sensitivity with respect to the detuning. The estimated magnetic sensitivity can approach 10^{-15}T Hz^{-1/2} in the strong coupling region, which is two orders of magnitude higher than that of the state-of-the-art magnetoelectric sensor. We derive the condition for the noiseless sensing performance. Purcell-like effect is observed when the PT symmetry is broken. A possible experimental scheme to realize our proposal is also discussed.

cond-mat.mes-hall↗

arXiv2019

Steady Bell state generation via magnon-photon coupling

We show that parity-time ($\mathcal{PT}$) symmetry can be spontaneously broken in the recently reported energy level attraction of magnons and cavity photons. In the $\mathcal{PT}$-broken phase, magnon and photon form a high-fidelity Bell state with maximum entanglement. This entanglement is steady and robust against the perturbation of environment, in contrast to the general wisdom that expects instability of the hybridized state when the symmetry is broken. This anomaly is further understood by the compete of non-Hermitian evolution and particle number conservation of the hybridized system. As a comparison, neither $\mathcal{PT}$-symmetry broken nor steady magnon-photon entanglement is observed inside the normal level repulsion case. Our results may open a novel window to utilize magnon-photon entanglement as a resource for quantum technologies.

cond-mat.mes-hall↗

arXiv2019

Non-Wigner-Dyson level statistics and fractal wavefunction of disordered Weyl semimetals

Finding fingerprints of disordered Weyl semimetals (WSMs) is an unsolved task. Here we report such findings in the level statistics and the fractal nature of electron wavefunction around Weyl nodes of disordered WSMs. The nearest-neighbor level spacing follows a new universal distribution $P_c(s)=C_1 s^2\exp[-C_2 s^{2-γ_0}]$ originally proposed for the level statistics of critical states in the integer quantum Hall systems or normal dirty metals (diffusive metals) at metal-to-insulator transitions, instead of the Wigner-Dyson distribution for diffusive metals. Numerically, we find $γ_0=0.62\pm0.07$. In contrast to the Bloch wavefuntions of clean WSMs that uniformly distribute over the whole space of ($D=3$) at large length scale, the wavefunction of disordered WSMs at a Weyl node occupies a fractal space of dimension $D=2.18\pm 0.05$. The finite size scaling of the inverse participation ratio suggests that the correlation length of wavefunctions at Weyl nodes ($E=0$) diverges as $ξ\propto |E|^{-ν}$ with $ν=0.89\pm0.05$. In the ergodic limit,the level number variance $Σ_2$ around Weyl nodes increases linearly with the average level number $N$, $Σ_2=χN$, where $χ= 0.2\pm0.1$ is independent of system sizes and disorder strengths.

cond-mat.mes-hall↗

arXiv2018

Current-induced skyrmion motion on magnetic nanotubes

Magnetic skyrmions are believed to be the promising candidate of information carriers in spintronics. However, the skyrmion Hall effect due to the nontrivial topology of skyrmions can induce a skyrmion accumulation or even annihilation at the edge of the devices, which hinders the real-world applications of skyrmions. In this work, we theoretically investigate the current-driven skyrmion motion on magnetic nanotubes which can be regarded as "edgeless" in the tangential direction. By performing micromagnetic simulations, we find that the skyrmion motion exhibits a helical trajectory on the nanotube, with its axial propagation velocity proportional to the current density. Interestingly, the skyrmion's annular speed increases with the increase of the thickness of the nanotube. A simple explanation is presented. Since the tube is edgeless for the tangential skyrmion motion, a stable skyrmion propagation can survive in the presence of a very large current density without any annihilation or accumulation. Our results provide a new route to overcome the edge effect in planar geometries.

cond-mat.mes-hall↗

arXiv2018

Atomic antiferromagnetic domain wall propagation beyond the relativistic limit

We theoretically investigate the dynamics of atomic domain walls (DWs) in antiferromagnets driven by a spin-orbit field. For a DW with the width of a few lattice constants, we identify a Peierls-like pinning effect, with the depinning field exponentially decaying with the DW width, so that a spin-orbit field moderately larger than the threshold can drive the propagation of an atomic DW in a step-wise manner. For a broad DW, the Peierls pinning is negligibly small. However, the external spin-orbit field can induce a fast DW propagation, accompanied by a significant shrinking of its width down to atomic scales. Before stepping into the pinning region, noticeable spin waves are emitted at the tail of the DW. The spin-wave emission event not only broadens the effective width of the DW, but also pushes the DW velocity over the magnonic barrier, which is generally believed to be the relativistic limit of the DW speed. While the existing dynamic theory based on the continuum approximation fails in the atomic scale, we develop an energy conversion theory to interpret the DW dynamics beyond the relativistic limit.

cond-mat.mes-hall↗

arXiv2018

Edge states in a two-dimensional honeycomb lattice of massive magnetic skyrmions

We study the collective dynamics of a two-dimensional honeycomb lattice of magnetic skyrmions. By performing large-scale micromagnetic simulations, we find multiple chiral and non-chiral edge modes of skyrmion oscillations in the lattice. The non-chiral edge states are due to the Tamm-Shockley mechanism, while the chiral ones are topologically protected against structure defects and hold different handednesses depending on the mode frequency. To interpret the emerging multiband nature of the chiral edge states, we generalize the massless Thiele's equation by including a second-order inertial term of skyrmion mass as well as a third-order non-Newtonian gyroscopic term, which allows us to model the band structure of skrymion oscillations. Theoretical results compare well with numerical simulations. Our findings uncover the importance of high order effects in strongly coupled skyrmions and are helpful for designing novel topological devices.

cond-mat.mes-hall↗

arXiv2018

Probing the Dzyaloshinskii-Moriya interaction via the propagation of spin waves in ferromagnetic thin films

The Dzyaloshinskii-Moriya interaction (DMI) has attracted considerable recent attention owing to the intriguing physics behind and the fundamental role it played in stabilizing magnetic solitons, such as magnetic skyrmions and chiral domain walls. A number of experimental efforts have been devoted to probe the DMI, among which the most popular method is the Brillouin light scattering spectroscopy (BLS) to measure the frequency difference of spin waves with opposite wave vectors $\pm\mathbf{k}$ perpendicular to the in-plane magnetization $\mathbf{m}$. Such a technique, however, is not applicable for the cases of $\mathbf{k}\parallel\mathbf{m}$, since the spin-wave reciprocity is recovered then. For a narrow magnetic strip, it is also difficult to measure the DMI strength using BLS because of the spatial resolution limit of lights. To fill these gaps, we propose to probe the DMI via the propagation of spin waves in ferromagnetic films. We show that the DMI can cause the non-collinearity of the group velocities of spin waves with $\pm\mathbf{k}\parallel\mathbf{m}$. In heterogeneous magnetic thin films with different DMIs, negative refractions of spin waves emerge at the interface under proper conditions. These findings enable us to quantify the DMI strength by measuring the angle between the two spin-wave beams with $\pm\mathbf{k}\parallel\mathbf{m}$ in homogeneous film and by measuring the incident and negative refraction angles in heterogeneous films. For a narrow magnetic strip, we propose a nonlocal scheme to determine the DMI strength via nonlinear three-magnon processes. We implement theoretical calculations and micromagnetic simulations to verify our ideas. The results presented here are helpful for future measurement of the DMI and for designing novel spin-wave spintronic devices.

cond-mat.mes-hall↗

arXiv2018

Antiferromagnetism emerging in a ferromagnet with gain

We present a theoretical mapping to show that a ferromagnet with gain (loss) is equivalent to an antiferromagnet with an equal amount of loss (gain). Our finding indicates a novel first-order ferromagnet-antiferromagnet phase transition by tuning the gain-loss parameter. As an appealing application, we demonstrate the realization as well as the manipulation of the antiferromagnetic skyrmion, a stable topological quasiparticle not yet observed experimentally, in a chiral ferromagnetic thin film with gain. We also consider ferromagnetic bilayers with balanced gain and loss, and show that the antiferromagnetic skyrmion can be found only in the cases with broken parity-time symmetry phase. Our results pave a way for investigating the emerging antiferromagnetic spintronics and parity-time symmetric magnonics in ferromagnets.

cond-mat.mes-hall↗

arXiv2018

Long short-term memory networks in memristor crossbars

Recent breakthroughs in recurrent deep neural networks with long short-term memory (LSTM) units has led to major advances in artificial intelligence. State-of-the-art LSTM models with significantly increased complexity and a large number of parameters, however, have a bottleneck in computing power resulting from limited memory capacity and data communication bandwidth. Here we demonstrate experimentally that LSTM can be implemented with a memristor crossbar, which has a small circuit footprint to store a large number of parameters and in-memory computing capability that circumvents the 'von Neumann bottleneck'. We illustrate the capability of our system by solving real-world problems in regression and classification, which shows that memristor LSTM is a promising low-power and low-latency hardware platform for edge inference.

cs.ET↗

arXiv2018

Interplay of Wave Localization and Turbulence in Spin Seebeck Effect

One of the most important discoveries in spintronics is the spin Seebeck effect (SSE) recently observed in both insulating and (semi-)conducting magnets. However, the very existence of the effect in transverse configuration is still a subject of current debates, due to conflicting results reported in different experiments. Present understanding of the SSE is mainly based on a particle-like picture with the local equilibrium approximation (LEA), i.e., spatially resolved temperature-field assumed to describe the system. In this work, we abandon the LEA to some extent and develop a wave theory to explain the SSE, by highlighting the interplay between wave localization and turbulence. We show that the emerging SSE with a sign change in the high/low-temperature regions is closely related to the extendedness of the spin wave that senses an average temperature of the system. On the one hand, ubiquitous disorders (or magnetic field gradients) can strongly suppress the transverse spin Seebeck effect (TSSE) due to Anderson (or Wannier-Zeeman) spin-wave localization. On the other hand, the competing wave turbulence of interacting magnons tends to delocalize the wave, and thus remarkably revives the TSSE before the magnon self-trapping. Our theory provides a promising route to resolve the heated debate on TSSE with a clear experiment scheme to test it in future spin caloritronic devices.

cond-mat.mes-hall↗

arXiv2018

Role of atomic spin-mechanical coupling in the problem of magnetic biocompass

It is a well established notion that animals can detect the Earth's magnetic field, while the biophysical origin of such magnetoreception is still elusive. Recently, a magnetic receptor Drosophila CG8198 (MagR) with a rod-like protein complex is reported [Qin \emph{et al}., Nat. Mater. \textbf{15}, 217 (2016)] to act like a compass needle to guide the magnetic orientation of animals. This view, however, is challenged [Meister, Elife \textbf{5}, e17210 (2016)] by arguing that thermal fluctuations beat the Zeeman coupling of the proteins's magnetic moment with the rather weak geomagnetic field ($\sim25-65$ $μ$T). In this work, we show that the spin-mechanical interaction at the atomic scale gives rise to a high blocking temperature which allows a good alignment of protein's magnetic moment with the Earth's magnetic field at room temperature. Our results provide a promising route to resolve the debate on the thermal behaviors of MagR, and may stimulate a broad interest on spin-mechanical couplings down to atomistic levels.

cond-mat.mes-hall↗

arXiv2018

Photonic orbital angular momentum transfer and magnetic skyrmion rotation

Magnetic skyrmions are chiral quasiparticles that show promise for future spintronic applications such as skyrmion racetrack memories and logic devices because of their topological stability, small size (typically $\sim3-500$ nm), and ultralow threshold force to drive their motion. On the other hand, the ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted a lot of interest. In this work, we predict a photonic OAM transfer effect, by studying the dynamics of magnetic skyrmions subject to Laguerre-Gaussian optical vortices, which manifests a rotational motion of the skyrmionic quasiparticle around the beam axis. The topological charge of the optical vortex determines both the magnitude and the handedness of the rotation velocity of skyrmions. In our proposal, the twisted light beam acts as an optical tweezer to enable us displacing skyrmions over large-scale defects in magnetic films to avoid being captured.

cond-mat.mes-hall↗

arXiv2018

Eavesdropping on spin waves inside the domain-wall nanochannel via three-magnon processes

One recent breakthrough in the field of magnonics is the experimental realization of reconfigurable spin-wave nanochannels formed by magnetic domain wall with a width of $10-100$ nm [Wagner \emph{et al}., Nat. Nano. \textbf{11}, 432 (2016)]. This remarkable progress enables an energy-efficient spin-wave propagation with a well-defined wave vector along its propagating path inside the wall. In the mentioned experiment, a micro-focus Brillouin light scattering spectroscopy was taken in a line-scans manner to measure the frequency of the bounded spin wave. Due to their localization nature, the confined spin waves can hardly be detected from outside the wall channel, which guarantees the information security to some extent. In this work, we theoretically propose a scheme to detect/eavesdrop on the spin waves inside the domain-wall nanochannel via nonlinear three-magnon processes. We send a spin wave in one magnetic domain to interact with the bounded mode in the wall. Two kinds of three-magnon processes, i.e., confluence and splitting, are expected to occur. The confluence process is conventional. We predict a stimulated three-magnon splitting (or "magnon laser") effect: the presence of a bound magnon propagating along the domain wall channel assists the splitting of the incident wave into two modes, one of which is identical to the bound mode in the channel. Micromagnetic simulations confirm our theoretical analysis. These results demonstrate that one is able to uniquely infer the spectrum of the spin-wave in the domain-wall nanochannel once we know both the injection and the transmitted waves.

cond-mat.mes-hall↗

arXiv2018

Twisted skyrmion at domain boundaries and the method of image

We predict a novel twisted skyrmion structure at the boundary of two antiferromagnetically coupled magnetic domains with antiparallel magnetization directions. Through this intermediate state, skyrmions with opposite polarities can be freely switched between each other by spin-polarized electric currents. Based on these findings, we propose the concept of double-track skyrmion racetrack memory and logic gates where the binary data are represented by skyrmions with different polarities. The dynamics of skyrmion polarity reversal is theoretically studied. Using the method of image, we derive the analytical formula of the repulsive potential when a normal skyrmion approaches the domain boundary. A harmonic attractive potential well is obtained for the twisted skyrmion across the boundary. Micromagnetic simulations compare well with theoretical predictions. The method of image skyrmion proposed in this work can be used to deal with a large class of skyrmion-boundary interaction problems.

cond-mat.mes-hall↗

arXiv2017

Energy repartition in the nonequilibrium steady state

The concept of temperature in nonequilibrium thermodynamics is an outstanding theoretical issue. We propose an energy repartition principle that leads to a spectral (mode-dependent) temperature in steady-state nonequilibrium systems. The general concepts are illustrated by analytic solutions of the classical Heisenberg spin chain connected to Langevin heat reservoirs with arbitrary temperature profiles. Gradients of external magnetic fields are shown to localize spin waves in a Wannier-Zeemann fashion, while magnon interactions renormalize the spectral temperature. Our generic results are applicable to other thermodynamic systems such as Newtonian liquids, elastic solids, and Josephson junctions.

cond-mat.mes-hall↗