Program

Session I. Chair: Kab-Jin Kim

Talk 1 [10:10 AM - 11:00 AM (GMT +9)]

Speaker: Yi Li (Argonne National Laboratory) 

Title: Time-domain Coherent Magnon Interference with On-chip Superconducting Hybrid Magnonic Circuits

Abstract: Cavity magnonics is an emerging research field which cultivates strong coupling of magnetic excitations, or magnons, with microwave photons for exploring their coherent interactions [1-2]. This field is a sub-branch of hybrid dynamic systems with primary applications in quantum information science, with the main goal of coherent manipulation of entanglements within decoherence time. Up to now, there are many cavity systems and magnetic systems that have been investigated for building hybrid magnonic systems. However, in order to manipulate magnon states in time domain, critical high performance and on-demand control of magnon-photon interaction is needed. Superconducting microwave resonators provide a new paradigm for building high-performance hybrid magnonic systems [3] because 1) they provide high quality factor and thus long photon coherence time; 2) they allow for flexible coplanar design and engineering of magnon-photon interactions with lithographic fabrications; 3) they are compatible for embedding superconducting quantum components such as qubits in the hybrid magnonic system.

Here, we develop a superconducting circuit platform, incorporating chip-mounted single-crystal YIG spheres that are mounted in lithographically defined holes on silicon substrates with superconducting resonators, for implementing microwave-mediated distant magnon-magnon interactions [4]. For a single 250-μm-diameter YIG sphere, we achieve a magnon-photon coupling strength of 130 MHz with both the magnon and photon coherence time approaching 1 μs at 1.6 K, corresponding to a cooperativity of 13000. In a two-sphere-one-resonator circuit, we achieve a resonator mediated magnon-magnon coupling strength of 14 MHz in the dispersive coupling regime where the magnon-photon frequency detuning is ten times larger than their coupling strength.

In addition, we use the two-YIG-sphere-one-superconducting-resonator system to demonstrate time-domain coherent magnon interference. Using two vertical microwave antennas that are placed adjacent to each YIG sphere, we can independently control and read out the magnon excitation state of the two spheres. By sending a microwave pulse to one sphere and measuring the time trace of microwave output from the other antenna, we can measure the Rabi-like oscillation of magnon excitations between the two remote YIG spheres that are strongly coupled from their dispersive coupling to the superconducting resonator bus. In addition, we show that by sending two microwave pulses with a certainly time delay, their interactions to the YIG sphere lead to coherent interference, i.e. constructive or destructive interference depending on the relative phase delay of the two pulses. This allow us to program the magnon-magnon hybrid states by changing the frequency of the pulse microwave and the time delay.

Our results provide a high-performance, circuit integrated cavity magnonic system for building coherent magnon networks and processing hybrid magnon excitations in the time domain. We anticipate the hybrid YIG-sphere-superconducting-resonator architecture to be applied in more realistic quantum magnonics study, particularly for microwave-to-optic coherent transduction and nonreciprocal microwave information processing.

Work at Argonne and UIUC was supported by the U.S. DOE, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under contract No. DE-SC0022060. Use of the Center for Nanoscale Materials (CNM), an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no. DE-AC02-06CH11357. K.-J.K. is supported by KAIST-funded Global Singularity Research Program for 2021 and the National Research Foundation of Korea (NRF) funded by the Korean Government (MSIP) under grant No. 2020R1A2C4001789, 2016R1A5A1008184. M.S. was supported by the education and training program of the Quantum Information Research Support Center, funded through the National research foundation of Korea (NRF) by the Ministry of Science and ICT (MSIT) of the Korean government under grant No. 2021M3H3A103657313.

[1] Y. Li, et al., Hybrid magnonics: Physics, circuits, and applications for coherent information processing, J. Appl. Phys., 128, 130902 (2020)

[2] B. Z. Rameshti et al., Cavity Magnonics, Phys. Rep., 979, 1-61 (2022)

[3] Y. Li, et al., Strong coupling between magnons and microwave photons in on-chip ferromagnet-superconductor thin-film devices, Phys. Rev. Lett. 123, 107701 (2019)

[4] Y. Li et al., Coherent coupling of two remote magnonic resonators mediated by superconducting circuits, Phys. Rev. Lett. 128, 047701 (2022)

Talk 2 [11:00 AM - 11:30 AM (GMT +9)]

Speaker: Geoffrey M. Diederich (University of Washington ) 

Title: Tunable interactions between excitons and hybridized magnons in a layered semiconductor 

Abstract: The interaction between distinct excitations in solids is of both fundamental interest and technological importance. One such interaction is the coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon, a collective spin excitation. The recent emergence of van der Waals magnetic semiconductors provides a platform for exploring these exciton-magnon interactions and their fundamental properties, such as strong correlation, as well as their photo-spintronic and quantum transduction applications. In this talk, I will demonstrate precise control of coherent exciton-magnon interactions in the layered magnetic semiconductor CrSBr. I will show how varying the direction of an applied magnetic field relative to the crystal axes breaks the rotational symmetry of the magnetic system. Thereby, one can tune not only the exciton coupling to the bright magnon, but also to an optically dark mode via magnon-magnon hybridization. Further, I will show how we modulated the exciton-magnon coupling and the associated magnon dispersion curves through the application of uniaxial strain. At the critical strain, a dispersionless dark magnon band emerges. The results I will present demonstrate unprecedented control of the opto-mechanical-magnonic coupling, and a step towards the predictable and controllable implementation of hybrid quantum magnonics.

Session II. Chair: Junho Suh

Talk 3 [11:45 AM - 12:15 PM (GMT +9)]

Speaker: Tomoki Hirosawa (Aoyama Gakuin University)  

Title: Magnetoelectric Cavity Magnonics in Skyrmion Crystals 

Abstract: Recently, a strong coupling between magnons and microwave photons in a cavity has attracted much attention. While previous works have focused on the magnetic coupling between magnons and photons via the Zeeman effect, multiferroic materials host electromagnons that can be excited by oscillating electric fields. Here, we present a theory of magnetoelectric magnon-photon coupling in cavities hosting noncentrosymmetric magnets. Analogously to nonreciprocal phenomena in multiferroics, the magnetoelectric coupling is time-reversal and inversion asymmetric. This asymmetry establishes a means for exceptional tunability of magnon-photon coupling, which can be switched on and off by reversing the magnetization direction.

Taking the multiferroic skyrmion-host Cu$_2$OSeO$_3$ as an example, we reveal the electrical activity of skyrmion eigenmodes and propose it for magnon-photon splitting of ``magnetically dark'' elliptic modes. Furthermore, we predict a cavity-induced magnon-magnon coupling between magnetoelectrically active skyrmion excitations. We discuss applications in quantum information processing by proposing protocols for all-electrical magnon-mediated photon quantum gates, and a photon-mediated SPLIT operation of magnons. Our study shows magnetoelectric cavity magnonics as a novel platform for realizing quantum-hybrid system.

Talk 4 [12:15 PM - 12:45 PM (GMT +9)]

Speaker: Sang-Koog Kim (Seoul National University) 

Title: Non-reciprocal, On-Off Switchable Negative Refraction through Photon-Magnon Coupling

Abstract: The rapid growth of metamaterials has led to unique electromagnetic properties surpassing natural materials' limits. One notable phenomenon is negative refraction [1,2], which allows for applications like superlensing and subwavelength imaging. Our study explores negative refraction in a photon-magnon hybrid system, specifically yttrium iron garnet films coupled with inverted split-ring photon resonators [3,4]. An equivalent analytical circuit model helps us understand and quantify our experimental observations, revealing that anti-damping [5], due to photon-magnon coupling, leads to a generalized condition [6] that only requires negative values in either permittivity or permeability. Our novel photon-magnon system displays nonreciprocal refraction, which can be switched on and off through magnetic-field control. This is due to the interplay between the coherent and dissipative coupling of photons and magnons. Our work provides insights into achieving negative refraction and offers a fresh perspective for developing practical negative refractive index materials, fostering advancements in communication networks and quantum technologies.

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (No. NRF-2021R1A2C2013543).

[1] Vesselago V. G., The Electrodynamics of substances with simultaneously negative values of ε and μ, Sov. Phys. Usp. 10, 509 (1968).

[2] Smith D. R., Padilla W. J., Vier D. C., Nemat-Nasser S. C., and Schultz S., Composite medium with simultaneously negative permeability and permittivity, Phys. Rev. Lett. 84, 84 (2000).

[3] Bhoi B., Kim B., Kim J., Cho Y.-J., and Kim S.-K., Robust magnon-photon coupling in a planar-geometry hybrid of inverted split-ring resonator and YIG film, Sci. Rep., 7, 11930 (2017).

[4] ] Bhoi B., Kim B., Jang S.-H., Kim J., Yang J., Cho Y.-J., and. Kim S.-K., Abnormal anticrossing effect in photon-magnon coupling, Phys. Rev. B., 99, 134426 (2019).

[5] Wang, Y.-P., Rao, J. W., Yang, Y., Xu, P.-C., Gui, Y. S., Yao, B. M., You, J. Q., & Hu, C.-M. Nonreciprocity and unidirectional invisibility in Cavity Magnonics. Physical Review Letters, 123(12). (2019).

[6] McCall M. W., Lakhtakia A., and Weiglhofer W. S., The negative index of refraction demystified, Eur. J. Phys. 23, 353 (2002).

Talk 5 [12:45 PM - 1:15 PM (GMT +9)]

Speaker: Jinwei Rao (ShanghaiTech University)   

Title: Pump-induced magnon mode and Magnonic Frequency Comb 

Abstract: Magnons in ferrimagnetic spheres have garnered significant attention due to their promising applications in quantum technologies. In 1957, L. R. Walker solved all possible magnetostatic modes in a ferrimagnetic sphere using Maxwell and Landau-Lifshitz equations, leading to the identification of these modes as Walker modes. Recently, we made an exciting discovery of a new magnon mode in a Y3Fe5O12 (YIG) sphere by subjecting the sphere to a strong microwave pump. We called this mode pump-induced magnon mode (PIM).

Unlike normal Walker modes, a PIM exhibits excellent tunability and intriguing nonlinear behaviors, as its effective spin number and mode frequency can be freely manipulated by the microwave pump. For instance, by adjusting the pump power, the coupling strength between a PIM and a Walker mode can be tuned following a fourth root relation of the pump power, expressed as g∝𝑃1/4 . Furthermore, when the coupled system composed of a PIM and a Walker mode is simultaneously driven by the pump and probe signals, the coupling strength becomes time periodic, resulting in the generation of magnonic frequency combs (MFCs). This nonlinear process reaches its maximum near the exceptional points (EPs) of the coupled system, and we have found a method to enhance the MFC generation by exploiting EPs. Consequently, our method significantly improves the efficiency of frequency conversion and produces MFCs with a record number of teeth (> 32). Our works establish a synergistic relationship between non-Hermitian physics and MFCs, offering advantages for coherent/quantum information processing and ultra-sensitive detection. 

Session III. Chair: Kab-Jin Kim

Talk 6 [3:30 PM - 4:00 PM (GMT +9)]

Speaker: Tao Yu (Huazhong University of Science and Technology)  

Title: Nonlinear and non-Hermitian topological control of magnons 

Abstract: In this talk, I will discuss our theoretical and experimental works on nonlinear and non-Hermitian topological control of magnons.

In the nonlinear regime, we studied magnon frequency comb [1] and phonon frequency comb [2] driven by magnetization dynamics. We found that the strong microwave drive can induce a new type of magnon state, and observed its energy level repulsion with other magnon modes in a waveguide rather than a resonator [1]. By adjusting the coupling intensity of the microwave drive, we achieve the exceptional points of magnons and realize the magnon frequency comb with 32 teeth enhanced by the non-Hermitian topology. We predict that the ferromagnetic resonance of magnetic nanowires under strong microwave drive can stimulate the second harmonic generation of phonons and the phonon frequency comb [2].

We systematically studied the chiral interaction of magnons [3]. In chiral coupled magnet arrays, we predicted the realization of one-dimensional [4] and two-dimensional [5] magnon non-Hermitian skin effects and a new topological characterization method [6], which makes it possible to achieve magnon accumulation and highly sensitive detection of microwave magnetic fields.

Talk 7 [4:00 PM - 4:30 PM (GMT +9)]

Speaker: Jinsheng Wen (Nanjing University)   

Title: Direct observation of topological magnon polarons in a multiferroic material

Abstract: Magnon polarons are novel elementary excitations possessing hybrid magnonic and phononic signatures, and are responsible for many exotic spintronic and magnonic phenomena. Despite long-term sustained experimental efforts in chasing for magnon polarons, direct spectroscopic evidence of their existence is hardly observed. I this talk, I will talk about our recent work on the direct observation of magnon polarons using neutron spectroscopy on a multiferroic Fe2Mo3O8 possessing strong magnon-phonon coupling. We attribute the formation of magnon polarons to the strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction. Intriguingly, we find that the band-inverted magnon polarons are topologically nontrivial. These results uncover exotic elementary excitations arising from the magnon-phonon coupling, and offer a new route to topological states by considering hybridizations between different types of fundamental excitations

Talk 8 [4:30 PM - 5:00 PM (GMT +9)]

Speaker: Irina Bobkova (Institute of Solid State Physics)

Title: Composite magnetic excitations in superconductor/magnet heterostructures

Abstract: The ability to control the dispersion law of spin waves is one of the most important requirements for the engineering of magnonic devices. We show that thin-film hybrid structures consisting of a ferromagnetic (F) or antiferromagnetic (AF) insulator and a superconductor (S) or normal metal (N) have broad prospects in this field. Structures with topological superconductors are also touched upon. Due to the presence of a surface exchange interaction between the magnetic insulator and the metal, an effective exchange field is induced in the latter. This leads to the appearance of spin polarization of the quasiparticles in the superconductor and the generation of triplet Cooper pairs in it, which create a rotational moment that acts on the magnetization of the magnet.

In structures with a ferromagnet and an antiferromagnet, due to the difference in their characteristic frequencies (GHz for ferromagnets and THz for antiferromagnets), the processes described above play a different role and the effect of a superconductor on a magnet looks different. In a typical ferromagnet, spin-triplet pairs exert the main influence. They dress the magnon, increase its mass, and screen the spin. The resulting composite particle was called magnon-Cooparon [1]. For structures with an antiferromagnetic insulator, the renormalization of the magnon spectrum involves both the polarization of quasiparticles and spin-triplet pairs. Unlike a ferromagnet, the spectrum of magnons in an antiferromagnet contains two modes, which for an easy-axis antiferromagnet are degenerate in energy at zero applied magnetic field. Interaction with a superconductor leads to the removal of this degeneracy [2]. One of the interesting practical results of our work is the proposal of a method for direct measurement of the exchange field induced by a magnet in a superconductor (or a normal metal, which is considered as the limiting case of a superconductor at high temperature). In structures with topological superconductors the presence of strong spin-orbit coupling brings additional new physics, which manifests itself as a coupling between magnons and collective excitations of the superconductor, Higgs modes.

The support by RSF project No. 22-42-04408 is acknowledged.

References

1. I. V. Bobkova, A. M. Bobkov, A. Kamra, and W. Belzig, Communications Materials 3, 95 (2022).

2. A. M. Bobkov, S. A. Sorokin, I. V. Bobkova, Phys. Rev. B 107, 174521 (2023).

Session IV, Chair: Se Kwon Kim

Talk 9 [5:15 PM - 5:45 PM (GMT +9)]

Speaker: Ka Shen (Beijing Normal University)   

Title: Magnon-Ferron Coupling Mediated by Dynamical Dzyaloshinskii-Moriya Interaction 

Abstract: As magnons in magnetic materials have been intensively discussed in the past decades, their counterparts in ferroelectric materials so called ferrons have been addressed recently. In multiferroic materials with both long range magnetic and electric orders, these two types of quasiparticles coexist. In this talk, based on a two-dimensional model, we will discuss the magnetic and electric control of the spin-lattice configuration and the magnon-ferron hybridization due to the dynamical  DzyaloshinskiiMoriya interaction connecting the magnetic and ferroelectric orders. We will see the transitions of the spin-lattice configuration introduced by external magnetic and electric fields can provides tunable enhancements of the electric and magnetic susceptibilities and efficient control of nonreciprocity in the magnon-ferro coupling. 

Talk 10 [5:45 PM - 6:15 PM (GMT +9)]

Speaker: Ryo Sasaki (RIKEN)

Title: Angular momentum conversion from acoustic wave to magnetization 

Abstract: The control of magnetization by angular momentum conversion from various systems, such as electrons, photons, and rotation of rigid bodies, has been widely studied.
Recently, the concept of angular momentum of "phonons" has been theoretically proposed, and the existence of phonon modes with finite angular momentum, so-called "chiral phonons," has been studied mainly using optical methods.
In order to better understand the physics of the angular momentum of phonons and to link it to various applications, it is helpful to investigate the phenomena of their conversion to other physical systems.
In this talk, we will present our work on angular momentum conversion between surface acoustic waves and the magnetization of ferromagnets. We observed that the direction of magnetization can be controlled depending on the direction of the angular momentum of the surface acoustic wave.

Session V Chair: Takehito Yokoyama

Talk 11 [6:30 PM - 7:00 PM (GMT +9)]

Speaker: Asle Sudbø (Norwegian University of Science and Technology) 

Title: Topological superconductivity mediated by quantum fluctuations in non-colinear spin states

Abstract: Topological superconductors are associated with the appearance of Majorana bound states, with promising applications in topologically protected quantum computing. In this Letter, we study a system where a skyrmion crystal is interfaced with a normal metal. Through interfacial exchange coupling, spin fluctuations in the skyrmion crystal mediate an effective electron-electron interaction in the normal metal. We study superconductivity within a weak-coupling approach and solve gap equations both close to the critical temperature and at zero temperature. Special features in the effective electron-electron interaction due to the noncolinearity of the magnetic ground state yield topological superconductivity at the interface.

Talk 12 [7:00 PM - 7:30 PM (GMT +9)]

Speaker: Gyungchoon Go (KAIST) 

Title: Topological Magnonics: Hall, Spin Hall, and Orbital Hall Effects of Magnons

Abstract: In this presentation, I will discuss my recent findings on topological magnon transport, specifically focusing on the  magnon Hall effect, magnon spin Hall effect, and magnon orbital Hall effect. At first, I will explain how the well-known magnetoelastic interaction driven by Kittel enables nontrivial topology of magnon-phonon hybrid excitations, even though each magnonic and phononic system is topologically trivial. Because the magnetoelastic interaction is ubiquitous in magnetic materials, our result indicate that existence of the topological magnon-polaron is not restricted to specific conditions but is a quite generic phenomenon. Next, I will discuss that the magnetoelastic interaction allows giant spin Nernst effect of magnon-polarons in the antiferromagnetic systems. Additionally, I will introduce a novel transport phenomena: magnon orbital Hall effect. Investigating honeycomb antieferromagnet, we discovered that the magnon orbital Berry curvature remains finite even in the absence of spin-orbit coupling related effects. This intrinsic transport property solely relies on the exchange interaction and lattice structure. Interestingly, due to the intrinsic nature of the magnon orbital Hall effect, the magnon orbital Nernst conductivity is estimated to be orders of magnitude larger than the predicted values of the magnon spin Nernst conductivity that require finite spin-orbit coupling. Because the magnetoelectric effect couples the magnon orbital and electric polarization, for the experimental detection of the magnon orbital Hall effect, I suggest the local voltage measurement which allows to detection of the magnon orbital accumulation.