arXiv subjects

Weichao Yu

Publications and source records attributed to Weichao Yu.

28 records · Page 2Linked to original sources

arXiv2020

A loop theory for the input-output problems in cavities

The input-output formalism is the basis to study the response of an optical cavity to the external stimulations. The existing theories usually handle cavity systems with only one internal mode. However, there is growing interest in more complex systems, especially the hybrid cavity-matter systems, which contains at least two internal modes, one or more from the optical cavity and the matter, respectively. Here we propose a graphical loop theory to calculate and visualize the reflection and transmission spectrum of such multi-mode cavity, resembling the role of Feynman diagrams in the quantum field theory. This loop theory gives a unified picture to interpret the experimental observations on a hybrid magnet-light system, and is extremely easy to apply to arbitrary complicated problems without any calculations.

cond-mat.mes-hall↗

arXiv2019

Realization of Attractive Level Crossing via a Dissipative Mode

The new field of spin cavitronics focuses on the interaction between the magnon excitation of a magnetic element and the electromagnetic wave in a microwave cavity. In strong interaction regime, such interaction usually gives rise to the level anti-crossing for the magnonic and the electromagnetic mode. Recently, the attractive level crossing has been observed, and is explained by a non-Hermitian model Hamiltonian. However, the mechanism of the such attractive coupling is still unclear. Here we reveal the secret by using a simple model with two harmonic oscillators coupled to a third oscillator with large dissipation. We further identify this dissipative third-party as the invisible cavity mode with large leakage in the cavity-magnon experiments. This understanding enables designing dissipative coupling in all sorts of coupled systems.

cond-mat.mes-hall↗

arXiv2019

Purely magnetic logic based on polarized spin waves

Spin wave, the precession of magnetic order in magnetic materials, is a collective excitation that carries spin angular momentum. Similar to the acoustic or optical waves, the spin wave also possesses the polarization degree of freedom. Although such polarization degrees of freedom are frozen in ferromagnets, they are fully unlocked in antiferromagnets or ferrimagnets. Here we introduce the concept of magnetic gating and demonstrate a spin wave analog of the Datta-Das spin transistor in antiferromagnet. Utilizing the interplay between polarized spin wave and the antiferromagnetic domain walls, we propose a universal logic gate of pure magnetic nature, which realizes all Boolean operations in one single magnetic structure. We further construct a full functional 4-bit Arithmetic Logic Unit using only sixteen spin wave universal logic gates, operating in a weaving fashion as a Jacquard loom machine. The spin wave-based architecture proposed here also sets a model for the future energy efficient non-volatile computing, the distributed processing-in-memory computing, and the evolvable neuromorphic computing.

cond-mat.mes-hall↗

arXiv2019

Topological spin Hall effects and tunable skyrmion Hall effects in uniaxial antiferromagnetic insulators

Recent advances in the physics of current-driven antiferromagnetic skyrmions have observed the absence of a Magnus force. We outline the symmetry reasons for this phenomenon, and show that this cancellation will fail in the case of spin polarized currents. Pairing micromagnetic simulations with semiclassical spin wave transport theory, we demonstrate that skyrmions produce a spin-polarized transverse magnon current, and that spin-polarized magnon currents can in turn produce transverse motion of antiferromagnetic skyrmions. We examine qualitative differences in the frequency dependence of the skyrmion Hall angle between ferromagnetic and antiferromagnetic cases, and close by proposing a simple skyrmion-based magnonic device for demultiplexing of spin channels.

cond-mat.mes-hall↗

arXiv2018

Manipulating spin wave polarization in synthetic antiferromagnet

Polarization is a key ingredient of all waves, including the electromagnetic wave, the acoustic wave, as well as the spin wave. Due to the fixed ferromagnetic order, the spin wave in ferromagnet is limited to the right circular polarization. The spin wave in antiferromagnet, however, is endowed with the full polarization degree of freedom because of the two identical magnetic sublattices. In the synthetic antiferromagnet, the two magnetic sublattices are spatially separated into two sublayers. The circular polarization of spin wave is partially locked to the magnetic sublattice of the antiferromagnet, thus to the sublayer in synthetic antiferromagnet. Based on this unique polarization-sublayer locking mechanism, we show that both the circular spin wave polarizer and retarder (wave-plate) can be straightforwardly realized using synthetic antiferromagnets by restructuring the sublayers, e.g. by removing or capping a portion of a sublayer. Manipulating spin wave polarization by geometrical engineering provides a simple yet powerful paradigm in harnessing the spin wave polarization for spin information processing.

cond-mat.mtrl-sci↗

arXiv2018

Nonabelian magnonics in antiferromagnets

We present a semiclassical formalism for antiferromagnetic (AFM) magnonics which promotes the central ingredient of spin wave chirality, encoded in a quantity called magnonic isospin, to a first-class citizen of the theory. We use this formalism to unify results of interest from the field under a single chirality-centric formulation. Our main result is that the isospin is governed by unitary time evolution, through a Hamiltonian projected down from the full spin wave dynamics. Because isospin is SU(2)-valued, its dynamics on the Bloch sphere are precisely rotations - which, in general, do not commute. Consequently, the induced group of operations on AFM spin waves is nonabelian. This is a paradigmatic departure from ferromagnetic magnonics, which operates purely within the abelian group generated by spin wave phase and amplitude. Our investigation of this nonabelian magnonics in AFM insulators focuses on studying several simple gate operations, and offering in broad strokes a program of study for interesting new logic families in antiferromagnetic spin wave systems

cond-mat.mes-hall↗

arXiv2017

Antiferromagnetic domain wall motion driven by polarized spin waves

The control of magnetic domain walls is essential for the magnetic-based memory and logic applications. As an elementary excitation of magnetic order, spin wave is capable of moving magnetic domain walls just as the conducting electric current. Ferromagnetic spin waves can only be right-circularly polarized. In contrast, antiferromagnetic spin waves have full polarization degree of freedom, including both left- and right-circular polarizations, as well as all possible linear or elliptical ones. Here we demonstrate that, due to the Dzyaloshinskii-Moriya interaction, the spin wave driven domain wall motion in antiferromagnets strongly depends on the linear polarization direction of the injected spin waves. Steering domain wall motion by simply tuning the polarization of spin waves offers new designing principles for domain-wall based information processing devices.

cond-mat.mtrl-sci↗

arXiv2017

Antiferromagnetic Domain Wall as Spin Wave Polarizer and Retarder

As a collective quasiparticle excitation of the magnetic order in magnetic materials, spin wave, or magnon when quantized, can propagate in both conducting and insulating materials. Like the manipulation of its optical counterpart, the ability to manipulate spin wave polarization is not only important but also fundamental for magnonics. With only one type of magnetic lattice, ferromagnets can only accommodate the right-handed circularly polarized spin wave modes, which leaves no freedom for polarization manipulation. In contrast, antiferromagnets, with two opposite magnetic sublattices, have both left and right circular polarizations, and all linear and elliptical polarizations. Here we demonstrate theoretically and confirm by micromagnetic simulations that, in the presence of Dzyaloshinskii-Moriya interaction, an antiferromagnetic domain wall acts naturally as a spin wave polarizer or a spin wave retarder (waveplate). Our findings provide extremely simple yet flexible routes toward magnonic information processing by harnessing the polarization degree of freedom of spin wave.

cond-mat.other↗

arXiv2016

Spin-Wave Fiber

Spin waves are collective excitations propagating in the magnetic medium with ordered magnetizations. Magnonics, utilizing the spin wave (magnon) as information carrier, is a promising candidate for low-dissipation computation and communication technologies. We discover that, due to the Dzyaloshinskii-Moriya interaction, the scattering behavior of spin wave at a magnetic domain wall follows a generalized Snell's law, where two magnetic domains work as two different mediums. Similar to optical total reflection that occurs at the water-air interfaces, spin waves may experience total reflection at magnetic domain walls when their incident angle larger than a critical value. We design a spin wave fiber using a magnetic domain structure with two domain walls, and demonstrate that such a spin wave fiber can transmit spin waves over long distance by total internal reflections, in analogy to an optical fiber. Our design of spin wave fiber opens up new possibilities in pure magnetic information processing.

cond-mat.mes-hall↗

arXiv2015

A spin wave diode

A diode, a device allowing unidirectional signal transmission, is a fundamental element of logic structures and lies in the heart of modern information systems. Spin wave or magnon, representing a collective quasi-particle excitation of the magnetic order in magnetic materials, is a promising candidate of information carrier for the next generation energy-saving technologies. Here we propose a scalable and reprogrammable pure spin wave logic hardware architecture using domain walls and surface anisotropy stripes as waveguides on a single magnetic wafer. We demonstrate theoretically the design principle of the simplest logic component, a spin wave diode, utilizing the chiral bound states in a magnetic domain wall with Dzyaloshiskii-Moriya interaction, and confirm its performance through micromagnetic simulations. Our findings open a new vista for realizing different types of pure spin wave logic components and finally achieving an energy-efficient and hardware-reprogrammable spin wave computer.

cond-mat.mtrl-sci↗